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Heat pump sizing

Dean Sandbo | Posted in General Questions on

So I have a completed manual J.

96k heating load, 47k cooling load. Big house renovation.

Am heavily exploring minisplit options for whole house… but could someone generally share with me how sizing would happen for a tradional ducted heat pump?

I will do a manual s and d, but would like to have a bit of direction on which way I am going to go before I choose.

So generally, how do you choose the sizing of the units? Do you size for the cooling load? The heating load? or somewhere in between?

Thank you!

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  1. Nate G | | #1

    If your heating and cooling loads are truly that high, it would probably be far more cost-effective long-term and yield far greater comfort to spend most of the money you were planning to spend on the HVAC system instead on insulation and airtightness improvements. Those are sky high loads unless the building is like 5,000 square feet. Look into whether those numbers are accurate too.

  2. Dean Sandbo | | #2

    I know its a little crazy... but the manual j was done by one of the most reputable hers raters in the country. It is almost a 9000 sf block and steel construction 1970's house that I am renovating. We are insulating and air sealing well. So, taking that into account, just looking for general ideas on how to size a heat pump for loads in general before I dive into the selection process.

  3. Nate G | | #3

    Let me guess--uninsulated walls, uninsulated foundation, minimally-insulated ceilings, and large expanses of glass?

  4. Tim Brown | | #4

    It's going to be asked by those in the know so I'll get to it so they have a little info to start..

    Where are you? (Fairbanks or Miami?)
    Zip code is important if you don't know your climate zone!

    House orientation... which side faces N/S etc?

    1970's house likely means 2x4 const... how many floors, basement or slab.

    What are you trying to achieve and how mush $$ and effort are you willing to put into the project.

    There is no point it sizing a HVAC system for the existing house if you are going to improve the envelope... air sealing, insulation, doors and windows...

    Several of the experts will chime in when you give them enough info to start a conversation...start with those basics..

  5. Dean Sandbo | | #5

    Sorry, I guess I was looking for a somewhat general approach to how to size heat pumps based on load, but here goes some details.

    It is a block and steel house. I gutted it. Zone 6.
    Putting a peel and stick wrb on outside of block then 2" of rigid continuos on the outside.. then brick.
    New stud wall areas will get zip sheathing r6 continuous with 3 inches of closed cell on the inside. fully enclosed ceiling ... r30 foam.
    Its a big ranch with a walk out basement.. basement is getting r5 xps on top of slab, r10 xps on walls..
    The majority of the east side of the house both floors is windows... a few windows on the west side... few on south.. none on north.
    windows are decent... u26 s21 low e 366 argon filled.
    1 have 4 foot soffits around the entire house except for one spall section.
    I have gone very far down the road considering whole house mini split, ductless and ducted...
    I just want to know how I should think about sizing a tradional ducted system (greenspeed). There are lots of other details I will ask about... I would love to get in to a discussion on what improvements I can make to lower load, but would first love to know how does one philosophically size a heat pump.
    thanks you so much you guys are awesome.. I have learned so much!

  6. D Dorsett | | #6

    2" of continuous foam doesn't come anywhere near IRC code min for a residential wall in zone 6. Even a mass-wall with continuous insulation on the exterior would need to be a minimum of R15, R20 if more than half the insulation is on the interior.

    With 2" of EPS on the exterior you could build a wood-framed non-structural batt-insulated studwall on the interior side of the block wall and meet code-min, but if it's steel studs it won't quite be there. That's sufficient exterior-R for dew point control on the insulated 2x4 wall without needing interior side vapor retarders (other than standard latex paint.)

  7. Stephen Sheehy | | #7

    Dean- heat pumps will deliver a specified amount of heat at say 70 degrees F for any given outside temperature. The sizing is pretty simple, although it depends on the room layout, duct size and length (unless ductless). If you need 100,000 btu/hr to meet your design load, you need heat pumps capable of delivering that. If the load is only 50000 btu/hr, you need half as much heat output. As you know if you have read a lot of the Q & A here, too much oversizing can be inefficient, so you need to get the heat load right.

    You need to calculate the total heat load for the building using Manual J. You'll also need the heat load for each floor and perhaps for each room. That depends on what your design temperature is, typically calculated for the low temperature expected to be exceeded 99% of the time. For example, in my part of Maine, the design temp is 0 F.

    The heat load also depends on a calculation that takes into account details about windows, insulation and other factors influencing heat loss and gain (like solar gain, although it doesn't sound like you get much of that.

    As others noted, no point in designing the heating/cooling system until you know what the loads will be.

