GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Replacing HVAC System

jplee3 | Posted in General Questions on

Hi all,

We are in the process of getting our HVAC system replaced (furnace broke down and both the furnace and AC are from the 80s so it’s time to go) and ducts replaced as well. We just had the contractor come out to get measurements for a room-by-room Manual J and should have the results next week hopefully.
Currently, I’m considering a hybrid system (2-stage heat pump + ultra low nox 80% gas furnace – the reason for this isn’t orthodox in terms of the original intended use case for hybrid systems btw… I’m quite aware of that. My intention here is really to have the flexibility of switching between natural gas and the heat pump based on the prices of electricity vs natural gas where I’m at. We may go solar in the near future too but even that is up in the air… that is a whole different discussion though. We are under SDGE, who has the *highest* electric costs in the nation. So this solution is really a hedge against that. Plus, we get a $3k rebate so it ends up costing less to install a hybrid system than it does a regular gas furnace/AC system).
I’ve  gotten feedback elsewhere (including on hvac talk) and used loadcalc and other tools and it seems the ‘consensus’ via loadcalc is that a 3-3.5ton system would be most suitable. We’ll see what the contractor comes up with. On that note: nearly every other contractor I’ve had come out with the exception of one (who also did a block loadcalc and determined 3tons) has told me that I need a 4ton system at a *minimum* and a 5ton system for “maximum comfort” lol. The current system we have is rated at 4tons btw.
I’m in Southern California (92653 zip) with a 1980 built 2-story home – double-paned windows, stucco exterior and Spanish clay tile roofs. Current temp outside is 80F and the inside thermometer I have setup on the second floor reads 71.6F while the downstairs thermostat temp reading is 70F. We haven’t been here through a full summer (just moved in Oct ’21) but the house seems to have the tendency to stay cool. The flipside is that when it’s cold outside, it gets quite cold inside too. The ‘characteristic’ seems to be that heat has harder time getting into the home and a much easier time escaping.

I’m not 100% sure what the insulation of the exterior walls is but the attic insulation I believe is R19 (it’s somewhere between 6-7″ thick and is the old yellow batts). There are a few gaps and areas that we should have rodent proofed and pretty much all the bathroom vents, recessed can lights, etc are areas that would need air sealing. I’ve gotten some quotes from attic cleaning/insulation contractors who have mostly concluded this. All but one of them have strongly suggested blown-in cellulose insulation because it performs the best. One contractor was saying he would just come in, air seal and rodent proof, clean up a little bit but leave the R19 insulation as-is and not change a thing regarding the insulation.
Our HVAC contractor is vehemently against cellulose though, saying that he has multiple clients who have had issues with dust and lung/health problems after having cellulose blown in…even with some amount of air sealing. He had done some testing (blower door I think?) and in many cases determined there were still unsealed fixtures, etc where dust would get pulled into the space when things like the range exhaust fan were turned on or if a bunch of doors in the house were shut. He warned that even if there is thorough air sealing done, over time you are still likely to have more gaps/cracks/etc develop so the concerns with cellulose particulates entering the inside living space will always be there. He was suggesting going with denim insulation – I looked at it’s near impossible to find that and even if I can it looks extremely expensive. I’ve read that adding a 2-3″ layer of spray foam may provide a more effective seal and then you can supplement with whatever other insulation material you want (including cellulose) and not have to worry as much about the attic infiltration issue.  I’m kind of lost in all of it and just want a solution that can address the attic infiltration concern and still provide decent insulation. Our HVAC contractor was saying the better you insulate too, the less strain there should be on our HVAC system and we could justify ‘downsizing’ (or right-sizing I suppose) much more… I’m not quite sure how much increasing from R19 to R38 would impact the heat load calculation.

I’m also open to DIY (if it makes sense, and would still need to research and prep a bit for that)

I’ve also asked about radiant barriers and gable exhaust fans and there seems to be a lot of debate for those things. Some contractors/ppl think radiant barriers are great while gable exhaust fans are nonsense and vice versa. Or some people seem to think that neither are really that effective at all. I don’t know what to make of it…

Just wanted to get some feedback on the best route to take here or if we even really need to do much.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Matteus_Olmedo | | #1

    My advice is to get a new HVAC contractor. You just need a heat pump. This will heat and air condition. There is no reason to get a furnace as well. Cellulose is not dusty except for during installation. Definitely air seal and repair leaky ducts but cellulose is your best choice for attic insulation for your health and your wallet. Attic ventilation is also important so you want to make sure there is adequate ventilation in place whether it be a gable fan or otherwise. There is plenty of great information on this site on all of these topics and if you read through you will learn a lot.

