Heating and A/C
I am building a new home in zone 5. I received a quote for our heating and a/c and I wanted to see if they meet my goals of efficiency and appropriate sizing. Here are the specs on our house:
1792 square feet plus full basement
3” foam on exterior(1.5” seams taped and staggered), osb air sealed with Prosoco seam filler and flashing. 2×6 studs 24” o.c. filled with damp cellulose. Cathedral ceiling (over living rm/dining rm/kitchen) with combination of cc foam and oc foam or possibly cellulose and remainder of flat ceilings with 16” of blown-in cellulose. The basement walls will be insulated also with rigid foam. The windows are from Alpen with 8 large picture windows on south end for passive solar effect.
A/c unit, Carrier 24ABC642A003—16 SEER-I thought this should be around 20?
Gas furnace, Carrier 59TP6A100E21-20 Performance 96%, 100k Btuh
ERV, Carrier ERVXXSVB1100-shows 79% heat recovery efficiency and I thought this was a little low.
Any advice is greatly appreciated!
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Replies
It would be nice to have heat load calculations, but 100k btu furnace sounds way, way too big, given what seems to be a small, well insulated, tight house. 20k btu is probably closer.
Michael,
Stephen is right. Your furnace (at 100,000 BTU/h) is grossly oversized.
Step one is always the same: Start with an accurate Manual J load calculation. This step is required by most building codes. Unfortunately, most HVAC contractors are unable to perform an accurate Manual J calculation. You need to hire an energy rater (energy consultant) or a mechanical engineer to help you.
For more information, see these articles:
Saving Energy With Manual J and Manual D
How to Perform a Heat-Loss Calculation — Part 1
How to Perform a Heat-Loss Calculation — Part 2
The 100K gas furnace is ridiculously oversized- going to be more than a 3x oversize factor for the heating load, and could be pushing 5x. With hot air furnaces that isn't particularly an efficiency issue, but something right-sized (no more than 1.4x oversize factor for the 99% design heat load) would be a lot quieter, more comfortable, and cheaper up-front.
A 2-ton Carrier Greenspeed heat pump would likely have sufficient heating capacity (a 3-tonner, at most), and is likely to be reasonably-sized for the cooling load.
But run the numbers first, using AGGRESSIVE assumptions for R & U-values, air tightness, and taking into account all shading factors, plug loads, etc. THEN you can figure out the appropriate mechanicals to support those loads. Using a P.E. or a RESNET rater rather than an HVAC contractor to run those numbers would be less likely to oversize. With a heat pump solution oversizing can have a significant (and negative) efficiency impact, so you want to know the real, aggressively assessed number rather than starting out slightly padded only to pick the next size up and end up 2x oversized.
I thought it seemed a bit excessive, something that would be in my current home built in 1903 with virtually no insulation and cool breezes from every corner. I had also considered mini-splits as a possibility. The responses are greatly appreciated!
When you have room-by-room load numbers you can then think about where a mini-split solution is a reasonable way to go or not.
Use only the 99% outside design temperature, and a code-min 68F for the interior design temperature, even though it gets colder than that, and even though you may prefer to keep it at 72F or whatever.
An all too common mistake is to see a house that size in US climate zone 5 with a head in every room tied to a 4 ton multi-split compressor serving a 1.5 ton cooling load, and a heat load that could have been handled by 2 tons of cold-climate mini-split compressor, with the individual heads sub-optimally oversized for the room loads.
With a mini-split solution it gets expensive and inefficient if you have a head in every room, but rooms with low loads may not really "need" it, even though code mandates that occupied rooms be capable of heating to 68F automatically, without auxiliary heaters. Small electric baseboards sized for the room load can be implemented to be able to satisfy code, at far less up-front cost than a ductless head, but you're not required to actually run them. Many ~1800' homes with reasonably open floor plans can be reasonably and efficiently heated & cooled by 2-3 ductless heads if properly sized & placed.
I am not finding a energy consultant in my area, so I am reaching out to St Louis but that is a couple of hours away. I got another quote from an HVAC contractor, here is what he said:
"I ran your loads a little light so that there was a little extra capacity if needed. I also ran the loads showing you operating costs if we were to bring in outside air and mix it straight into the furnace. We can supply any type of erv or hrv that you would ever want. just remember that they will recover up to 60% of your energy, that's on a good day. My best guess is maybe on average due to the temp swings we have 40%. Most of the time we just mix o.a with return air due to the costs of the hrv units. The question on the 20 seer equipment, the larger the tonnage of equipment the harder it is to achieve 20 seer. York equipment can not do it until you get to three tons or less. The problem is that it would cost an extra $2500.00 to achieve this with no payback (return on investment). I showed you the operating cost difference between a 13 seer and a 15.5 seer. Your loads have come from the massive glass on the south. A triple pane is a good window with low-e but if you look up the true r value it will be around r-5 and a double pane will be around r-3."
He did run heat load calculations (per wall), I will attemp to upload a pic of that.
Michael,
I think Alpen would take offense to their windows being estimated to have a U-Value of .72. Alpen lists their 625 triple high solar gain fixed as .18.
Ha ha, no kidding. If I am reading the rest of the report correctly, he is showing that we require 41,126 btuh and the three York units that he proposes are a heat pump(YHE48B21) with backup(Tm9E080C16MP11) of 47,500 btuh, and 2 conventional furnaces running 77k-78k btuh. It seems like the heat pump is close but I'm not sure why the conv. furnaces are so large.
Michael,
Also, I don't see where you are getting credit for the exterior insulation and the reduced framing factor with the 24" OC studs.
Another way to get a sanity check is to look at what the PHIUS Climate Specific heat load limit is for your area. I don't know what city you are in so I just picked Pittsburgh which is also in Zone 5 and the 99% Peak Heat Load is 4.6 Btu's / sf. So if you were 3k SF including the basement which is in your thermal envelope then your peak heat load would be 13.8 kBtu's if you were building a PH. I have a similar plan to your's and I am meeting the PH requirement, but I am in Zone 4A. I would say you are pretty close for Zone 5, but let's say worse case you are 20 - 25K Btu's.
Since heating and cooling is in my mind the most important thing to get right and the most likely thing to get wrong, I decided to hire a PE to run Manual J,D and S. I think it is money well spent.
Your contractor's proposal for ventilation is exactly what my HVAC contractor proposed. I asked if he meant CFIS, but he had no idea what I was talking about. He had no intention of installing any type of controls.
If you have your contractor supply an HRV or ERV, make sure you are on the same page about the associated ductwork before you agree to a quote.
Garbage-in = garbage-out. This is a prime example of why you don't want the HVAC contractor doing the load calculations. The assemblies described in the calculation bear no resemblance to the description in the original post.
The U-factor of a 2x6 24" o.c. wall with 3" of exterior foam is between U0.030 and U0.035- he's assuming ~2x the heat loss from walls with the U-factor of U0.065.
The U-factor of a CODE MINIMUM window is U0.35, less than half the U0.72 used in the calculation.
Your attic R is about 1.5x what they used in the calc.
They bent over backward to come up with something more than 2x oversized for YOUR house.
Thanks for all the responses, I really appreciate the feedback. I heard back from the first HVAC contractor and he said he sized the furnace a little big for the A/C part of it, for dehumidification of the basement.
The bigger the A/C the LESS dehumidification you will get, because it runs shorter periods. Dehudification takes time and you want your system to run as long as possible. An oversized system equals humid air and mold. Design your system to run 24/7 during the hottest part of the year.