Heat Pump Size and Airflow
abaerlo
| Posted in Mechanicals on
We’re building a passive house in New Brunswick, Canada (climate zone 6). Our energy modelling shows a heating load of 13,434 Btu/hr and cooling load of 6,986 Btu/hr. The house is 3,000 sq ft over three stories. We’re going with a ducted, heat pump (either 1.5 or 2 ton). Some HVAC providers have suggested the air flow might be too low (650-800 CFM) with a smaller heat pump. Could this be an issue? Do we need to oversize the heat pump for adequate air flow? Thanks for your help!
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When it comes to heat or cooling houses don't have air flow requirements they have BTU. The airflow is there to deliver the required heat and that is about it.
The air flow issue is a hold over from old days with leaky houses. You wanted "adequate" air flow for good mixing to even out room temperature. This is one of the reasons you see larger than required furnaces in a lot of houses which ironically makes them less comfortable as they short cycle.
Pick the heat pump that covers your load and don't worry about the air flow. The air flow is an output from the furnace selection and it is only used for designing your ducting and registers. It should not be an input for picking a heat pump.
With a 3 story structure make sure to design in a large return near the ceiling on the third floor. Without this you'll get a lot more stratification and cooling suffers. This is a big duct and takes up space but a must for a new build.
P.S. Most cold climate heat pumps deliver well above their nameplate rating even down to -15C. Check that actual max output of the unit you are looking to select, some 1 ton units deliver above 1.5 tons of heat. This a good resource for comparing units:
https://ashp.neep.org/#!/product_list/
Ok, great. Thanks for your help!
With 3 stories the challenge will be keeping all the floors at the same temperature as the warmest air will want to rise to the top and the coldest air will want to get to the bottom.
You may want to consider mutable systems or zoning.
Consider selecting the smaller heat pump and expecting the resistant backup heat to run a handful of hours a year. The smaller unit will remove more humidity when cooling.
Walta
Yes, they've recommended three zones, one per floor. Maybe these could just be manual dampers? Thanks for your feedback!
I think manual dampers will work if you are ok with having several degrees of temperature variation between the different floors at times.
Walta
I’d consider two ductless heads, one on the first floor sized to handle the heating load and one on the 3rd floor sized for cooling. Assuming you have a well designed mechanical ventilation system and PH level insulation and air tightness, those btus will get distributed and you won’t have comfort issues.
This would work for heating especially if you have a post heater on the ERV. If you are in a place with a lot of cooling, this won't be great if you ever want to sleep with closed doors.
I think for a new build, go for ducts. Bit more work but better air distribution plus less maintaince. It is much easier to change an air filter once in a while than to clean the coil and blower wheel on a wall mount.
This was proposed as an option but it was suggested ducts would likely be more comfortable (and require less maintenance). We do plan on having a separate, ducted ventilation system, though.