heat pump system maintenance, thermostat settings, operational efficiency

[Jordan Van Voast]

My house has sixi Mitsubishi split heads on three floors and one large compressor connected to all of them - installed in fall 2020.  I purchased an annual maintenance contract. I've only used it a handful of times for cooling in the summer. Some back story: When I was a child, my father would always set the oil heater in our Maine house at 60 degrees during the night and then turn it to 72 in the morning.  Whenever we left a room, we were taught to turn off the lights to conserve energy.  Yesterday, the maintenance person was here and he said the system was running fine. Recently, I've realized that on cold winter days in Seattle, it can take 8 to 10 hours to bring a room to the desired (warm) temperature if the heat pump thermostat is set too low the previous night. So I've resigned myself to go against my conservation training and set the thermostats higher in those cases.

The technician suggested that I keep all my units running at a more consistent temperature so that the system doesn't have to deal with huge temperature variances. I have an AirBNB (yeah, I know, political no-no right now) in my basement. It's been empty 95% of the time since early December. Mostly I've left those units off and the basement temp can get down into the low 40s in a cold snap. The question I'm getting at here is how do others deal with this question of increased electricity costs and larger energy usage versus taking care of the needs of the system?  Is there any legitimacy to my technician's advice? The maintenance contract is ~$287 per year, paid in advance. There's the cost of repair (so far zero), versus the cost of electricity for heating and cooling a large house. In the summer - for at least 4 months, I typically turn the system off, but now I'm not sure what is best. Thanks for any advice!

--

Jordan Van Voast, Licensed Acupuncturist
on Duwamish/coast Salish traditional land

2109 31st Ave. S.

Seattle, WA 98144
206.860.5009

[Mike O'Brien]

Hi Jordon-

Good to hear from you. 

A couple quick thoughts but I’m happy to talk more. 

On the maintenance side, these units should have a 10 or 12 year parts warranty from Mitsubishi. That warranty is not dependent on you running the machine on any frequency. That said, like any equipment, it’s probably good to run it periodically. 

On the set back question, I’ve seen mixed data but generally the cost/energy benefits of night time setbacks for heat pumps are limited or perhaps negative, so I generally try to encourage people to let their comfort drive setbacks decisions, not cost. I do think large setbacks such as 10 degrees is probably not ideal for over night for the reasons you are observing. When the machine is working really hard to make up ground it uses a lot of energy and does put come additional wear on the equipment. It’s probably best to keep the temp constant all night or set it back a couple degrees if you want to sleep cooler. If you go away for a week, then go ahead and set it back 10 degrees. 

Hopefully that helps. 

Mike

Mike O'Brien

Co-owner, EcoSpark Heat Pumps

mi...@ecosparkheatpumps.com

On Feb 20, 2025, at 7:11 AM, Jordan Van Voast <jorda...@gmail.com> wrote:



My house has sixi Mitsubishi split heads on three floors and one large compressor connected to all of them - installed in fall 2020.  I purchased an annual maintenance contract. I've only used it a handful of times for cooling in the summer. Some back story: When I was a child, my father would always set the oil heater in our Maine house at 60 degrees during the night and then turn it to 72 in the morning.  Whenever we left a room, we were taught to turn off the lights to conserve energy.  Yesterday, the maintenance person was here and he said the system was running fine. Recently, I've realized that on cold winter days in Seattle, it can take 8 to 10 hours to bring a room to the desired (warm) temperature if the heat pump thermostat is set too low the previous night. So I've resigned myself to go against my conservation training and set the thermostats higher in those cases.

The technician suggested that I keep all my units running at a more consistent temperature so that the system doesn't have to deal with huge temperature variances. I have an AirBNB (yeah, I know, political no-no right now) in my basement. It's been empty 95% of the time since early December. Mostly I've left those units off and the basement temp can get down into the low 40s in a cold snap. The question I'm getting at here is how do others deal with this question of increased electricity costs and larger energy usage versus taking care of the needs of the system?  Is there any legitimacy to my technician's advice? The maintenance contract is ~$287 per year, paid in advance. There's the cost of repair (so far zero), versus the cost of electricity for heating and cooling a large house. In the summer - for at least 4 months, I typically turn the system off, but now I'm not sure what is best. Thanks for any advice!

<mitsubishi.png>

--

Jordan Van Voast, Licensed Acupuncturist
on Duwamish/coast Salish traditional land

2109 31st Ave. S.

Seattle, WA 98144
206.860.5009

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[hollytownes]

I live in Seattle and have a full house Mitusbishi. I do set my house back at night ftom 68 to 63.I had a gas furnace before and that I turned down to 55.  I set it to heat the house back up by 8. It is set to come on as needed depending on thr outside temperature.  Generally it comes on about 6 am. This saves energy since it is off from 11pm to 6 AM ( 7 hours). The house is a very old house but has been retrofitted with double pane windows, attic insulation and some wall insulation. How long it takes to heat up depends on the air tightness and windows and insulation. I would experiment to see how it does.

