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Furnace sized based on air handler required for cooling?

Steve Mackay | Posted in Mechanicals on

I’m new to furnace and AC terminology so before I ask my question I had to figure out why AC units talk about tons.  It’s not tons of air. I just learned why they use tons to describe air conditioner capacity from:
https://blog.micrometl.com/hvac-tonnage-simplified-explained-defined/

Basically it takes about 12,000 BTU/h to melt a ton of ice.  So to determine an AC capacity in tons take the rated cooling capacity and divide by 12,000.

Now that I got that out the way I’m trying to correlate furnace size and air handler size to the heating and cooling requirements.

Typically you need more volume of air to cool the house than to heat it.  So the cooling requirements determine the size of the air handler.  However the air handler comes with the furnace.  So the air handler size determines the furnace size.  This logic forces me to buy a furnace of a much larger capacity than I need.

Why can’t you size the air handler independent of your furnace? Is it a cost issue in that a furnace comes with an air handler so it’s much cheaper to deal with an over sized furnace than to buy right sized furnace and air handler independently?

Here is what my HVAC design guy set up for my house.  The manual J shows I have a heating load of 53,400 BTU/h and a cooling load of 36,400 BTU/h (3 ton).  So they are recommending a Lennox furnace with 85,000 BTU/h output and a Lennox AC unit and coil with a 47,000 BTU/h cooling capacity (4 ton) both at 2000cfm.

Is there a way for me to match my furnace to be closer to 55,000 BTU/h and my cooling equipment closer to 40,000 BTU/h?

I guess I don’t understand why my HVAC contractor is recommending he installs equipment so over sized.

Steve

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Replies

  1. User avatar
    Jon R | | #1

    I recommend you get a furnace/AC with heating capacity of about 70 Kbtu/hr and cooling capacity of 36 Kbtu/hr. Verify that you can maintain the correct CFM/ton (350-400 for AC in humid areas). It should be about right in this case (1050 to 1200 CFM, nowhere close to 2000 CFM).

    Explore the advantages of variable/two stage and heat pumps (aka dual fuel). Different fan speeds help when the numbers don't happen to work out.

  2. User avatar
    Dana Dorsett | | #2

    It's possible to buy air handlers that come with both an R410A refrigerant coil for both heating /cooling, plus a hydronic coil for heating/cooling, with multilple speeds (even continuously variable), if the goal is to burn fossils for heat rather than going full heat-pump. The varialbe speed FirstCo 36HBQBR comes with a 3 ton refrigerant coil and will deliver 52K out of the hydro coil at an EWT of 160F, 63.6K at an EWT of 180F:

    https://www.firstco.com/documents/ProductDocuments/hbqbr718.pdf

    The 2 speed 3-ton 36HBQB delivers 49K @ 140F, 63K @ 160F.

    https://www.firstco.com/documents/ProductDocuments/hbqb718.pdf

    A small condensing boiler under outdoor reset control can be set up to run in condensing mode more than 90% of the season with either of those air handlers. A condensing tank water heater with a 100KBTU/hr burner can get more than 50K out of the 2-speed if need be and still have capacity left over for domestic hot water.

    A 53K+ load for a new house implies a pretty big house, if real. Who did the Manual-J?

  3. Steve Mackay | | #3

    Jon R,

    If the heating requirement is 53,400 BTU/h why go with a 70,000 BTU/h furnace. Is there a code requirement that it needs to be oversize?

    Dana,

    The manual J is something I have been struggling with. I had a local guy who only does HVAC design (no installs) (Perfect Home HVAC) do the manual J. We had a discussion on it here:

    https://www.greenbuildingadvisor.com/question/understanding-hvac-and-heating-cooling-zones

    With my original guy being unresponsive following our discussion above I wanted a second opinion. I received a recommendation on HVAC engineer (EnergyWise) from Rich McGrath and they came back with higher requirements 71,600 BTU/h heating and 55,000 BTU/h cooling.

    It's a 4,000 sqft home with lots of east glass and not nearly as much on the south, west or north.