    1. jplee3 | | #2

      I've already signed the contract with him - he is the most experienced of all the contractors I've had out to get quotes and the only one actually willing to do a room-by-room load calc. He just presented hybrid as a possible option but didn't pressure me to do it in any way. I was researching this beforehand and that was something I had thought of on my own. Unless we go solar (which is not a guarantee at this point), running a heat pump in the winter is going to do a number on our electric bill - our current rates are as high as 50 cents/kwh! Again, the plan for this is quite unorthodox - it's not about having a gas furnace that can kick in when it gets to below-freezing temps outside (because that should never be the case where I am). It's about having the freedom to choose between fuel and electricity for heat based on rates. If we already had solar at this point, I probably would have just gotten a heat pump. I'm saying right now that we want to put solar up within the year or two but that may not happen... it may not happen for years. And if NEM 3.0 passes before I ever get around to it, it may not ever happen! The hold-up with all this right now is an issue we're having with electric panel relocation - SDGE is notorious for dragging their feet and taking a long time for things to get going. This is compounded with it not being trivial finding a very easily suitable place to actually relocate the panel due to trenching challenges which substantially increases the cost of the project 2-3 times what it normally would cost just to do a simple panel upgrade! Anyway, the decision around this is really based on rates than it is efficiency (I know this is a site about efficiency but not all of my decisions are going to only be based 100% on that)

      The concern with cellulose isn't so much how dusty it gets around installation time. It's when the stuff finds it's way into your home through gaps/cracks that develop *after* the install (and air sealing) whenever there's negative air pressure. This could be years down the line as it starts degrading in conjunction with gaps/cracks/etc forming in the ceiling/corners as the house settles and shifts.

      1. Matteus_Olmedo | | #3

        There are people much more experienced than me on this forum, but I have insulated several houses with cellulose both blown in and dense pack, and I can’t imagine it would ever behave as you described, whether it is the day after you install or 50 years down the road. As far as the furnace, I can guarantee you will never get back the price of the furnace no matter how high electricity prices are. The bottom line is that over the next decade, gas prices will rise since it is a limited resource, and electricity prices will decrease as solar tech and economies of scale in regards to renewables come into play.

        1. Deleted | | #8


        2. jplee3 | | #9

          Seems like it depends on how thorough you are with air sealing everything before blowing the stuff in. I was just reading a review of Sanctuary on Home Depot where the person blew it in and it started leaking out of a light fixture lol. So it seems like as long as you're very good and very thorough about identifying ALL sources of leaks in the ceiling and sealing them up very well, you'll probably be fine.
          BTW: with the contractor I'm using, the cost of installing a hybrid system vs heatpump is no different. Either way it's less than going with the standard gas/AC setup. So it's not more expensive paying for the furnace in this case. The math is something like this: standard gas/AC might be $10k. Adding a heat pump to the equation (whether you choose an air handler or furnace, per my understanding...I'll double-check on this) ups it by $100o so it's now $11k. But there's the $3k rebate incentive so in reality it's actually $8k after rebates. So it's still $2k less than the gas/AC install.

          I don't have as much faith when it comes to the cost of electricity going down... unless we all become grid independent. These utility companies/pocos are getting super aggressive with all their Net Metering initiatives to try to reclaim "what's rightfully theirs" lol. I just wonder how this is all going to play out when most people end up going solar. But it's very clear that these initiatives they're trying to push through right now are heavily favoring them and not people who are trying to go solar.

  2. Expert Member
    Akos | | #4

    Most likely your highest on peak $.50 rate is in the summer. You are running the AC at that time anyways, so heat pump vs furnace is moot. If anything, you want a high quality modulating heat pump as these tend to have a much higher SEER rating and will cost way less to run in the cooling season.

    For the rest of the year, your electricity is probably average, this puts a heat pump at a COP of 3 slightly cheaper cost than gas. Since you are paying extra for the gas, I can't see any ROI for having it. Add in the meter fees and gas is costing you even if you don't use it.

    Now is the time, ditch the fuel burner. I'm in much colder climate (north edge of Zone 5), even up here NG heat is starting to make less and less sense.

