Some data on heat pump performance today

Submitted by happyhourscience t3_10t0xj8 in vermont

I know there is some curiosity about how heat pumps do in the cold, so I thought I'd provide some data from today, just for fun. Remember that humidity, wind, solar gain will all influence the results, so this is just a snapshot.

First some background: our house is a mid-90's two story house in Burlington. 1400 square feet plus a semifinished basement.

We have three heat pumps, all Mitsubishi cold climate. Two single zone 12,000 BTU and one multizone 36,000 BTU (but interior units only put out 18k + 9k). Total nominal capacity is 51,000 BTU.

I set the heat pumps to keep the house around 70f last night before the temps dropped. I have been tracking the outside and inside temps all day and here are the results. I am reporting actual outside temperatures, but note that the wind chill has been consistently 20-25f below the actual temps.

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https://preview.redd.it/0zgn4bdgl2ga1.png?width=402&format=png&auto=webp&v=enabled&s=460874c5dbf1e5adc6b8034b99a4ef07e4fe5224

So the heat pumps were able to keep the house within a degree or so of the intended temperature until the sun went down and the temp dropped below -10f. They are still putting out heat, but just to be safe, tonight I'll turn on a space heater that has an automatic thermostat.

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Unique-Public-8594 t1_j74ofys wrote

Great to see this level of detail. Nice post. Thank you.

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zoomwojo t1_j74uli6 wrote

I have a Mitsubishi hyper heat. The way I understand it, under -13° it's basically just running the compressor constantly to produce enough heat to defrost the condenser. Not sure if this is accurate or if there's enough temperature difference between the refrigerant and the outside air to pull any heat. Basically, I know nothing.

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Careful_Square1742 t1_j74vt46 wrote

you're close - it ramps the compressor in bursts to defrost the coil.

I have a Daikin system that doesn't have a hyper heat function. it's still able to pull heat from outside air at -15 pretty efficiently. below -20 it works hard but it's kept up as low as -26 before.

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wrenchindaddy802 t1_j76cz1l wrote

This is where the geothermal setups really shine.

Fuck, I'd love me some heat pumps, let alone a geo setup.

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Careful_Square1742 t1_j76eyxb wrote

geo's awesome, if you have the right conditions (and bank account).

-17 at my house this morning and the heat pump system is purring. it goes into defrost for 5 minutes every 45 minutes or so, but the temp inside is within a degree or two of set point.

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zoomwojo t1_j74w0un wrote

As in, shakes the frost loose from the coil by abruptly starting?

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Careful_Square1742 t1_j74xbf3 wrote

nah a buzzer goes off to tell me to go out there with a hairdryer

it defrosts by pulling heat from inside briefly. like cooling mode but it doesn't run the fans on the evaporator (inside unit)

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abitdaft1776 t1_j76tmt9 wrote

I also have a Mitsubishi. The house was about 65 this morning. The pellet stove went out probably around 2am judging from the ash box. They are working okay. I think if we consistently had these temps I would get another stove but we are far from freezing

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Dangerous_Mention_15 t1_j74q25i wrote

How about power consumption? Heat pumps typically have a significant drop in efficiency the greater the temperature difference and eventually converges on resistance heating.

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happyhourscience OP t1_j74s1f9 wrote

Yes, I would like to have had the power consumption data for this post, but the BED consumption tracker does not always update in real time. I may have to wait until tomorrow for the data.

According to the usage tracker, last year on a similar day (Jan 21, High of 6, low of -14), I used about 90 kwh, but I charged my car and used a space heater. Assuming $0.16 per kwh that works out to about $15 for that day.

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Curious_Buffalo_1206 t1_j75i1bj wrote

You should really get a pellet stove for supplemental heat… 90 kWh for a single day is horrific.

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Abitconfusde t1_j75j2xi wrote

Charged a car and used a space heater. That could be 30-40kwh right there.