    Steve

  4. User avatar
    Jon R | | #4

    > If the heating requirement is 53,400 BTU/h why go with a 70,000 BTU/h

    Your 53,400 should be for design day. It gets colder than design day, so you need more capacity to account for this. Or you could under-size (say 62 Kbtu/hr) and just be somewhat cold now and then. Or you could occasionally use some other supplemental heat source.

    Sounds like you would be happier with two stage heating/cooling and a variable speed fan.

  5. User avatar
    Dana Dorsett | | #5

    >"It's a 4,000 sqft home with lots of east glass and not nearly as much on the south, west or north."

    53.4K would be a credible heat load for a house of that description (71.6K probably not unless it's really a HUGE amount of glass).

    Assuming the HVAC designer's Manual-J is correct a modulating 5 ton Carrier GreenSpeed or a 5 ton Trane XV18 can probably heat & cool the place efficiently if your 99% outside design temp is in positive double-digits. Either is good for 50K+ on heat pump alone at +17F, and can modulate down in the 2 ton range even at a modest 82F outdoor temp:

    https://ashp.neep.org/#!/product/29549

    https://ashp.neep.org/#!/product/27775

  6. Steve Mackay | | #6

    Zone 6B. the 99% design day is 2.6F the 99.6% day is -1.2F.

    I'd be okay with being a little cold on the colder than design day. We have a direct vent fireplace in the main room to supplement heating on the super cold days.

  7. User avatar
    Dana Dorsett | | #7

    At single-digit design temps any heat pump solution would call for a true cold climate heat pump, or a least a product that used the same sort of vapor injection scroll compressor technology.

    If it's broken into zones (perhaps by floor?) you can get there with multiple independent heat pumps to get sufficient capacity rather than zoning with zone dampers.

  8. jrpritchard | | #8

    Steve,

    What model numbers does your manual S suggest? Lennox blowers usually have 5 speed taps for their standard ECM motors so you can have different blower speeds for cooling and heating. Lennox uses the nomenclature XE to mark their standard ECM blower motor which means you could have a two stage unit but the blower speed will always be at the speed you have selected on the board irregardless of whether first or second stage is calling. the XV model has a lot more capabilities with regards to blower speeds and allows you more control over blower speed for each stage of heating or cooling. When your systems get commissioned you can set your blower speeds to maintain the same delta T for each stage. Lennox BTU ratings are also somewhat misleading, a 45K furnace will have an input of 44, output close to 42, their 70K model has an output of about 63 and their 90K model in the neighborhood of about 84. My favorite furnace right now is the EL296XV which gives you two stages of heat with their V drive blower motor. I am not a big fan of their SLP98 which is their full modulating unit

  9. Jamie B | | #9

    Hey jrpritchard,

    Out obviously have hands on experience with these lennox units. I'm curious as to why you like the 2 stage over the modulating.

    Philosophically, a fully modulating unit is better than a 2 stage. But we all know units in real life have quirks and most things are not as it's stated on paper.

    Care to elaborate on your experience?

    Jamie

  10. jrpritchard | | #10

    Jamie

    My own cowboy math shows a very long payback period based on the jump in price. Also I see the warranty parts come through for the SLP models and the cost to replace a modulating control board vs a two stage board out of warranty is substantial. Also very seldomly are these units controlled properly. If you put a nest thermostat or any non Lennox thermostat on a modulating unit it turns them into a timed 3 stage unit meaning they run each stage for a certain time and then moves into the second stage and finally third stage. The modulating Lennox thermostats and outdoor sensors are expensive which is why I see a lot of systems wired with conventional t stats. In the long run most people end up with a very expensive 3 stage unit that will be very expensive to fix and service. Two stage V drive furnaces give people an efficient option that is serviceable, economical, and still allows for a wide range of applications such as zoning or matching up equipment sizes in my opinion

  11. Jamie B | | #11

    jrpritchard,

    Beautiful, thank you!