    One benefit that most people don't know about is the comfort from a modulating heat pump. A right sized unit will be delivering some amount of heat all the time, you never have blast of hot followed by the long waits between thermostat calls you typically see with a gas burner. Nice even heat all the time. Just for this it is worth it.

    1. jplee3 | | #7

      I just checked again - so the rates up to 130% of baseline are 39 cents. Anything over 131% is 49 cents. So it's the same regardless of the time of year. Is a modulating heat pump one of those fancy variable speed ones? We are going with Carrier - the top of the line Infinity heat pumps would probably end up costing us at least several thousand more. I've heard, financially, it just doesn't make that much sense given we're in a very moderate climate. We currently have other gas appliances in the house right now (stove/oven, dryer, and water heater), so we're paying the gas fees regardless. I really don't want to go all out right now and replace perfectly functional appliances for the sake of going all electric (on that note, I've seen people point out that going all electric might feel good until you realize the power plant a few hours away providing most of the power in your area is burning a ton of natural gas). Here natural gas runs anywhere from $1.50-1.75/therm. It's significantly less right now to run NG but there is some speculation that CA will wean people off this by increasing costs year over year.

      For the comfort thing, I've heard different - it seems a bit subjective. Some people prefer the blast of dry hot air and will say that heat pumps produce more a "humid heat" - obviously I wouldn't know the difference but I've heard, anecdotally, of some switching from gas furnace to heat pump and absolutely disliking it when it comes to heat. I don't know the science behind everything but I would guess that the climate you're in also can make a difference with all this.

      1. Expert Member
        Dana Dorsett | | #29

        The Infinity w/Greenspeed (also now with some Midea/Toshiba innards) have lots of bells & whistles that you probably don't need. The simpler mid-static 3 ton Fujitsu unit would likely be cheaper and run about as efficiently.

        A 3 ton Carrier branded Midea would probably cut it too:!/product/36747

        ... but a 3.5 ton Toshiba/Carrier is likely to be both more efficient & more comfortable due to the bigger turn-down ratios + lower minimum output modulation levels, with a bit more margin on the peak cooling:!/product/32099

        (All Midea units use Toshiba compressor technology. Midea & Toshiba have been in bed together for a couple of decades or more, with Carrier snuggling up with them for about a decade.)

    2. user-5946022 | | #16

      And the story your contractor told about cellulose is definitely NOT my experience. I have loose fill blown in my attic and it is fine - great actually - much better than itchy fiberglass, and probably healthier.

      It is highly unlikely your house will get so tight that when you turn on the range exhaust it will backdraft, and if it does, you can put in makeup air.

      I'm wondering though - are the ducts for your HVAC system in your attic? If so, you have a different issue - you don't want to run conditioned air in ducts through unconditioned spaces.

      1. jplee3 | | #17

        I'm wondering if there are shady companies out there who do a half-a job with air sealing. I could see how, if you don't air seal well, the cellulose could start leaking through the gaps. I'm wouldn't be surprised if there are a number of people out there who have hired companies to blown in cellulose who either don't air seal or do a pretty poor job of it.

        Yes, the ducts for the system are in our attic - this is a pretty common thing here in SoCal if not the furnace/blower being also located in the attic. In our case, the furnace/blower is currently located in our garage and the ducts in the attic. Where else would ducts normally go? Perhaps in moderate climates it's not as big of a deal?

        1. user-5946022 | | #19

          You need to be wary of this HVAC contractor.
          Ducts with conditioned air inside running through an unconditioned attic is a far bigger issue than any of these other things, except maybe air sealing.

          Yes, most companies out there do a half job with air sealing, because most of them are hired to do a specific portion of the job, so have no responsibility for the actual results. They just don't give a ----

          1. jplee3 | | #20

            Where else are you supposed to run the ducts? Through the basement? This is a 2-story home with no basement and no crawlspace. When the builders built the house, the attic is where they put the ducts. We aren't going to hang ducts inside our rooms. And going minisplit would be cost prohibitive. In terms of ductwork in the attic - this is how many homes in the area are. In fact, I've never heard of homes *not* having ductwork in the attics here in CA at least. And in many cases, most homes built in the 90s and later also have the furnace unit located in the attic out here! What I have heard of people doing out here to help with the temp issues is installing gable exhaust fans and also a radiant barrier.