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wrenchindaddy802 t1_j76d68n wrote

We used to have a shitty little 1br apartment in Burlington, and our heat was about on par with that per month. Of course the upstairs neighbors were only paying 100/mo because they got all of our heat 😭 couldn't get out of that place quick enough between the heat fuckery and lack of driveway maintenance. I used to have to pull everyone out of the driveway in my outback, otherwise everyone would get stuck and gridlock the parking lot.

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Gilashot t1_j74sw0s wrote

Yes, but totally depends on the unit. My Gree Sapphire is still super efficient at -17f. My Mr. Cool is no good at all below -12

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Careful_Square1742 t1_j74umkz wrote

my 36kbtu heat pump system, on a day like this, costs about $10 to run. I'll check my GMP account when it updates tomorrow

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Careful_Square1742 t1_j767ot3 wrote

just checked GMP. I used 69 kWh yesterday. at $0.18, that's about $13 for heat, lights, fridge, stove, etc

69kwh is roughly 235,000 BTU

a gallon of propane contains about 93,000 BTU

assuming 96% efficient burn in a boiler/furnace, that's about 2.8 gallons of propane

at $3/gallon that's about $8.50.

so yesterday my heat pump was roughly on par with propane heat when you factor in the non heat electric use. today is supposed to be 10 degrees warmer, so I'm confident my heat pump will "win"

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thisoneisnotasbad t1_j76gaod wrote

Before I got my heat pump I looked at historic electric outages and temperatures over the past 10 years. It was a clear winner in terms of efficiency and reliability of infrastructure.

I grew up with wood.I have a pellet stove currently but heat pump tech has come a long way and they truely are hard to beat at this point

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Careful_Square1742 t1_j76i0y8 wrote

if you live in an area with frequent power outages, a wood stove for backup heat is a great idea.

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thisoneisnotasbad t1_j76jmi2 wrote

Very true. Which is why it is important to check that before investing in a heat source.

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abitdaft1776 t1_j76u12j wrote

I can’t imagine living up here without a generator. Power where I am doesn’t go out much, but if it went out this weekend because of wind…

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Careful_Square1742 t1_j76y3sq wrote

def a risk management calculation to be made here. Where I live, the power is super stable - maybe goes out 2-3 times a year for an hour, max. low likelihood of it going out, but whoa man it'd be bad if it did today!

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abitdaft1776 t1_j76tw51 wrote

Last winter I averaged 1000 a month in propane. Plus about 200 in electric. This month my electric bill is about 400. Even if you add in 100 for pellets when I use my stove I’m way ahead this year due to my mini splits

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kellogsmalone t1_j7fd43g wrote

$1000 in propane??? What factors led to this?

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abitdaft1776 t1_j7fl96z wrote

A very large, very poorly insulated house with an inefficient burner over 30 years old

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kellogsmalone t1_j7fniwn wrote

Sorry to prod just, my wife and are house shopping and trying to gage expectations for bills, renovations and upgrades and what to incorporate into our budget.

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t59599 t1_j74verx wrote

Thanks. Good information and happy to see how well they’re working.

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NJRMayo t1_j74piq8 wrote

Very nice!

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qrqrqrqr4 t1_j74y4xc wrote

Nice data! Ours is 1200 Sq ft house with 18,000 BTU centrally ducted. We are having similar results as you (and have discovered today that the relay on the in-unit electric resistance coil is broken).

Keeping up ok but definitely drooping.

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relativevelocity t1_j76b13q wrote

Great post, /r/heatpumps would appreciate the data too. I didn’t track it to your degree, but I noticed similar performance with my hyper heat (single zone 18k BTU).

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Vermontess t1_j76zzg1 wrote

How did it do overnight?

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happyhourscience OP t1_j771rsg wrote

Woke up to a 65 degree house, which is the temp I normally keep it at. No frozen pipes or other problems.

I did keep a space heater in the basement, set at 65 (so not constantly on), so I can't say how it would have gone with heat pumps alone. In general, I'm pretty happy with how they performed yesterday. Still waiting on energy usage data.

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redwolf1430 t1_j78mv9e wrote

Heat pump and space heater. Inside did drop to about 60F

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