    In the position I'm in, I never get real and educated feedback from guys in the trenches like this. About a year ago I almost went the the Lennox SLP98. I went with a whole other system instead and its proving it's own challenges, that no one warned me about, and I'm still stuck with dealing with the manufacturer trying to solve the issue.

    So advice like yours is priceless.

  12. User avatar
    Jon R | | #12

    Using jr's favorite, I see these models fit most of the numbers being discussed:

    EL296UH070XV36B
    EL296UH090XV36C

  13. Steve Mackay | | #13

    jrpritchard,

    That is great info thank you.

    My heating guy recommended
    EL296UH090XV48C but that's only a 80% AFUE and 90,000 BTUH.
    I much prefer Jon R suggestion above.

    I'd like to use a 4 ton unit for the cooling requirements. Considering my cooling design point is 95F outside and 75F inside my installer suggested making sure I go above 3 ton to ensure that when conditions are outside the design points I can still get adequate cooling.

    I think this points me to the EL296UH110XV48C?

    jrpritchard or Jon R any comments on that unit for my loads or should I stick with a 3 ton?

    Steve

    1. User avatar
      Dana Dorsett | | #14

      >"I'd like to use a 4 ton unit for the cooling requirements. Considering my cooling design point is 95F outside and 75F inside my installer suggested making sure I go above 3 ton to ensure that when conditions are outside the design points I can still get adequate cooling."

      That's not necessarily a good idea (usually not). Unless the bulk of your cooling load is from conducted heat rather than windows bumping to a 4 ton isn't going to help. Peak loads are more about peak solar gains, not outdoor air temperatures. The sun isn't any brighter on days when it hits 105F than on days when it's "only" 95F.

      Even if you're sized EXACTLY for the load at 95F outdoors/75F indoors under peak insolation levels, setting the thermostat to 70F earlier in the day to pre-cool the thermal mass of the house & contents will in most cases prevent blowing by 75F during peak load hours, even when it's 105F outside. In most zone 6B locations your latent loads are usually negative, even on screaming hot days.

      If you are planning on holding Polka parties with a dozen 250lb Czechs dancing carrying a keg of beer on one shoulder when it's hotter than 95F out, maybe a 4 tonner would be the right choice. Otherwise, probably not.

  14. Steve Mackay | | #15

    Good point about the solar gains. 44% of my heating load is a result of glazing. I can't tell what percentage of that is through conduction but most of it is radiated gains.

    Cheers

    Steve

  15. jrpritchard | | #16

    Steve,

    Here is a quick breakdown of lennox model numbers

    EL - Elite Series Unit
    296 - the 2 is for 2 stage, 96 is the effeciency
    UH - upflow or horizontal
    090 - nominal BTU/h output ie 90,000
    XV - blower type in this case the V drive ECM
    48C - 48 for a 4 ton blower and C for the 21" cabinet width

    On most lennox units with a 4 ton drive furnace, with a decent A coil and pretty much any 3 ton unit you will actually max out at around 34,800 BTU/h. Going up to a 3.5 ton unit such as the 14ACXS042 condenser will get you about 40,ooo BTU/h of cooling potential. Dont worry about going up to a 110,000 BTU furnace just make sure you select the 90K version that ends in 48C rather than 36C. If you trust your manual J i would go with the 3.5 ton unit but if it were my house i would try to air seal or insulate just a little bit better to get you to a 3 tonner with the 70K furnace. I would also look over the manual J carefully. A lot of times in my area even 'aggressive' manual Js assume 'average' construction tightness. Also a lot of times they assume way more ventilation than you need which comes with a large energy penalty especially if they selected demand or supply only ventilation as opposed to something with an enthaply core. On my build for example the penalty was close to 2k BTU which is a crazy high when you compare it to the overall load. Also if your house is zoned and you keep the zones at different temps that can decrease your load which is something that is usually not taken into account during the manual J.

  16. Steve Mackay | | #17

    One critical piece of information I forgot to mention is that my house sits at 7,000ft. How much do I have to de-rate the air handler and furnace for high elevation?

