Did I size this Mitsubishi Hyper Heat pump correctly?
I’d like to replace our furnace with a Mitsubishi hyper heat system and would love a sanity/reality check. The house is 1,000SF plus unfinished basement in zone 5b. The design temps are 0º and 90º.
Following Mr. Dorsett’s method I took the highest consecutive two-month usage period (135 therms over 63 days) and calculated 19 and 21k btu/h for 65º and 60º, respectively. Or ~25k btu/h ASHRAE.
Does this seem ballpark for a 1962 R-11 wall/R-30 attic house or did I make an error somewhere? We did do a thorough exterior remodel last year with new windows/doors and saw a nice heating bill drop from the prior year.
If I could do some more infiltration work and insulate the basement it would be great to be in the 1.5ton class for a heat pump i.e SVZ-KP18NA-SUZ-KA18NAHZ
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Since a heat pump is a long term commitment, it would be wise to base it on the house as it is to be long term. Insulating the attic and air sealing ought to be done first, to my thinking.
When I worked on my house some years back now, I changed it from an energy pig that needed intervention in very cold weather[leave certain faucets dripping etc] to a house where I have no idea what temperature it is outside -10 to 95.
Where does your house sit in that continuum?
Frozen pipes and cold spots/rooms are frequently air leaks.
Houses with a single heat pump as a heat pump are usually very well air sealed and highly insulated
Peak load is also the thing that you need to think about
Heat pumps lose heating capacity at very low temps, so from a purely heating perspective, a bit of oversizing is not a bad thing. Too much it will cost you money in efficiency and to varying extent comfort in cooling season.
Other than basics being met i.e. no frozen pipes or freezing kids, it's hard to tell where we are as the PO put in a 110KBTU furnace in 17'. Imagine standing behind an F18 taking off a couple times a day....
Anyways, sizing for our near future plans was my hope.
You could wait for a day at or close to the design temp and record the run time percentage at that temp.
Consider the greenest thing you can do is to not replace anything unless absolutely necessary. By upgrading early, you are needless forcing the mining manufacturing and transportation of this equipment many years sooner than necessary. Reduce and reuse before you recycle.
Do plan ahead so you know what you want the day this equipment does die.
Walta
I appreciate this point and would go another route if it were possible.
Btu per sqf is not the best measure but can tell us if we are in the ballpark. Say the basement is equivalent to 1/2 a floor, so that puts you around 14btu/sqft which is reasonable for a house with insulated walls and new windows.
Generally the fuel based load calculation is accurate, about the only thing it does not take into account is solar gain.
As is, the SUZ-KA18NAHZ would foot the bill, if you are planning to do some more upgrades (say insulating rim joist area and maybe top 2 feet of foundation), it will drop your heat loss even more. No need to upsize.
In case you are concerned, add in a strip heater. The installed cost is not all that much plus it avoid the argument with the HVAC tech about their perceived undersized heat pump. Your heat loss is about 5kW, there is no need to install a strip heater bigger than that.
Without any practical experience on heat pumps I just thought to size for extremes i.e heat pump capacity for safety then backup strips and fireplace supplement for comfort. But reading the above I may have been thinking about this wrong i.e the strips should replace most of the HP load.
I'm betting our rim joist area is responsible for the majority of our current heat loss. The solar gain is a good point, thanks.
I don't think you need a strip heat. Sometimes people prefer to install one for peace of mind and to cover those polar vortex days. Lot of times I see techs install strip heaters that are larger than the heat loss of the place which is not needed.
Since the heat pump is already right sized, my rule of thumb for strip heat is to size it to keep your place from freezing in case your heat pump fails. This generally means a strip heat about 80% of your space heat load. In your case, 20000BTU=5.8kW so 80% is about 4.6kW strip heat. Again this is not needed, install it just for a piece of mind and to keep the installer complaining about the "woefully undersized" heat pump.
In terms of operation, the heat pump and strip heat can run at the same time. Unlike older heat pumps whose efficiency dropped off in cold weather, most modern units manage a COP of around 2 even in extreme cold. Since worse case they use 1/2 the electricity of stirp heat, you want to run the heat pump as much as possible and the strip heat at the same time but only to assist if really needed.
This makes sense to me, thanks. It seems the SVZ-KP18NA does not have a temp setting for the strip heater but they cite an "Electric Heat Lockout Control" accessory.
I don't like undersizing the heat pump and relying on strip heat. My house has twice the heat loss of yours in a milder climate (17 °F 99% design point), but look how ugly the power use vs temperature graph is for my undersized heat pump (attached). With mass adoption of heat pumps, which should be the goal, we'll all be hitting those extreme peak power draws at the same time. I'd save the strip heat for emergencies.
Edit: But bigger capacity doesn't automatically help. For example, the suz-ka24nahz gives 23 kbtu/hr at 5 °F, while the suz-ka18nahz gives 21.6 kbtu/hr. The larger unit uses 454 W more, which is more than the difference in capacity. The smaller unit is actually pulling in more outdoor heat.
The way I see it there is almost no downside to letting the HVAC guy have this win and allowing him to put in the largest strip heat package that available for your unit. The operating costs will be exactly the same as the correct size. The bigger heater will use more power but run for less time but both will make the same number of BTUs and use the same number of kWh to do so. The price differential between the two units is a rounding error. Yes, you may end up buying one or two more circuit breakers and some more wire. If putting in the oversized strips gives your HVAC guy the confidence to install the HP size you want and he thinks is undersized I call that a huge win.
The strip heat penalty is not so great when you consider the strips will be locked out unless it is under 5° or so and at that point most heat pumps CPO will be under 2 and dropping fast. If your HP makes 95% of the BTUs needed to heat your home, does it really matter if the last 5% costs twice as much?
Walta
It doesn't matter much to the homeowner, but what happens when everyone has heat pumps and is using strip heat on the coldest day of the decade? It breaks the Texas grid, but New York is going to do better?
On average it does not matter to the grid the same number of kWh must be produced and distributed to the same number of homes.
If all the heat strips were synchronized to come on at the same moment that would be a problem but that are not. maybe after a power outage but the grid is down at that point.
I do recall trying to flush all the toilets in a large hotel at the same moment but it did not work.
Walta