ducting retrofit for ASHP
We are talking to a contractor about replacing the gas furnace in our ~1500 sq ft, 1929 southern Ontario (CZ6) home with a ccASHP.
The contractor is suggesting a 3.5 ton unit. (He has experience with Mitsubishi and is now also selling Moovair. As far as I can tell, he’s been doing ASHPs for several years, and geothermal for decades.)
The static pressure will be too high in our current ductwork, so we’re looking at enlarging it. Our walls are plaster/lath, so redoing ducts to the second floor is infeasible. Our basement is unfinished, so we can do rework down there.
The contractor is suggesting that we can enlarge the main supply duct in the basement, add another vent in the basement, and add few more vents to the floor of the first storey.
Does this seem like a sensible way to address the problem of static pressure in
the new system?
Is having a lot more vents on the first storey (where the thermostat is) than on the second story going to cause comfort problems on the second storey?
How is static pressure in the system affected when air moves from the main supply duct into the smaller ducts that feed the room vents?
[I’m not sure I buy the sizing being suggested, but that, with more info, will be another post.]
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
How sure are you that 3.5 tons is the correct size? Have you done a fuel use study to confirm 3.5 tons is correct?
Have you made all the logical improvements your buildings envelope?
I agree that connecting to poorly designed undersized ductwork is a bad idea.
I like the idea of keeping all the ductwork inside the conditioned space.
If the contractor is going to do a manual J calculation for each room and then use that data to do a manual D calculation in order to deliver a calculated number of CFMs to each room to provide the BTU needed that is great. Understand most contractors only calculate the invoice and simply look at the old equipment and bid something a little bigger after a few rounds the old ductwork is way undersized and make a new design on the back of an envelope.
If you really want both the upstairs and down stairs the be the same temp in both the winter and summer, I think 2 systems or zoning is a must unless you are going to make seasonal adjustment. Note do not allow equipment or ductwork in the attic.
If you are on city gas switching to a heat pump is likely to increase your operating costs.
Does the heat pump you are considering have vapor injection IE “hyper heat” each brand has a different trademarked named for the technology.
Walta
+1 on Fuel based load calc. Run through the math here and see where you end up:
https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler
The chances of a 1600sqft house needing 3.5 tons of heat pump is pretty slim. A house that big generally will have at least a 60000BTU furnace, so ducting should be sufficiently big for a right sized heat pump.
2nd floor cooling problems are common in older houses. They are usually a combination of too small of a return on the 2nd floor and ceiling air leaks.
Of course fixing both is the best for energy efficiency and comfort, but just sealing the ceiling (blower door directed attic sealing) will make a big difference. Enlarging the existing return and moving to near the 2nd floor ceiling is a bigger job but helps a great deal.
Thanks. I plan to make another post about trying to get sizing right.
I guess my question here boils down to: If I need to enlarge ducts for whatever we end up getting, is the strategy of enlarging in the basement and only adding new vents to the first floor a reasonable one?
We do have an air return on the second floor, but it's in the hall floor. I wonder if the expense/mess of moving it up would be worth it.
In my experience, it's hard to imagine any old ductwork that is a good match for a well-designed new heat pump installation. But, these days sheet metal and labor are both crazy expensive so it's worth salvaging where possible. For all the other great reasons to go with two zones, the static pressures of two independent duct supplies and returns means perhaps you could use the old downstairs ducts with fewer modifications? Work with a contractor who can make reasonable calculations about this.
Would two zones require two outdoor units?
Not necessarily. A single outdoor unit can serve two or more indoor units (a "multi-split") and typically costs less, but there are downsides: less efficiency, comfort problems, no redundancy. Long discussions on this site regarding this. There are other major cost factors to your upgrade: oversized equipment means you're paying substantially more for capacity you don't need; higher-powered mid-static equipment also costs more. Simply starting out with right-sized equipment could save enough money upfront to pay for a new duct layout.
The way I see it you are putting the cart before the horse.
You can’t begin to decide if the current ducts are undersized until you decide how many tons of heat pump your home will require and decision depends on if you are going to air seal and the current upgrade your insulation.
Walta
The rule of thumb is you need to move 400 cubic feet per minute for each ton of equipment size moving at less than 900 feet minute. You can find the calculator to determine the number of square inches required to move the air and decide if the ducts are to small.
If you move less than 400CFM the equipment does not work correctly and if you exceed 900FPM people complain about the noise and draft.
The smaller blowers used in Asian equipment are not powerful enough to play by this old rule of thumb and require more careful designs for the ductwork.
Walta
“Would two zones require two outdoor units?”
2 independent systems would require 2 outdoor units.
A single system with zoning controls would not but tend to cost almost as much as independent systems and run less affectively, less efficiently and you lose the redundancy of the independent systems.
Walta
Thanks everyone. We have had an energy audit done (for Canada's Greener Homes incentive program) and have done a fuel-based load calculation.
The energy audit report specifically says not to use its heat load to size hvac equipment, but to have your hvac contractor do a proper load assessment (which up here is F280). But guess what contractors actually do?
The energy audit says 19 kW ~= 65,000 BTU. Our gas furnace is 60,000 BTU.