          2. user-5946022 | | #28

            I think we are too deep into the discussion so the site is not giving me the option to reply to your response.
            The typical solution in your situation is to either build a small enclosure in the attic in which the unit and the ducts are enclosed and enclosure becomes part of the conditioned area of the house, or you build a soffit drop from the ceiling for the ducts, or a combination of both.

            If you replace your traditional unit with ducted minisplit, those can often be installed in the ceiling of a closet - they are very small. Then distribute the duct upstairs in a dropped soffit. The mistake often made is thinking the supplies need to be at the exterior - if your house is properly air sealed you can deliver the supply near the center of the house, which is usually near the corridor, so sometimes only the corridor ceiling is dropped, and the ductwork is minimal.

  3. jplee3 | | #5

    I just checked again - so the rates up to 130% of baseline are 39 cents. Anything over 131% is 49 cents. So it's the same regardless of the time of year. Is a modulating heat pump one of those fancy variable speed ones? We are going with Carrier - the top of the line Infinity heat pumps would probably end up costing us at least several thousand more. I've heard, financially, it just doesn't make that much sense given we're in a very moderate climate. We currently have other gas appliances in the house right now (stove/oven, dryer, and water heater), so we're paying the gas fees regardless. I really don't want to go all out right now and replace perfectly functional appliances for the sake of going all electric (on that note, I've seen people point out that going all electric might feel good until you realize the power plant a few hours away providing most of the power in your area is burning a ton of natural gas). Here natural gas runs anywhere from $1.50-1.75/therm. It's significantly less right now to run NG but there is some speculation that CA will wean people off this by increasing costs year over year.

    For the comfort thing, I've heard different - it seems a bit subjective. Some people prefer the blast of dry hot air and will say that heat pumps produce more a "humid heat" - obviously I wouldn't know the difference but I've heard, anecdotally, of some switching from gas furnace to heat pump and absolutely disliking it when it comes to heat. I don't know the science behind everything but I would guess that the climate you're in also can make a difference with all this.

    1. Expert Member
      Akos | | #10

      Modulating and variable speed are the same thing. The Carrier unit is a great option in your climate.

      There is a lot of myths about forced air circulating, usual one is it dries the air. First time I heard that it adds humidity. Either one is impossible, the only thing a heat pump or furnace does is heat up the air, it does not add or take away moisture.

      The common issue is hugely oversized units. Since even the low modulation on these could be well above the house loads, the units will tend to cycle which defeats the purpose of a modulating unit. So make sure the unit is sized correctly. A good way to check is to run through the calculations here using an old gas bill:

      Utility bills, at least here, tend to be pretty obfusticated to hide costs. I would take a high usage natural gas bill, divide the total cost by the therms used to see what it really is.

      Keep in mind that you can always produce electricity yourself but not natural gas. A heat pump running from electricity generated by a gas plant still uses less gas to produce the same heat than burning gas directly.

      Replacing perfectly good natural gas appliances when they are working is no a good investment. If they do need replacement, look for electric options.

      1. jplee3 | | #13

        The variable speed unit is significantly more expensive. The cost differential between a single-stage and 2-stage system is maybe a couple thousand. To go up to the 5-stage FROM the 2-stage it's $3k more and then to go up to the full variable speed (also from the 2-stage) is $5k (so upgrading from a single-stage the the variable is nearly $7k)! Not sure that the efficiency savings would really even out for us for the significantly added expense whether we are coming from single-stage or 2-stage (I would wager that a major repair on the variable speed unit would also probably occur before we can even break even if it even lasts that long... the price you pay for comfort and sake of "efficiency" I suppose). I was doing some math on the therms usage compared to what it, in theory, would be for electrical and it's less. I don't have all of it in front of me but running the numbers along with a couple others on some financial forums we all concluded the same that at least with my local utilities (Socal Gas for Gas and SDGE for electric), it costs less to heat with gas than it does electricity. I also ran the numbers through and the Carrier calculator - so from a financial perspective, at least right now, there's not a ton of incentive or motivation to go *all in* on going green... *eventually* we'll get there though, so we hope ;) The only saving grace here would be solar IF we end up doing it. And right now that's a big IF with the logistical problems we're running into...

    2. exeric | | #11

      " Just checked again - so the rates up to 130% of baseline are 39 cents. Anything over 131% is 49 cents. So it's the same regardless of the time of year."

