Is this post-ASHP electricity use expected?
Summary: Our electricity use roughly doubled this winter compared to last after installing an ASHP (from around 500 kwh per winter month to 1100-1200 kwh). In the previous winter, we used between 0.5-0.8 kwh per heating degree day, this winter we’re at 1.3-1.6 kwh/HDD. I’m trying to understand whether this is normal, or if I should investigate further. More details below.
Details: We swapped out our forced-air gas furnace for a Carrier 38MARBQ24AA3 heat pump with a 40MBAAQ24XA3 air handler unit. The unit was deliberately ever-so-slightly undersized based on heat load estimates from our historical gas usage (heating load of about 26k BTU/hr at a design temp of 0F). During the polar vortex days we’ve had in MA this winter, the decrease in capacity occurred pretty much as expected (couldn’t keep the 68F set point below roughly 8F outside temp). This was the only time that we used the 10kw electric heater kit, for no more than a couple of hours.
I did install a Panasonic IB 100 ERV half-way through this winter, which is installed using the simplified approach (tied into the existing ductwork and circulated with the air handler fan). But the doubling in electricity use is visible for the month prior to installing the ERV as well.
Edit: as was pointed out below, it doesn’t make sense to compare kwh/HDD before and after swapping the gas furnace for the ASHP. The correct comparison is: (BTUs embodied in the gas)/HDD to (BTUs embodied in the kwh)/HDD.
If I do this, I see that my energy use fell from 7’000-15’000 BTUs/HDD to 1’600-3’300 BTUs/HDD when we moved from gas to the ASHP.
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I wouldn't focus on the doubling so much as the actual increment. In dollars, is your combined electric and gas bill lower than a year ago?
It's an unfortunate comparison because electricity rates in the ISO New England grid have increased so much. Electric bills increased from $150-190 to $550-650 (corresponding to the incrase from 500 kwh/month to 1100-1200 kwh/month). Gas bills went from $150-250 to $16 (corresponding to going from 100-150 therms to 2 therms for the dryer).
Total $ are often deceiving: energy unit rates and COP can turn these into $/MMBtu. So for example: $.10/kWh and a COP of 3.5 would be 293 x $.10 /3.5 = $8.37/MMBtu. The fact that my gas rates change monthly only adds confusion :).
So you are using about 650 kWh/mo for heating, or an average of 900 W (0.9 kWh/h). That's about 3000 BTU/h energy input to your heat pump.
What's the average temperature been during those months? Let's say it was 35 F, so about 30 F below a base temperature of 65. If you need 26 kBTU/h at ΔT = 70, you need 11 kBTU/h at ΔT = 30 F. 11 kBTUh output at 3 kBTU/h input is a COP of 11/3 = 3.67, which is pretty great. So it seems like your system is working well from those rough guestimate numbers.
Using the same 11k BTU/hr I get about 80 therms for the month for gas.
Dividing out the bills I get gas is $1.50 per therm and electricity is $.50 per kWh. One therm is about 29 kWh. Even with a COP of 3.67 one therm is about 8 kWh. If those rates are right the electric is going to be about three times as much.
$0.50/kWh is right for MA if you don’t pick an electric provider. At least for the winter when generation prices are at their highest. The electricity is probably 0.33 and the rest is delivery.
Those numbers are not inspiring me to go to a heat pump yet.
They do make solar look attractive though!
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Thank you both, Charlie and DC, I've learned a lot from your answers.
Based on the HDD data for my area, I get observed COPs of 3.0-3.9 for the coldest months, and 5.6 for October/November. So I guess that is better than expected!
Yes, The ERV is responsible for some of the increased usage. Maybe a lot of it depending on how much air you are moving.
If you are using more kWh per degree day. The elephant in the room is that your system may have developed a small leak that has reduced the COP of your heat pump.
It is also possible some other variable has changed like your thermostat is set higher. It is possible someone mistakenly set the thermostat to E Heat turning off the heat pump.
It is also possible your local weather is colder this year lowering your COP.
At this point we can’t be sure all the extra kWhs went into the heat pump and not into some other loads.