Our fuel-based calculation, which was done on data from after we did some air sealing and replaced most windows and doors, was ~33,000 BTU. (We also did the calculation with data from before those upgrades, but we had to estimate how much gas went to heating for that period, so possibly less reliable. The result was ~= 38000.)
I'm not confident enough to just use the 33k and go on my merry way, so now I'm looking for an hvac engineer. I'm hoping I can find someone to do the F280 and also help design the system. I'd rather spend a little more money up front to get the right system.
I was really expecting the fuel based number to come back under 30k. What was your blower door number, there might be some low hanging fruit in terms of air sealing.
The 33k puts you well with the range of a SVZ-KP36NA SUZ-KA36NAHZ which in southern ON climate should put out just a bit above 3tons of heat. If you can do a bit more air sealing or say insulating your basement, that would drop it down to even the 2.5ton unit.
The good news is either unit is way cheaper than the PUZ/PVA 3.5 ton unit your contractor initially recommended.
Either 2.5 ton or 3 ton unit, your existing ducting with a 60k furnace should be fine. Your HVAC tech can do a quick check with a pressure gauge on the supply and return ducting and see where it is. Even if close, lot of time just upgrading the to a larger air filter is enough.
I don't think there is all that much design a HVAC engineer can do in this case except provide a number in case your installer is hesitant to install the smaller unit. One way around this problem is to spec the smaller unit but allow the installer to put in a backup strip heater to give them a piece of mind.
Before doing any duct relocation, you need to figure out how much flow rate there is in your 2nd floor return. If there is good airflow, moving it is relatively simple upgrade.
The blower door number on the initial energy audit was 2561. I think we've hit the easy sealing opportunities. We spray foamed the underside of the roof (we couldn't find anyone who would remove the loose attic insulation and seal the 2nd floor ceiling). We replaced the basement windows, which had a lot of leaks around the edges. And we repaired some holes in the basement walls up around the rim joist that had been made to run wiring and such. Insulating the basement would mean finishing it, which is not in the cards right now.
Part of the problem is that there's not much insulating we can do. The exterior is brick and the interior is plaster/lath.
The contractor assessed the ducts by measuring the supply duct and using a cardboard slide rule type thing. I'm not sure they're going to do much more measurement. One of the things I'd be looking for from an engineer is another blower door test.
We have been playing around with the NEEP tool described here https://www.greenbuildingadvisor.com/article/using-neep-software-to-size-a-heat-pump and finding that the ZUMAs have relatively high minimum capacities, putting us below the modulating zone a surprisingly large portion of the time. Some of the Moovair models we're looking at seem to do better at middling temps. (But we're still playing around with the graphs because Canada is not included, so we have to fudge our climate info.)
As for the 2nd floor, there are two main supply branches going up there. The one that goes to the front of the house (the problem area), is closer to the furnace. The other one is closer to the end of the supply run and feeds the back rooms. The back rooms get plenty of air. This is why I think a different duct configuration might solve the problem.
Now let me get this straight. You want to replace a 60k furnace in CZ6 1500 sq' house with a 2 1/2 ton heat pump?
Maybe I'm not following correctly, but 3 1/2 tons for heating sounds way too small. That's the size of the air conditioner I need for my 1700 square foot house in Iowa. If you have a 60,000 BTU furnace now, wouldn't that require a 5 ton unit when heating? The air is going to favor the path of least resistance and not be balanced between the first and second story. I've installed ductwork for homes like this and always installed either a zoning system or a manual damper to restrict the flow upstairs in the winter.
The gas furnace is certainly way oversized.
Why do you say that? What are the run times on the coldest days?
Mike, have you read the article Akos linked to in post #2. Sizing a heat pump based on the rated capacity of the current furnace is a pretty lousy way to size a system.
I'm also wondering how your 3.5 ton system for 1700 sq. ft. in Iowa was sized.
I winged it. Happens all the time in the hvac industry.. Previous a/c was 4 ton and performed ok, but I dropped down 1/2 a ton. Indoor humidity rarely exceeds 40% now. Long run times..
Furnace was 80% 100K NG and I dropped that to a 96% 75K which barely kept up last time we saw -40, so I'm in the sweet spot. And correction.. My home is 1900 sq' 1970's
So I would feel comfortable using 6-7 ton heat pump, but my duct work wouldn't.. Just guessing now again, as I haven't installed hvac for decades.
Don't size based on whatever the old system was or on square foot rules of thumb, is the message. Wing it if that suits you, but I don't think that fits the definition of 'advisable.'
Yeah that's fine. But Id bet a hundred bucks a 2.5 ton heat pump wont keep up in the OP's home when the polar vortex hits. Make it 200
That is what backup strip heat is for.
People tend to overestimate how cold it gets and for how long, in my area with warmer climate we have barely seen any days bellow code design temps in the past three years. Even looking back a decade, there has been only a couple of days here and there with a min 10f bellow design temp.
Even if the outdoor temperature dips say 20F bellow your design temp, it doesn't mean your house will be 20F colder. It will slowly get colder at night (even a standard stick built home has a surprising amount of heat capacity), but will recover a
fair bit during the day as daytime temps tend to be higher plus you get some solar gain even on those stormy days.