    Steve

  17. Steve Mackay | | #18

    Any word on de-rating factors for air handler and furnace up at 7000ft instead of sea level?

  18. jrpritchard | | #19

    Steve,

    I am at sea level so I have not run into this. See below - it looks like you will need to change out your pressure switches but should be able to accommodate the altitude with the factory gas pressure adjustments. 7500+ will require an orifice change. All the blower data was the same irregardless of altitude. I am assuming that is because the charts / CFMs are all based off of external static pressure which should already account for the density of air.

    Hope this is what you were looking for

  19. User avatar
    Jon R | | #20

    Altitude de-rating (part of Manual S) is a good point - looks like about 23% (heating and cooling).

    You might want to post your full Manual J and Manual S data for review.

    The EL296UH090XV48C is 96% AFUE and seems to fit your numbers (with de-rating).

    1. Steve Mackay | | #21

      No manual S has been done. Manual J for the only floor with HVAC is attached.

      What's the thoughts on using the hvac system to distribute ERV fresh air? Seems like that's not a good idea because they are 2 different sized systems for two different functions and I don't think you can balance the flow to each room appropriately. The HVAC guy is pushing me that way.

      Steve

  20. Steve Mackay | | #22

    I've been out of town for several days. We hadn't made a decision on the size of furnace and I come back and they've already installed a 5 ton air handler (EL296UH090XV60C. My manual J calls for a 3 ton. I was figuring I should over size to 4 ton because of the 7500ft elevation.

    What are the downsides to having a 5 ton air handler when a 4 ton would be better and likely a 3 ton would have worked?

    Should I live with the 5 ton air handler or have them change it out?

    Steve

    1. User avatar
      Jon R | | #24

      As long as the right sized AC is still compatible, looks like it has enough enough blower range to not matter. But installing equipment without a Manual S is sloppy.

      I'm curious if they have a heat pump that makes sense for heating during mild weather. Might pay for itself. Also consider some zoning.

      https://www.lennoxpros.com/docs/Technical/210619.pdf

      Re ERV: running the blower continuously at low speed to make shared ducting work should cost < $100/yr and with the right ERV, can be fine in terms of ERV balance. Distribution probably isn't a problem (with enough air circulation, there is lots of mixing and it all gets evened out). I doubt you have a "high SHR with continuous AC blower" humidity concern in your climate. Also keep in mind that separate ERV ducts also have issues (like cost and sufficient airflow to control Co2 in closed off rooms).

      1. jrpritchard | | #25

        It will be fine - that blower is capable of about 950CFM to 2000CFM depending on static. If the ducts are correct and the units get commissioned properly it will be ok. If you post the A coil and condenser model i could check the AHRI and performance data for your whole system

  21. User avatar
    Dana Dorsett | | #23

    >"What are the downsides to having a 5 ton air handler when a 4 ton would be better and likely a 3 ton would have worked?"

    In more humid climates than your's it's usually a recipe for lousier latent load handling. Most locations in the US at 7500' of elevation have negative latent loads, so I wouldn't sweat that aspect too much.

    The other issues are things like noise, higher air-handler driven infiltration rates, or with a low-swing thermostat potential for short cycling the compressor.

    You'll be OK. At my house (at 600' of elevation in a much more humid climate) a 5 ton AC (predates my involvement) and a <1 ton design load. I have to resort to a half-ton window shaker during the sticky but not so blistering days to control humidity, but on 1% design day or hotter it keeps the place comfortable cycling at a ridiculously low duty cycle. With a longer cooling season or bigger load it might be worth fixing, but even with something like 25 years of service it has run so few accumulated hours it's almost "like new". (I did get rid of the ludicrously oversized boiler driving the hydro-air coil in the air handler, and am running it at condensing temps during the heating season.) Your mismatch is nowhere near as a bad as mine- it'll just cycle some, but you probably won't need a window-shaker AC in your climate. As long as it's not short-cycling it'll be fine.

    1. Steve Mackay | | #26

      With it being variable speed I should be able to dial it in over time so it is not short cycling.

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