  8. Dean Sandbo | | #8

    I am sooo sorry I had r6 on my mind, my home is located in zone 3. zip 28104.. So I believe I meet or exceed local codes. ( I might have to up the ceiling to r38 from r30 based on your chart, think my county is still in the last code r30) Stephen, I have a full blown manual j calc already done by a very reputable company. I have R10 exterior continuos where there is existing is on block (mass) wall continuos, and I will play around with adding insulation to lower load... would love your help with ideas.. but what I am doing meets or exceeds code in zone 3 where I am.. so sorry!... I didn't want to put any insulation on the inside of the block, to negate the mass wall advantages, but maybe I am wrong on that? From the Manual J most of the load is coming from the windows... ( and heat load in basement is from the floor even though I am adding r5 to the slab.)

    So as to my original question, let me break down to a floor to make it a little easier. 3000 sf Basement load is 24k btu heating, 11k cooling ( about 1500sf per ton) I know that we would need more info on the specific equipment, but in general would you size the unit for 2 tons? in essence oversizing the cooling load? Or would you size it at 1 ton and make up the heating differnce with strip heaters or furnace? Or would you size it at 1.5 tons and make up the difference with strip heater, or furnace? Just wanting to know in general how you approach sizing when the heating and cooling loads are so different?

  9. GBA Editor
    Martin Holladay | | #9

    It isn't always necessary for the homeowner to perform all calculations. Since sizing a heat pump is new to you, you might want to hire a mechanical engineer to help you. If you try to master this new (to you) field of knowledge, you might make some mistakes along the way.

    Here are links to two useful documents that you may want to study:

    How do I size a heat pump?

    Right-Size Heating and Cooling Equipment

  10. Dean Sandbo | | #10

    Thank you for the links!

  11. D Dorsett | | #11

    Code min is the crummiest building that's legal to build, and 2" XPS would be a lousy solution, since it would underperform code min in 50 years.

    In zone 3 using 1.5" or 2" foil-faced polyiso would meet code min for a mass wall, and would be far greener than 2" of XPS due to the fact that XPS is blown with an extremely powerful greenouse gas (HFC134a, ~1400x CO2 for 100 year global warming potential), compared to the pentane (7x CO2) used for blowing polyiso. With a 0.5-1" sized air-gap to the brick veneer it adds about R1 to the total stackup too due to the low-E facer. As XPS loses it's climat damaging blowing agent over 5 decades R10 XPS drops to about R8.5 in performance too.

    I zone 3 even with 2" (R12) polyiso on the exterior it's still worth insulating any interior studwalls with R11, or bumping up the exterior rigid foam to 3". At that level of insulation it's pretty straightforward to get to Net Zero Energy with rooftop PV solar and heat pumps, assuming the rest of the building upgrades are also judicously brought up to something better than code. For guidance, refer to Table 2 of BA-1005 (note, those are "whole assembly-R" numbers, with all thermal bridging factored in, not center-cavity R.):

  12. Dean Sandbo | | #12

    I have already framed the areas that butt up to the block walls and planned on 2" rigid on the exterior of the concrete walls at the transition. e Where I have block walls, I have planned on 2" rigid on the exterior, I have some brick ledge depth issues, and the transition to zip r6 framed walls are already set. I was planning on just furring strips on the interior of the block walls then sheetrock. I probably could add some rigid on the interior without too many adjustments to what I already have in place. If I put 1" of rigid on the interior of the block wall then furring strips, will I negate the benefit of the mass wall adjusting to the temperature of the interior? So i would have two inches of rigid on exterior, then 8" block, then 1" rigid. Or I could cut in rigid between the furring strips..Or will this mess with the perm ratings too much?

  13. D Dorsett | | #13

    If you have only ~1/3 of the total R on the interior side of the thermal mass you actually get MORE benefit out of that mass than if there were no interior insulation, since the thermal mass is then "allowed" to swing through a somewhat larger temperature range. If you installed even 3/4" of continuous foil faced polyiso trapped to the wall with furring, the low-E foil + air gap gets you to more than R5 performance, and isolates the moisture susceptible wood furring from any moisture wicking up the block wall.

    Don't sweat the perm ratings, especially when all of the moisture-susceptible wood is on the interior side of the insulation and low-perm foam. But if it's extremely tight on both sides any wood resting atop the CMU is at least theoretically at risk. Going with foil-faced goods on both sides of the wall means the masonry will have a higher moisture content. If you used 2" Type-II (1.5lbs density) EPS on the exterior would deliver R8.4 and have over 1-perm of drying capacity toward the exterior. The CMU itself is worth about R1, and the 3/4" of polyiso + air gap is ~R5 or a bit higher. With the thermal mass benefit of the CMU that would be a very decent performance wall for the climate.

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