      Maybe I'm reading more into your statement than is warranted but it sounds like you're saying that you normally consume greater than 130% of baseline in electricity. Is this correct? If it is then it sounds like your home would have some low hanging fruit that could reduce your electricity consumption. What are the main causes of this excess electricity consumption? It seems like this would be a valid assumption by me since the range/oven, dryer, and water heater use NG. Rather than configure your HVAC solution to your (apparent) overuse of electricity it might make more sense to reduce your current electricity consumption through whatever means is available. This could mean giving more priority to solar than you currently have. You really want to move to a lower tier of electricity consumption. Once you have then a heat pump will make perfect sense again. Again, if I'm reading more into that quote than is reasonable let us know.

      I concur with Akos. I used to have a propane fireplace and replaced it with a mini split. The house is much more comfortable now in my northern California location.

      1. jplee3 | | #15

        We just moved in here back in October, so I really have no good baseline to go off of so far. We were going over our baseline at the apartment we were renting though - they probably had a 4-5ton unit running in a 700sq ft second floor apartment unit LOL, so it was easy to go over. From the limited time I've been here though, and I have to check our bills, but I think we may have gone over baseline once or twice probably when we were running the heating/cooling. I'm not 100% sure on the envelope of the house at the moment but I do think it's leaky. We should know more after doing HERs testing too. We're between 500-600kwh month-over-month at the moment - I don't know if that's high, average or below average.

        EDIT: I just checked the bills and our baseline allowance ranges from anywhere between 275kwh-350kwh. Honestly, I don't know how that's determined or if that's the same for all customers...? It says our billing voltage level is 'secondary' so I'm not sure if that actually indicates that we are above baseline. What's weird is that our electric charges seem to indicate the 39cent/kwh 130% baseline charge. There are added electricity generation charges which are 15 cents on-peak, 8 cents off-peak and 6 cents super off-peak. But on every bill I've also received a "baseline adjustment credit"

        Hard to really understand the breakdown but if I simply divide our total cost by the total kwh used, I get: 42 cents/kwh for Feb, 43 cents/kwh in Jan, 38 cents/kwh in Dec, 36 cents/kwh in Nov, and 36 cents in Oct.

        For gas, the total cost paid per therm has ranged between $1.65-1.83/therm where 'normal' usage (not much furnace) is around 50 therms/mo and higher usage (while the furnace was working) was around 75 therms/mo. So using the furnace through the coldest part of the year seems to have added 25 therms in a month (but there certainly could have been other factors too like cooking more, running more loads of laundry, etc)

    3. exeric | | #12

      NEM 3 isn't going through after all:,production%20back%20to%20the%20grid.

      There was a ground swell of opposition due to its basic unfairness. I'm feeling immense relief about that.

      1. jplee3 | | #14

        I wouldn't count it out that they would push hard for the next revision of it. Another NEM just passed in Florida -

        The more they pass these things, the more it just disincentivizes people from going solar. I see this being a major battle here in CA but the way CPUC seems to be (asleep at the wheel), I wouldn't be surprised if another one passes even if it's "not as bad" - I mean, if they signed off on SDGE raising their rates to be the highest in the entire nation (and after publicly being called out, SDGE can't give a valid reason as to why lol - obviously it's a monopoly so very much greed-based), I wouldn't count anything out...

  4. Deleted | | #6


  5. bigred | | #18

    I have had a couple of insulation contractors not recommend cellulose in hot humid climates. And recommend fiberglass instead. In less humid areas they like cellulose.

  6. Robert Opaluch | | #21

    Your zip code is San Diego. Attached are the temps, cloud cover and solar radiation for your zip code from, a great resource for all of us who are trying to figure out requirements for heating and cooling, and how much solar potential we have in our area.

    I apologize for being a bit rude here, but you appear to have been misled. You seem to be worried about the wrong things. You should have minimal needs for heating and cooling in your fairly benevolent climate IF your home had a well sealed and insulated building shell. You have 50F average mid-winter low temps (that's not considered cold weather for most of us), high's midsummer well under 80F (that's not considered hot for most of us), and tremendous solar potential. This is not the types of difficult climates most of us face when we need to air seal, insulate and heat/cool our home interiors. (I lived in LA metro 7 years and loved the climate vs. colder climates of Colorado, New England, New Jersey, and Germany).

    Go solar now. The potential is there, cash in.