I like to watch the temperature difference between the supply and return ducts when my HP is on low speed 13-15°
Walta
"I like to watch the temperature difference between the supply and return ducts when my HP is on low speed 13-15°"
Mind explaining what information you get by watching this?
This is very low delta t - especially for a mini split. Fujitsu wnd Mitsubishi want to see at least 30-35f in the real world with 1-3f superheat and roughly 140f compressor discharge temp
greenright,
I thought mini-splits typically had lower delta T's given the modulation and all that? What am I missing?
No... they run absolutely hysterical delta t... 35-40f is what mostly seems they target regardless of indoor fan speed.. I know for a fact Fujitsu and Mitsu air handlers target 35f delta t when operating in the real world. Wall mounted indoors can go even higher- at 70f intake it is not uncommon for them to run 120-130f discharge air...
I believe that modulation in heating is really a fancy word for "maintain 35f delta t in any of the 3 speeds of the indoor fan and outdoor temps range". The modulation algorithm is really not that complicated either- "controlling the compressor rps (usually from 20 to 100) target 420-ish psi high pressure and controlling the ev obtain 1-3 degree superheat". Thats about it in simplified terms.
hmm. This goes against everything I have ever understood about mini splits.
Why is there the common notion that heat pumps are a low temp (but more constant) heat source?
If the unit is always maintaining a certain delta T, then it seems you are implying the only way they target set points is by changing fan speed... but fan speed can be set manually. And it further implies that the bottom end of modulation is set by the CFM of low fan speed and absolutely nothing else (unless the outdoor compressor bottoms out first)?
You seem like you know your stuff, but this just doesn't make sense to me nor have I ever heard anything like it... I welcome further clarification.
Hmmm.. it seems that I cannot respond to maine_tyler (too deeply nested answers?) so here it is:
Yes, the regular heat pumps tend to be low temp, long run devices. The vapor injected current crop of minisplits do not operate that way - they seem to always try to maintain the same discharge temp unless too cold outside and then they start to discharge proportionately cooler air
On fan speeds- yes- the fan speed can be constant, but delta-t and compressor rpms to maintain it are not if you are targeting certain discharge temp. As you are approaching setpoint the delta t to target discharge (say 100-105) is getting smaller and since the refrigerant pressure delta gets smaller too (low side gets higher) the compressor can back off a bit.. or a lot. Often when properly sized, when say the setpoint is 1 degree off the modulating algo can hit a sweet spot where the 105 degree discharge at current air flow equals heat loss of room and thus run time is extended greatly.
So overall there is no black magic... if you connect a power monitor to your outdoor and you have a return and discharge air temp sensors you can get a good idea of how the modulating algorithm works. I even can tell you by watching the power consumption every second if the unit is undercharged or overcharged or just right...
Generally speaking the Mitsubishis and the Fujitsus will ramp up the compressor for 10-15 minutes to a point when enough hot vapor circulates and then will start the injection cycle. As that happens the high side pressure will go up significantly and very quickly to around 450-475 and then the compressor will back off a bit (from 60 rps to about 20)... then it will wait for a minute or two and will pick up rps that brings pressure to 425 for Fujitsu and 410 for Mitsu... at that point small steps of rps up/down will be made to target said pressure AND vapor injection and EV will be controlled to maintain 1-5f superheat. As the room heats up and delta-t gets smaller the compressor will back off a bit over time until setpoint is hit.
The simple answer here is that YES, you should expect your electric usage to increase significantly after switching from a gas furnace to an electric heat pump. The reason is that you're now using electricity to run ALL of your heating equipment (note that I said "equipment", not "BTU" here), instead of using gas as the heat source for the equipment. You should expect an increase in your electric bill and a decrease in your gas bill as a result.
The big question then becomes "will my COMBINED gas+electric operational costs be LESS than what they were before I put in the heat pump?". That's usually the goal, and often also the case you find since heat pumps are usually cheaper to operate overall, since they move BTUs from outside, but they use electricity to do that. That means more electricity to run the equipment, but less input energy to produce the actual heat compared to using gas to make ALL the heat instead of a heat pump which scavenges heat from the outdoors.
