Whole House HP Selection
We’re about to finalize the HP size for the whole house system. I’ve run my own calculations, paid energy consultant to model from plans, and now awaiting the HVAC contractor numbers. The energy consultant model and my own were close at approx 38,000 BTU design heat loss/hr. HVAC contractor initial walkthrough suggested we will see some inefficiency in system due to the number of 90-angles and layout of the house. Their walk through guess is 4-ton to allow for loss mentioned. The systems they specialize in only come in whole ‘tons’, so I’m between 3 and 4 tons, but not sure where the line would be, once I see their results. Assuming their result agree @ 38k BTU, but want to account for said layout, is undersize 3-ton still the right path or go 4-ton? Will be a variable speed air handler.
The House
Zone 6
Design temp -18C/0F @ 99.6%
4,000 HDD @ 18C which I think is 7,200 for 65F??
Slab on grade with 2.5” foam under entire slab with 2.5” on edge to frost wall
Walls R20+R7.5 ci
Attic R60 (considering more)
20” heel truss
R6.5 triple pane windows (WWR of 13% mostly east facing)
Unfortunately minimal s. facing windows
Slab is 2,500 sqft
bonus room above garage 700 sqft with R-40 under floor
HRV
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Replies
Do you have a cooling load? Oversizing is more of an issue with cooling, because it results in poor dehumidification and resultingly poor comfort.
What outdoor temperature are those ton ratings given at? Often they use 47F which is going to be very different from 0F.
I don’t have a cooling load, but AC would have minimal to no use based on coastal climate.
Not sure the rating at the tonnage. Technical spec provides COP at 47F and 17F of 4.2 and 2.8.
The way I see it undersized HP with acquit resistance backup is not a bad setup. If the HP covers 90% of the load the backup will only kick in when it is pretty cold outside and the COP of the HP is likely down to about 2 so it is only twice as expensive to run as the HP the few hours you need.
Seems to me your 65° indoor set point is unrealistic most people will be wearing a sweater at 68°
My guess is insulation beyond R60 will not make economic sense and will not improve your comfort levels.
My list for a modern HP
1 Variable speed vapor injected compressor.
2 Variable speed indoor fan motor.
3 Electronic expansion valves.
4 Communicating thermostat made by the unit’s manufacture.
If the bonus room seems likely to be rarely used having a separate system would give you the option of having a lower set point. It seems very unlikely the bonus room will get R60 insulation making it very different than the rest of the house and a thermostat in the main room will not control that room very well.
Walta
15BTU/sqft seems awfully high for your proposed assembly.
Lets try some rough math, assuming wall area is about the same as house footprint and say 20% glazing.
-wall losses 70F*2500*0.8/R24=5800
-ceiling 70F*2500/R60=2900
-slab (assume 50F bellow slab) 20*2500/R10=5000
-glass 70F*2500*0.2/R6.5=5300
So about 19000BTU. Add in some ventilation losses and air leaks, still not seeing anything near 38000BTU.
As for the HVAC tech. First time I've heard about heat not flowing around bends. Heat does not care if it has to go through a 90. About the only thing a lot of bends do is add extra equivalent length, provided the ducting is sized and designed properly, this is not an issues. Definitely no reason to bump up your heat pump size "just in case".
The fact that the tech is saying the units come in one ton increments implies a domestic heat pump. Most of these tend to fall flat in cold weather, very few are a good fit in zone 6. You want a real hyper heat cold climate unit, unfortunately most of these are from overseas manufacturers. The good thing is all of them come in 1/2 ton increments. Figure out what your real heat loss is and go from there. This is a good resource for checking the cold weather performance of units:
https://ashp.neep.org/#!/
For a new build, you are looking at a single zone centrally ducted unit. Lot of times these are referred to as multi position air handler. Some of the units that would work are Mitsubishi Zuba, Gree Flexx or Dainkin Skyair unit.
Hi Akos. A few clarifications to try and get it aligned with your assessment.
Slab is 2,500 but second story over garage adds 700 sqft, so 3,200 total.
700 sqft floor loss over garage needs to be added.
The house has high ceilings and a wall area of 4300 sqft.
The glazing is 13%
Assuming framing @ 25% of walls.
In modeling, used set temp at 76F an air infiltration at 1.5
-wall losses 70F*4,300*0.75*0.87/R24=8,200
- Framing @ 25% would be 70F*4,300*.25*.87/R12=5,500
-ceiling 70F*3200/R60=3730
-slab (assume 50F bellow slab) 20*2500/R10=5000
-glass 70F*4300*0.13/R6.5=6,000
- floor over garage 70F*700/R40=1,200
Total 30,000
As noted in my post, virtually no South facing windows for solar gains.
The 90’s are not flexible bends, rather the 90’s in rigid ducts to get from one side of house across 9’, 10’ and 14’ ceilings.
Akos,
Based on my clarifications, any change in your feeling it’s way over rates?
Thanks
You’ve got a bad HVAC contractor - you’ll have to lead them to the right answer kicking and screaming as 90 degree angles don’t increase heat loss. Or try the honey approach. They’re probably skeptical of heat pumps and while foolish, you’ll need them to comply. An easy compromise is installing a small resistance strip backup - 3kw is probably sufficient. That way, you placate them and get the right sized equipment. You can then shut off the resistance heat.
Another option is using a separate system for the bonus room - a small ductless or ducted unit should do the trick.
Akos is right here - what’s the total square footage and how many sqft of windows? What infiltration value did you use? This is where the calculators usually fail.
I agree with Akos. These numbers don't pass the sniff test.
If you have your own model, you should share it.
See above. Tried to align by clarifying a few assumptions. Rough calculations come out to ~30k, without air leak, HRV loss etc. So, modeling at 38k isn't wildly off.
I’ve reviewed everything again, and my calculations and energy audit align close enough for design heat loss when considering the slab, wall, roof insulation and windows. We both come in around 38k btu/hr using 1.5 air changes. I won’t know the actual air changes until construction is done. I followed the suggestions here to aggressively push the calculations, as did the energy advisor from what I can tell.
It will be a whole house ASHP, but I’m struggling whether it’s the 3-ton or 4-ton. Assuming a target temp of 76F, 78F, and the btu/hr creep up.
Will have HWHP, so should I be considering what it will steal?
Ceilings are 10’-14’ with total sqft at 3,200, and volume around 31,000 cubic sqft.
Starting to worry I’m over thinking this.
Your are right about at the 3.5 ton range. You can look at the P series unit:
https://ashp.neep.org/#!/product/34536/7/25000///0
These are semi commercial so tend to be a bit more expensive.
I think a better option would be the residential 3 ton hyper heat unit (SVZ-KP36NA & SUZ-KA36NAHZ) with a separate wall/floor/slim ducted unit for the bonus room. The combination of the two should be more than enough for your heat loss and give zoning control for the bonus room which tend to be uncomfortable when ducted to a central air handler.
You can also install the 8kW strip heater into your main ducted unit which would add an extra 27000BTU which can easily carry you through a polar vortex plus provide backup heat.