    Yes, air sealing and insulation work needs to be done before you discuss heating and cooling systems. Installing heating/cooling first is the wrong sequence. OR if you decide to install the system first, DO NOT use the current Manual J as a guide. Your system will be way too oversized, and thereby won't dehumidify as well in summer. A heat pump water heater might help you dehumidify as well as cool your home.

    Your biggest cooling issue might be solar gains through windows, especially any unshaded west and east-facing windows. There are many shading options (e.g., trees), don't let one unappealing option end your search for others.

    Manual J has a lot of subjective choices, and I don't think an HVAC contractor will right-size your equipment. They tend to oversize by a large margin. Especially when you have gotten advice to avoid some insulation materials that are very popular and cost-effective. 4ton/5ton in...San Diego??

    Best of luck, whatever you decide to do. But in engineering, it typically pays to keep it simple. I suspect that a single ductless mini split would be sufficient for your needs, and would be inexpensive compared to alternatives. One head downstairs, maybe one ducted head upstairs for bedrooms. Others here know more how to size a system once you get your air leakage and insulation work done, and share more information about your room layout etc.

    1. exeric | | #22

      A big +1 to Bob's reply. The beginning of the solution to any problem is to learn what you do NOT know. In other words, not knowing something that is important to your situation (but you don't know it's important!!) can severely harm your interests. Ask Vladimir Putin.

    2. jplee3 | | #23

      I'm not sure why it thinks that zip is SD - I'm in South Orange County (Laguna Hills). The second choice from the dropdown is in fact Laguna Hills.

      Our closest weather station is El Toro. I've attached the SNA one as well.

      Right now I don't know how well-sealed or insulated our home is. All I know is that even on warmer days it tends to stay cool inside (72-74F normally while it could be anywhere from 80F-90F outside).

      I am trying to get onto the solar train but the prerequisites to doing so are difficult - first we need to take care of relocating and updating our 100a Zinsco electrical panel to 200a. This is going to require trenching and breaking of concrete across our driveway (the hand hole is 65-70ft from the current panel location and on the other side of the driveway - the furthest possible location you would ever want it!). On top of that, SDGE's lead times for these projects can end up making this a 2-3 month ordeal. We won't be able to consider solar before changing the panel (it seems pointless *not* to but our insurance called out the Zinsco panel as a fire hazard and are forcing us to change in order to remain insured). I'm getting quotes on trenching/breaking concrete right now - this isn't going to be an inexpensive thing to do. Coupled with solar, even after a tax credit/rebates, we are going to be out probably $20k *minimum* and that's if I'm extremely lucky.

      The home is a 2-story 2680sq ft 4beds/2.5ba + loft. Vaulted ceilings in the master bed and also in the living room area which the loft overlooks into. I'm not sure where the upstairs minisplit would go because all the rooms are closed off... unless it's possibly to install one in the ceiling?

      I've attached the room layout as well

      Minisplits aren't something we've considered either way. We were planning just to replace the existing central HVAC system that's currently installed. The results of the contractor's Manual J point to a 4ton system and 3.5 if we improve the insulation, etc. If we were to go with a 2-stage, however, there is no half-ton sizing so he would suggest 4ton if we were to install the 2-stage system.
      There are obviously tons of differing opinions and approaches on this whole topic, so "keeping it simple" isn't that simple ;)
      That said, I guess we'll continue playing it by ear and keeping our options open especially because at the moment, the waitlist for these heatpumps has us looking at getting something installed in the summer!

      1. Expert Member
        Akos | | #27

        For PV, in most cases you don't need to upgrade your service. You do a lot of times have to upgrade your electrical panel.

        For net metered setups, most panels are limited by the busbar current ratings, this means you generally can't install more than a 3.8 kW PV system onto a 100A rated panel.

        What you can do, for much less cost than upgrading your service to 200A, is to swap out the panel for a 100A PV ready panels or for a 200A panel but swap out the main to a 100A breaker. This can still connect to a 100A service but eliminates the main busbar bottle neck and you can install a much larger array.

        Either option would let you install ~20kW array even with the existing 100A service.

        It takes a lot of electric loads for a house to get need more than a 100A service. Even a heat pump barely budges the power use. As long as you don't have a big pool/hot tub or more than one EV charger, a 100A service will work.