DC did a pretty good job explaining how the COP comes into play here regarding exactly how much kWh worth of electricity it will take to produce a given amount of BTUs worth of heat for your home. If you are finding that your actual electric consumption has increased more than what those predictions would show, then you MIGHT have a problem. I would suspect either the auxillary heat is kicking in more than you thought, or you are running a lot of defrost cycles. Those are the two things I'd look for.
ERVs are usually pretty small electric users, so even if they run 24 hours a day, the total cost shouldn't be that much to keep them running. If you have an ERV that uses 100w, for example, and run it 24x7, you be payin $36/month to run it assuming a 50 cent per kWh electric rate. If your rate is 33 cents per kWh (note that I'm pulling those rates from earlier posts in the thread), then you're looking at $23.76/month to run the ERV.
Bill
Bill maybe we are reading different questions.
The way I read it last winter this HP in this house used between 0.5-0.8 kwh per heating degree day and this winter they required 1.3-1.6 kwh/HDD
The only notable change is the addition of an HRV but there are many other variables.
I agree with your analysis for the power consumed by the HRV itself but I disagree that that number is the total cost of operation for an HRV.
No HRV can extract all the heat from the air it is expelling so their must be some loss. The HRV proponents tell us the units recover well over 90% of the energy. Frankly I say BS maybe in some lab under the ideal conditions for some short amount of time but not in the real world when it is 15° cooler outdoors than in my guess is the expelled air is no more than 2° cooler than the indoor air making the recovery under 14%.
Walta
Walta,
I think you are reading a different question.
From the OP:
"Our electricity use roughly doubled this winter compared to last after installing an ASHP"
To be fair, I don't know why the OP compared last winter to this winter using kWh per heating degree day since they used to run gas...
Well, there’s electricity being used for other things. Probably best described as kWh/month.
Right my point was the use of HDD specifically. To me that implies a comparison between two electric based heating systems. For the year without the heat pump, there shouldn't be a tremendous association between HDD's and electrical usage.
The comparison between total (or monthly) electrically usage is fine and makes sense in order to compare the increased electrical energy input compared to previous gas energy inputs.
Yeah, I saw the "replaced the gas furnace..." part and was thinking "more electric since the heat pump is running now instead of the gas". If the comparison is between heat pump last year and heat pump this year, I'd look more at the average outdoor weather conditions for each heating season, assuming the indoor temperature set point was the same both years. It should be relatively easy to check on the ERV though as a possible cause -- compare the months before the ERV to the months after. If you see a big jump in your electric use immediately after the ERV is installed, then that MIGHT indicate the ERV is the culprit, but you'd still have to check if maybe the weather got a lot colder in that same month...
Bill
" 15° cooler outdoors than in my guess is the expelled air is no more than 2° cooler than the indoor air making the recovery under 14%."
I guess it is possible to unbalance an HRV to that point but even a crappy install will get much better than that.
I've monitored the temperatures on my properly balanced ERV and I get nameplate efficiency out of the unit. When real winter hits here with near single digit temperatures, the ERV temperature efficiency is about 70% which matches the spec sheet quite well.
Sorry everyone for the confusing way I've posed my question.
Tyler is of course right that the comparison of kwh/HDD before and after the ASHP install does not make a ton of sense, because we used to heat with gas until the spring of 2022. The division by HDD was my attempt at controlling for the heat load due to different average temperatures. But in the numerator I should have compared the BTUs embodied in the gas for the previous winter to the BTUs embodied in the total kwh for this winter.
If I do this, I see that my energy use fell from 7'000-15'000 BTUs/HDD to 1'600-3'300 BTUs/HDD when we moved from gas to the ASHP.
I do worry a little bit that the implied COP has gone from 4.1 to 3 this past month, even though we've gone from 1070 to 920 HDD. I might get an electricity monitor for the house to be able to track the specific use of the heat pump.
>"because we used to heat with gas until the spring of 2022."
