Heat Pump Wall Unit Placement (with existing electric central furnace)
Hello all,
Here are some details of my home:
– Year: 1971
– Type: Bungalow
– Ft^2: 1900
– Climate zone: 6? (Ottawa Canada)
– Insulation Quality: Poor walls/windows/doors, 2×4 framing, attic redone with >R60, I should also note the living room is under a flat roof with unknown insulation value, see layout description/image.
– Heating System: 20kW Electric Central Furnace, Single Stage, located in a “tall crawlspace” under living room
– Home Layout: Somewhat open concept, large living room ~450 ft^2, large kitchen ~350 ft^2, 3 bedroom, 4 exterior person doors + 1 door to garage (unheated), Majority of living space is over unfinished basement – exception is the living room which is over “tall crawlspace” that houses furnace. See attached image, I modelled my home’s layout and ductwork to run some computational fluid dynamics but can’t get the simulation working… Restating here that the living room is under a flat roof with unknown insulation value. Basement is heated, but I have no problem with it being much cooler than upstairs, it’s currently just storage/workshop.
– My goal is to reduce heating costs (electricity bill in winter is way too high) and potentially comfort (some areas are a bit chilly, mostly the living room.
And here is my situation:
Problem:
After spending the past year getting quotes from contractors to install a central air sourced heat pump, I am coming to the conclusion that it is far too expensive ($15k-$20k) to justify, even with the new federal program ($5k rebate), and wall units ($3k-$5k installed myself, no rebate). I’ve gotten quotes for both systems eligible for the rebate and systems not eligible. The size that I need is 2.5 ton, and I understand that “bumping” up to 3 ton isn’t a problem (assuming that I go with a variable unit and not a staged one) so that is for what most of my quotes are. One of the issues is the low clearance in the “crawlspace”, although it’s 5-6 ft high, the air handlers with most/all the units are 4-5ft and have require ducting over and under to work in the vertical configuration; adding the requirement for the unit to lay on its side and extra duct work.
Solution:
As I mentioned above, I’ve almost reached the conclusion that wall mounted units are my best option. I am considering a 3 ton, 3 indoor wall mounted unit DIY system, (I understand that getting someone to fill my lines may be an issue and I intend to find someone/equipment before placing an order). I’ve also considered using the concealed duct type and placing 2 of them below the floor (ceiling of basement/crawlspace) with a few ducts running to some strategic areas.
I intend to have the central system blower running at all times to circulate at least some air. I’d have its temperature set at ~20°C and the heat pump wall units set for ~22°C so that the furnace would run infrequently/never (I hope). My current thinking is for 2-3 wall units, 1 in living room, 1 in kitchen, and possible 1 in dining room.
Questions:
– Is it absurd to install wall units when I have a central system? Salesmen say it is, but they have an obvious conflict of interest…
– Where should I place the wall units, should they be as close as possible to the returns so the furnace distributes their air?
– Should they be a certain distance from the central system’s thermostat so they don’t give it a false reading of satisfactory warmth in the house?
– Any other input/suggestions?
If you made it this far, thanks for all your time! Any input/suggestions would be greatly appreciated!
Thanks in advance,
Nick.
Replies
I know you said you are in Canada, but I would look into a system similar to this
https://www.highseer.com/collections/ducted/products/pioneer%C2%AE-36-000-btu-18-seer-ducted-central-split-air-conditioner-heat-pump-system?variant=31950430994543
Could use your existing ductwork and wiring. You said you living room tended to be colder, so during installation you could increase the duct size/airflow to that room.
I suspect that the lines being prefilled is why they cant ship across the border, there might be certain regulations in place for that, so try to find a local wholesale dealer.
Going full minisplit would give you great zone control & cost savings, but would be much more work to install.
I also support your decision to overspec the heat pump a bit since they put out less heat as it gets colder, though it should always use less energy per unit heat produced vs the resistance electric furnace.
Good luck.
p.s. Nice 3d model!