        1. jplee3 | | #30

          The issue we have is that even if we want to update the panel to a newer/safer 100a or 125a panel, our utility company is mandating that we bring the utility closet to code (it would need to be lengthened, widened and deepened and the gas meter relocated outside of it). The overhead cost to do this would probably end up being just as much as or not far off from what it would cost to relocate and update the panel to the outside wall (on the backside of the closet) and then trench and lay new conduit to this location. Right now, lowest bid I've gotten on a 200a panel relocation + conduit is $6500 and trenching (including breaking concrete, backfilling, and then patching) is $4000. So over $10k. If we had to make physical modifications to the closet (and bring in a framer), we might save a little but that would have been an awful lot of trouble to go through just to upgrade to the same sized panel.

          Considering the amount of re-work that has to be done to bring the closet to code OR relocate the panel, I figure relocating AND upgrading the panel is probably the most cost-efficient solution in this situation, especially considering that we would only be 'future proofing' and hopefully adding additional value to the home.

          1. Expert Member
            Akos | | #32

            Sounds like no option would be simple. In most cases the issue with gas meters is the vent for the regulator which can be plumbed sideways to increase clearance. Cheaper than moving the meter.

            Even with a 200A service a PV ready panel would be a good upgrade. A standard 200A panel with 200A main is still limited by the busbars, you can install a 7.6kW array max.

            7.6kW is a big array, depending on your roof, you might not have space for more anyways but extra capacity is a good thing if you ever want to install a battery system as well down the road.

      2. Expert Member
        Dana Dorsett | | #31

        Per your attached ASHRAE sheet you're looking at a 99% design temp of 41.4F, a 1% design temp of 85F. If the Manual-J used temps higher or lower than that, go for the 3 - 3.5 tonner. The ACCA's design numbers for El Toro MCAS are 43F & 89F, only a few degrees higher than the ASHRAE sheet.

        Latent cooling loads for El Toro are negative at 55% RH, barely positive at 50% RH indoors, so literally any AC would more than cover the latent loads, making very high SEER modulating equipment just fine for the local climate, whereas it could leave you clammy & moldy in the Gulf Coast states.

        Regarding 2- stage options:

        Though it's somewhat counterintuitive, bigger is the OPPOSITE of better. Whenever in doubt, go smaller- it'll be more comfortable. Most people gravitate toward bigger, fearing that they'll be too hot or cold at the temperature extremes, but oversizing only makes the indoor temperature swings between cycles larger and more noticeable.

        The cooling output of a 3 tonner is nominally 36,000 BTU/hr at ARHI test conditions (95F outdoors, 80F in), and it will usually have marginally more capacity at your mid to high 80sF design condition. The 3 ton 2-stager would in all likelihood cover the 85F cooling load, and would absolutely be able to cover the heating load at 40F. Running long/very long cycles even during the shoulder seasons is both more comfortable and more efficient, and even more comfortable at the peak 99% & 1% conditions than a 33% oversized equipment (the 4 tonner) would be, since it almost never cycles off. A 4 ton 2-stager would probably (almost) never need the high stage during heating season. If the Manual-J determines the equipment load is around 40KBTU/hr @ the 1% outside design temp, a 3 tonner is a much better fit than a 4 tonner.

        If there's even 10% 0f padding in the calculation that came up with a 40K cooling load, the 3 tonner could be close to perfectly sized, or VERY modestly undersized. With slightly undersized equipment, when the forecast temps are going to be hotter than the 1% design temp, pre-cooling the house in the late morning or early afternoon to 2-4F cooler than than you normally run it will usually keep the house in the comfortable zone, even if can't quite keep up with the lower thermostat setting.

        Modulating units with a 3:1 or larger turn down have less of a comfort hit from 30-33% oversizing than with 2-stagers, but some people still insist on installing ludicrously oversized factors (especially with multi-zone ductless systems) that only make it more expensive and less comfortable.

  7. Expert Member
    Dana Dorsett | | #24

    What they said.

    A 3 ton heat pump for a normal sized house in SD is likely to be at least 2x oversized, unless there is a huge expanse of "sunset view" west facing window area driving excessive peak cooling loads.