I also save energy if I only use one room in the evening in the finished basement. I turn down the system and use a little electric heater in the TV room. I calculated the difference in energy use and it saved energy to do that. I would turn off or down parts of the house that you aren't using for several hours or when you leave the house for several hours.

Turning down and off can save energy.

I do not pay for a maintenance contract since there is little that can go wrong with a relatively new heat pump as long as you change your air filters and keep your outdoor unit free of debris. Also you need to check that your condensate pump pipe is free from blockage.I do mot think annual checks by a technician are worth the money. I did do that on my gas furnace as it got older.

Hope this helps.

Holly Townes 

Mechanical Energineer specializing in Building Efficiency 

Sent from my T-Mobile 5G Device

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[Rob Harrison cPHc]

Another way to look at it. Setbacks work well for gas forced air furnaces because the difference between the temperature of the heated air coming out of your forced air vents (110°—170° depending on the age and efficiency of the furnace) and the temperature of the air in the room (say 55°—65°) is large. The heat energy in that hotter air transfers more quickly. (Like if you’re boiling water—you don’t set the stove to simmer if you want the water to boil quickly.) That scenario is what we’re used to and what prompts the idea of saving energy with night setbacks.  Ductless minisplits on the other hand, distribute air in the 90°—110° range. So they warm a house more slowly. As Mike says, heating quickly bumps them into less-efficient electric resistance mode. 

As Holly noted differences in air tightness and insulation levels, as well as the type of heat pump, make a difference in how well night setbacks work. A drafty, poorly insulated house will cool off quicker, but will save energy with night setbacks because it’s just losing heat all the time. A well insulated, air sealed house will “coast” for much longer without the heat on. 

Rob

Rob Harrison cPHc

HARRISON architects

(206) 794-2738

via iPhone….

http://harrisonarchitects.com/

lyrical sustainable design  ::  passivhaus

On Feb 21, 2025, at 10:19 AM, 'hollytownes' via Western Washington Home Electrification <seattle-home-e...@googlegroups.com> wrote:



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[Jonlin, Duane]

But, we're able to set the heat pump operating mode so that it won't kick into supplementary electric resistance mode, right?

And in that case, it would seem possible to do a setback, and adjust the morning startup time so that the temperature would rise to the desired temperature by the time the alarm went off in the morning.

DJ

	Duane Jonlin, FAIA Energy Code and Energy Conservation Advisor City of Seattle Department of Construction and Inspections P.O. Box 34019, Seattle, WA 98124-4019 P: 206.233.2781 |  duane.jonlin@seattle.gov Facebook I X I Blog
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Subject: Re: heat pump system maintenance, thermostat settings, operational efficiency

 

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[Jordan Van Voast]

Very helpful again. Thanks Rob. I mostly have a well insulated house with double pain windows - except for one large (thick?) single pane picture glass window in the living room (6' X 12' approx). I pull the curtains at night at least. 

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--

Jordan Van Voast, Licensed Acupuncturist
on Duwamish/coast Salish traditional land

2109 31st Ave. S.

Seattle, WA 98144
206.860.5009

https://communichi.org/

https://dharmafriendship.org/

airbnb.com/h/urbanshangrila

[Jordan Van Voast]

Thanks Mike too for your initial explanation!

[hollytownes]

Good explanation Muke thanks. I should note that I have no electric strip backup in my system. So it never goes to the inefficient mode. It is always in heatpump mode. Alot of systems are set to not bring on strip heat until very low temps like 35 or 40. Also some mini splits have no electric strip heating. I know my son's do not

 

Holly

Holly

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[Jonlin, Duane]

So I'm still utterly confused by this conversation.

Given a system that's in no danger of going into electric resistance mode:

Seems to me that if the interior temperature is several degrees lower at night (when the exterior temperature is also lower), and then starts ramping back up to occupied temperature at 5:00 AM or something, there would be energy savings, right? This is why codes mandate setback capable thermostats.

Is the advice to just "set it and forget it" for optimal energy efficiency incorrect?

DJ

	Duane Jonlin, FAIA Energy Code and Energy Conservation Advisor City of Seattle Department of Construction and Inspections P.O. Box 34019, Seattle, WA 98124-4019 P: 206.233.2781 |  duane.jonlin@seattle.gov Facebook I X I Blog
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From: 'hollytownes' via Western Washington Home Electrification <seattle-home-e...@googlegroups.com>
Sent: Friday, February 21, 2025 2:07 PM
To: Jordan Van Voast <jorda...@gmail.com>; Rob Harrison cPHc <r...@harrisonarchitects.com>

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Subject: Re: heat pump system maintenance, thermostat settings, operational efficiency

 

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[Thad Curtz]

My installers explanation for this rule of thumb was that our Daikin mini-split is running a lot less efficiently when it’s operating at its maximum output than when its puttering along. (There’s no electric strip backup in it, just the compressor and the air handler.)

Best wishes,

Thad Curtz

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[Jordan Van Voast]

That makes sense, though I'm still trying to figure out the balance point between maximizing energy/equipment efficiency versus electricity cost and any increased maintenance costs due to higher wear and tear. Do I turn off the heat in a room and close the door if I don't expect to use it for 48 hours, 72 hours or more? Or, if I anticipate using it, do I just keep the heat set at the lowest setting (61 degrees) to avoid taxing the system when I need to heat it up from 40/45 degrees to 75 degrees?  

[Jonlin, Duane]

Jordan,

One reason not to close off a room from the heat during winter is that you can get condensation resulting in mildew and mold.

DJ

	Duane Jonlin, FAIA Energy Code and Energy Conservation Advisor City of Seattle Department of Construction and Inspections P.O. Box 34019, Seattle, WA 98124-4019 P: 206.233.2781 |  duane.jonlin@seattle.gov Facebook I X I Blog
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From: Jordan Van Voast <jorda...@gmail.com>
Sent: Friday, February 21, 2025 3:49 PM
To: Thad Curtz <oly...@gmail.com>
Cc: Jonlin, Duane <Duane....@seattle.gov>; Rob Harrison cPHc <r...@harrisonarchitects.com>; hollytownes <holly...@comcast.net>; Electrification Western Washington Home <seattle-home-e...@googlegroups.com>

[Jordan Van Voast]

You are talking about condensation/mold in the conduits? Or the room itself?

[Jonlin, Duane]

In the room itself, especially on the interior face of exterior walls.

	Duane Jonlin, FAIA Energy Code and Energy Conservation Advisor City of Seattle Department of Construction and Inspections P.O. Box 34019, Seattle, WA 98124-4019 P: 206.233.2781 |  duane.jonlin@seattle.gov Facebook I X I Blog
	Helping people build a safe, livable, and inclusive Seattle.

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From: Jordan Van Voast <jorda...@gmail.com>

Sent: Friday, February 21, 2025 3:53 PM
To: Jonlin, Duane <Duane....@seattle.gov>
Cc: Thad Curtz <oly...@gmail.com>; Rob Harrison cPHc <r...@harrisonarchitects.com>; hollytownes <holly...@comcast.net>; Electrification Western Washington Home <seattle-home-e...@googlegroups.com>

[hollytownes]

Hi all

Do not shut room off permanently during winter. 

For saving energy do setback at night and when gone several hours or m9t on use for several hours.

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[Mike O'Brien]

Hi folks - there are certainly others with deeper knowledge on this and I defer to their expertise, but my understanding is that one of the drivers for the lack of savings with setbacks is the power consumption curve of variable speed heat pumps. 

Page 31 of this study (or labeled page 22) https://www.nrel.gov/docs/fy22osti/80802.pdf shows the power consumption of the outdoor fan motor and different rpms. 

At about 450 rpms it uses 50 watts and around 900 rpm it’s using 250 watts. It is not linear. 

Even without electric resistance heat strips (which we almost never install) the heat pump itself uses significantly less energy when it’s running at half capacity than at full capacity. The energy use will depend on a number of factors but on a mild winter day in the PNW, you can imagine that the heat pump could cruise along all evening at 50% capacity just siping energy to maintain temperature. With a setback in a well insulated home it may not run at all during the night, but then when it ramps up in the morning, it would be running at near full capacity for a bit to make up the temp, and at least on this graph, full capacity is using 5 times the power as half capacity. 

The other challenge with setbacks is on the coldest days of the year. If your heat pump is sized properly it should have enough BTU capacity to just barely maintain your temp on the coldest day of the year. But if you start it in the morning at a large temperature deficit it has to work hard enough to not just maintain temperature, but make up lost ground,  and it just may not have enough capacity to do that. 

Mike O'Brien 

On Feb 21, 2025, at 11:43 AM, 'Jonlin, Duane' via Western Washington Home Electrification <seattle-home-e...@googlegroups.com> wrote:



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[Jonlin, Duane]

Thanks, Mike!

	Duane Jonlin, FAIA Energy Code and Energy Conservation Advisor City of Seattle Department of Construction and Inspections P.O. Box 34019, Seattle, WA 98124-4019 P: 206.233.2781 |  duane.jonlin@seattle.gov Facebook I X I Blog
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From: Mike O'Brien <mobse...@gmail.com>
Sent: Saturday, February 22, 2025 8:58 AM
To: Jonlin, Duane <Duane....@seattle.gov>
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