Thanks for advice! I should have been a bit clearer with my "clearance issues" in my original post. I attached a quick sketch that shows my height restrictions in the vertical config (horizontal would give me a bunch of ducting work to figure out). If the air handlers return is vertical in the bottom and supply is vertical out the top, the max height it can be is ~26" so that I have room for 90° turns within the limits created by the joists above and the floor below. If a unit has the option for the return to be cut in the side (and bottom blocked off), the max air handler height I could use would be ~46". If both the supply and return have the options to go in/out the sides at the bottom/top then I could use anything up to 62". It seems like it is not possible to have either top nor bottom cut out, salespeople will say it is over the phone, and then when a tech comes to see/quote, they'll say it can't and talk about horizontal config with elaborate ducting.
One thing that stood out to me was the low CFM (941) for the unit you linked, I haven't come across one that low before now, might be why the say it's for 1200 ft^2 which is very low for a 3 ton.
It seems like it's a bit of a challenge to get these systems shipped to Canada still. There are a few suppliers that I've found that might have solutions but I'm not willing to risk whatever the costs are on a "might"...
I'm starting to reconsider my requirement for a variable stage system at this point. I could get a single stage central system put in for $5-7k and it would function efficiently to about -15°C. I've done a pretty thorough analysis of my history of energy usage vs temperature (past 2 years only so I validated the data with publicly available daily temperature averages for the area) and my conservative estimates (assumed cut-off at -10°C where electric furnace is used) tell me that the energy savings from single stage to variable stage will be very small (under $200 CAD per year) and that additional upfront cost is not financially justifiable.
But.... seeing the prices of the walled units keeps pulling me back... Pros: Cheaper than single stage, better efficiency, localized adjustments. Cons: Visible wall units, risk that heat won't be sufficiently distributed through house.
Anyway, thanks again for your help. Please feel free to provide any more advice/insight!
Full Disclosure: I am not an HVAC professional, just an avid DIY'er who has installed a couple minisplits myself who is also passionate about comfortably heating a home for as little $$$ as possible.
I'm guessing that you might need to do a horizontal mount and some fancy ductwork for a central system.
I have seen pretty small air handlers for lower btu units, but yours is so small right now because it only has resistive electric heaters. Check out the installation manual for more details.
https://www.pdhvac.com/site/downloads/manuals/DR_AH_IM.pdf
Though this unit is just an example from a company I like, feel free to shop around.
The actual CFM of the system can be a moot point since the air handlers are designed to work as efficiently as possible with the compressor. What matters is the amount of BTU produced per unit energy (COP/HSPF). It is worth looking into if the fan can reach its rated CFM with your current ducting system since that will improve efficiency and reduce strain on your equipment.
If you go the minisplit route you will have temp issues as long as you have walls and doors separating any unheated zones. Running the furnace fan will mitigate this, but will also drop your efficiency through any duct leaks and the energy required to run the fan. For example my furnace fan uses about 550watts. Running it 24 hours a day to regulate temperature would use ~13kw a day that I could have used to heat the house.
I estimate that putting that energy into my heat pump would yield 1-2 therms of heat.
(13.2kw x 3412btu,kw x 3COP = 135kbtu)
Most of the time a single stage will do fine, more moving parts usually means more complexity to break down the road, but could be worth it dependent of energy costs and time of use costs.
If your backup is electric heat you can run your heat pump compressor as low as the manufacture will allow. For example my single stage dakin heatpump at -10c will still do a COP of over 2, meaning that I will get twice the amount of heat per energy inputted. My electric heat only comes on when the heatpump cant keep up and temp starts dropping in the house.
If you're not sure what you're doing when it comes to duct work, electrical, and load calculations I would recommend going with a HVAC contractor. It is expensive since these guys are running a business and put in enough overhead to make their time worth it. DIY just means that you pay with your time.
If it is any consolation usually hvac upgrades are reflected into the value of your home if you own it.
Also homes with central heating and cooling tend to be valued higher than minisplit homes.
I am a big fan of mini splits and personally don't mind the way they look, but Im also a fan of using what you have to your advantage, which is this case is a duct running to every room in your home.
Going the mini split route I would personally start small and get one unit+compressor installed in your living room. That will help with the temp imbalance and give you a good idea of what it is like to install this kind of system, plus hopefully save a little money this heating season.
Obviously there are alot of different ways to tackle your issue, but it's all trade offs for cost, ease of install, and efficiency, and I am not saying anything I suggested is the gospel & way to go, just some surface level observations. I hope someone else comments on this thread and provides more insight!