That implies that most of last heating season, you were heating primarily with gas, with the heat pump pickup up the load only as the weather was warming up in the spring of 2022. That means that during the current heating season, you've been heating with the heatpump through the fall, and the winter so far as well. That's likely a LOT more heating "work" being done by the heat pump this year, which would explain the higher electric bills over the same period.
To make any comparisons, you have to hold everything as constant as possible so that you know what you're measuring. That means, in this case, average outdoor conditions, indoor setpoints, and time duration of operation. If any of those are significantly different between your two sample periods ("this" and "last" heating season), then you don't have directly comparable data to work with.
Bill
How about this?
How much did you pay for electricity Oct 21 through Feb 22?
How much did you pay for gas during the same period?
How much did you pay for electricity Oct 22 through Feb 23?
How much did you pay for gas during the same period?
What is the likely increase in power use due to the Panasonic ERV? Here you go:
Panasonic FV-10VE2 is rated at 80% efficiency tested at 53 CFM, using 29 watts of power as tested here: https://www.hvi.org/hvi-certified-products-directory/section-iii-hrv-erv-directory-listing/)
Case 1: 0 F outside ambient day at 80% efficiency = 56 F incoming fresh air.
801 BTU or 235 watts which is 5.64 kWh/day to heat the incoming fresh air to 70 F.
At COP of 3, then 1.88 kWh/day to heat the incoming air.
.7 kWh/day to power the unit.
Total power use running at 53 CFM on a 0 F day, 2.58 kWh/day
Note that defrost cycles which start at 14 F, would actually increase power use by the unit to around 80 watts.
Case 2: 32 F outside ambient day at 80% efficiency = 62 F incoming fresh air.
457 BTU or 133 watts which is 3.2 kWh/Day to heat the incoming fresh air to 70 F.
At COP of 3, then 1.1 kWh/day to heat the incoming air.
.7 kWh/day to power the unit.
Total power use at 53 CFM on a 32 F day, 1.8 kWh/day
If your unit was running at 75 CFM (max flow) I would add about 55% to those values, and at at 100 CFM, add about 110% as efficiency will drop on the core to somewhere in the 65-70% range as air flow increases.
Op might not realize any $$$ savings by switching to ashp from gas… just saying. Ashp are awesome alternative to oil and resistive electric heat, but gas in terms of btu per dollar is still ahead even at current prices. Just saying
Thanks everyone for your responses so far. I am attaching a spreadsheet that shows my calculations of the estimated energy use and COPs for anyone that's interested.
My conclusion from all this so far is that I am observing a performance from my heat pump that is in line or better than expected. The increased electricity use is therefore no greater than expected (unfortunately it coincided with a substantial increase in electricity costs per kwh). My remaining concern is that the COP seems to have degraded during the most recent billing period, even though the number of HDDs was lower.
In response to Bill's concern that I am not comparing apples to apples: the numbers that I've reported here are *per billing period*, and I've made sure to compare roughly the same range of winter months for 2021/22 and 2022/23. I've cumulated the HDDs for each billing period, so I think that should control for differences in average outdoor temperatures. I also believe that we've used the same temperature setpoint on our thermostat. It's of course possible that we've had other differences in usage that are more difficult to control for.
Spreadsheet is great, but you need to add in the Electric Bill for last year so you can see total energy costs when you were using gas for heat.
Humid weather around freezing will do wonders to cop in a bad way. Add some precipitation and those heat exchangers get plugged in a hurry. Normally defrosts are every 3 hours or as needed. During conditions as described above I have seen defrosts every 45 minutes… for 5-10 minutes each. That is a significant hit on performance
Hasn't your area had some extra cold weather over the past month or so? That would also contribute to higher costs to run the heat pump, especially if the auxillary heat had to kick in on ocassion.
In my area, you need at least about a COP of 3 for gas and electric to start to cost about the same. If you're under that, gas is cheaper to run. If you're over that, then electric is cheaper, and the heat pump saves you money on input energy. Keep in mind that the COP is NOT constant, either, it varies based on outdoor air temperature (assuming a constant indoor setpoint). Frequent defrost cycles will also act as a hit to your overall efficiency too.
Bill