    For properly sizing heat pumps, the freebie BetterBuiltNW load tool is quicker, easier to use, and BETTER than the other free online tools (LoadCalc, CoolCalc, etc):

    This tool was developed & financed by a consortium of electric utilities that had a history of ratepayers and other stake holders dissatisfied with the performance of "as-installed" heat pumps that were sized by contractors, who were almost universally oversizing them. The target audience & original intent was an easy to use sanity check tool for HVAC contractors, to get them to abandon seat of the pants rules of thumbs and inherent biases/anxieties driving oversizing. The resulting tool is easy enough for newbies to get it mostly-right, most of the time (at least for the heat load/sizing part, if not the duct design part.)

    For reference, I live in a 1920s vintage 2x4 framed 2400' (+1500' of insulated full basement) sub code 1.5 story bungalow with and wouldn't need more than a 4 tonner to cover the heat load at my 99% design temp of +5F with margin- many 3 tonners could even handle it. At SD style 99% & 1% outside design temps (low-mid 40s, mid-80s F) the gas furnace makes no sense. A right-sized modulating heat pump would provide more comfort, and wouldn't be appreciably more expensive to heat with even at SD's insane residential retail rates. The seasonal average heating COP of a decent ducted mini-split would be north of 4 if right-sized for the load.

    The installed cost of rooftop solar is reasonably low in your area (averaging an unsubsidiesed $2.77/watt for 10kw systems in March 2022, per Energy Sage), with a levelized cost of electricity less than half the current retail price, and the seasonal output in your area is also quite good- high enough that less-than-super-performance homes with simple roof lines and reasonable shading factors can hit net-zero. Even one of Tesla Solar's cookie-cutter residential solar systems(+ battery) might even get you there. At $0.35+/kwh retail, by 2030 it will become cost effective to pull the meter oversize the PV array and cut the cord as battery costs (particularly no-nickel no-cobalt LFP chemistry batteries, and maybe CATL's Na-ion in the near future) continue to fall.

    1. jplee3 | | #25

      So far with the numbers I've put in (rough room dimensions, window sq footage and assuming 2x4 weatherized with vinyl windows), the cooling load is right at around 40k btu - already we're not far off from what our contractor came up with on his Manual J using this tool so it seems to me that things are checking out... unless one of my inputs is grossly off. I'm using Tustin/Irvine Rch as the climate and not SD btw - Winter 34F/Summer 89F is more in line with the climate here in Laguna Hills based on my research.

      How much is it to installed a ducted mini-split system, on average btw?

      As far as solar, I am trying to get something going. The other challenge is that we have a Spanish clay tile roof so extremely delicate and anyone who walks on it let alone touches it ends up breaking it most of the time. I found a roofing company who also does solar and has been doing both for a long time, and their quote is pretty competitive considering they would avoid the inlay and install the panels on top of tiles (there's a lot of debate between inlay vs tile hook/floating panel installs too btw, which is another thing I've had to navigate). Anyway, all of it is dependent on us figuring out the panel situation.

      1. Expert Member
        Dana Dorsett | | #26

        Per ACCA's data sets Irvine's 99% outside design temp is 40F (but it's located at 34 degrees north latitude). Tustin MCAF's 99% outside design temp is 41F. These temps are in the heating efficiency sweet-spot for modern heat pumps.

        A 40K (3.3 tons) of cooling load @ ~90F for a 2680' house seems on the high side but not crazy high, at about a ton per 1250'. Are the windows & window orientation the primary drivers? For sun-exposed windows a low SHGC low E coating (or exterior shades/awnings) can make a real difference.

        The cost of ducted mini-split installations varies with the complexity of the duct design. If it's reasonable to re-commission existing ducts it can be pretty cheap. Most "mid-static" ducted cassettes can handle up to 0.8" w.c. static pressures, and if the existing system was oversized the static pressures will be lower at the lower cfm of a right-sized system. A reasonable design target for a new duct system would be 0.3-0.5" w.c..

        Something like the 3-ton mid-static Fujitsu AOU/ARU36RGLX could be about right:!/product/25352

        That thing could come close to covering MY home's +5F heating load, and would be loping along at about 1/3 speed at your likely design load at a COP of about 4. It delivers nearly 40K of cooling @ +95F (which is above your 1% design temp), so ~40K @ ~90F is pretty much assured.

        There are others.

        With a ceramic roof the peak cooling loads are often a bit lower than most load calculators indicate, due to the thermally massive roof delaying peak ceiling temps. It takes a more sophisticated model than Manual-J and more data on the materials to hit that accurately. With solar panels shading part of the roof it gets even more complex, but the cooling load will usually go down a bit with the addition of solar.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |