Zoning vs. multiple heat pumps
TL;DR: for a central ducted heat pump system in a large house, is it better (performance-wise, more cost-effective) to use a single heat pump +/- zoning for each floor, versus having two heat pumps that cover different areas?
For more background: high-performance/net-zero-ish project with a plan to use central ducted air-source heat pump(s) for the heating (and cooling) with electric back-up. The house is located in climate zone 7a.
Heat-loss calculations (F280) estimate need for 46000 BTU/h overall on coldest day of year (-30°C), divided into 9500 BTU/h for upper floor, 12500 BTU/h for main floor, and 17000 BTU/h for basement.
Would a single 4-ton unit (48,000 BTU/h) sufficient? Ideally, if we went with a single unit, we’d want to have it zoned 3-ways (upper floor, main floor, basement). However, if zoning into 3-zones would add too much complexity/decrease life span of equipment, could go 2 zones (upper floor, then main floor + basement), or no zoning at all. In this scenario, it would be nice to be able to run three thermostats (in series?) to turn on the heat pump whenever it gets too cold. (Proposed model of heat pump has variable compressor and variable-speed air handler.)
The HVAC supplier isn’t crazy about the idea of going to a single unit, and is continuing to recommend two units. We haven’t received the formal proposal yet, but presumably a 2-ton unit for the basement and a zoned 2.5-ton unit for the main/upper floors, based on the heat loss calculations.
So our options are:
- 2 heat pump units, divided somehow between the upper, main, and basement floors, presumably with a zoning to split one of the units between 2 floors.
- A single 4-ton heat pump unit, with separate zones for 3 floors.
- A single 4-ton heat pump unit, without any zoning, but separate thermostats to ensure that each floor is sufficiently comfortable.
What’s the best option here? From a cost perspective, 1 is doable, but obviously if 2 or 3 are similar and won’t pose reliability issues, they’d love to take the cost savings.
Thanks for your thoughts in advance.
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Replies
Both work just as well. It really comes down to what your installer is comfortable with.
My preference would be two units. One for basement/main and one for the 2nd floor. The reason for this split is the basement runs are easy to add on as the ducting will run through there anyways and by having a unit on the 2nd floor, you don't need to run any supply and return truncks through the main floor. It also allows you to cool the 2nd floor more during the summer which is important for comfort.
If you do go for a 2nd floor unit, make sure to select a downflow unit so you can run the ducting through the floor joists (floor trusses with a duct chase are your friend for this). You don't want any ducting in the attic.
As for the unit, this is a good resource.
https://ashp.neep.org/#!/product_list/
You are looking for either "centrally ducted" or "compact ducted". You can use the sliders to select heat at 5F, most units loose about 20% of their 5F capacity at -30C.
Once you select a unit in the corner, there is box called "advanced sizing tool". You can pick a zip (you'll have to find a place somewhat equivalent in the US) and enter you heat load. It will show you how much the unit will cover and how much backup heat you'll need.
Something like this can cover you main/basement:
https://mylinkdrive.com/viewPdf?srcUrl=http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10006/M_SUBMITTAL_PVA-A42AA7_PUZ-HA42NKA1_en.pdf
There are equivalent units from Fujitsu/LG/Carrier.
P.S. Your basement loads seem very high. Is this some raised bungalow type with all above grade with big windows?
If the cost numbers work for you, I would go with 3 separate units because each floor has very different heating and cooling needs and timing. In summer, you could easily have the top floor asking for cooling most of the time, the first floor sometimes and the basement almost never.
I agree with Akos, the basement loads seem unlikely. With earth sheltering, you should have much lower temperature difference between inside and out, meaning less heating and cooling demand. If the difference is driven by low insulation levels in basement walls, add insulation to bring your heating demand down to reasonable levels. You get some cost reduction in your HVAC installation and operating costs to make up for the extra insulation.
I was on the same path as you once, regarding zoned systems. I've learned that Zoned controls are complicated and not great, hard on the equipment, contractors hate them and apparently they don't work well. You can't do things like cool the 3rd floor and heat the basement at the same time. Etc.
I'm a big fan of dedicated units per floor.
Each floor has their own intricacies with light, windows, doors, appliances, people, stack effect etc. controlling each floor independently is ideal.
Its not always feasible, but very doable with new builds and smaller units.
Will the basement really be occupied enough hours a day to justify the expense of a third system?
Without the equipment it is unlikely to be more than a degree maybe 2 at most different than the main floor would that differential matter given its intended usages.
If I went with a zoning system, I would avoid the third party systems and only consider ones made by manufacture of the HVAC equipment you selected as the controls will be fully integrated.
I like the redundancy separate systems give you and think it is worth a somewhat higher cost.
Walta