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Heat Pump Truth or Myth#1 - Keeping it running 24/7 uses less energy

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(@kev-m)
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Starting an ASHP from cold uses lots of energy so you’re better off leaving it on 24/7. Truth or myth?  Let’s see.

I’m running on weather compensation only, keeping the whole house at about 21 deg.  I turn the heating off with the timer 12-5am.  The house temp drops 2-4 degrees and then slowly builds up again from 5am.  This suits us as it’s cooler at night and in the morning when we’re  active or out. 

These are the results from a recent, cold, 5am wake-up for the ASHP.

Cold Start

In the 90 minutes shown, the first peak gets the flow up from 17 deg to the WC target of 42 deg.  The second peak is a defrost. It then settles down to a steady state to maintain the flow at 41-42 deg. The ambient was -2 deg throughout.  

The 90 minutes used 5.48kWh. If the ASHP had been running steadily with no cold start and no defrost, at the same level as at 6:30am, that 90 minutes would have used 4.36kW.  So it took an extra 1.12kWh to wake up the ASHP and  get the flow up to target compared with running constantly.

Had the ASHP been running at the 6:30am level for the 5 hours it was off, it would have used c. 14.5kWh. There are a couple more adjustments (defrosts, Freeze Stat) I could make to the numbers but I think they would make the argument stronger.  The conclusion is clear; switching off overnight saves me energy. 

Of course the big caveat here is that this isn’t heating up the house, just the radiators.  The house does heat up but takes 3 or 4 hours. Like I said, this suits us. If I (or the ASHP) bumped up the flow temperature to heat the house up quicker then it would be different.


   
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(@andris)
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I found the same results. We like it cooler a wee bit at night too. Also have the advatage of a well insulated house wich can be heated a bit more during the day like a battery and just very slowly cool at night. My results are in now for a test 1 whole year.

Withnwheater comp.  I used to use 12500 kw a year. 

Now 10000kw/ year.

2 changes i made to get the results.

1. Higher fixed flow using thermostat: 35c for 4-5 hours the warmest part of the day.

2. Hot water gets heated twice a day: only mornings and evenings.

16kw Samsung TDM ASHP. 8.4kw PV, power optimizers 20×420watt panels 6kw SolarEdge inverter.


   
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cathodeRay
(@cathoderay)
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Posted by: @kev-m

The conclusion is clear; switching off overnight saves me energy. 

Very interesting Kev. There has always been a lot of smoke and mirrors on this subject, extra energy need to raise rather than maintain temperature etc, but intuitively (or perhaps relying on basic physics), it must take more energy to maintain a body, in this case a house, at a constant temperature, than to allow a period or relative cooling. Heat loss is proportional to the temperature difference between the body and its environment (houses loose more heat in cold weather), therefore, if you decrease that difference, you will decrease the heat demand, and, in a properly set up system, the energy consumption. You have very neatly provided some empirical evidence that this is so.

And then it gets complicated: defrost cycles, house like mine that are very slow to reheat (a night time setback may reduce my energy use, but the price is a cool house for most of the day).

It would be helpful if your findings could be replicated. On that, I wonder where you got the data for the chart? My blasted Midea app only goes down to hourly data, and that gets wiped the following day, really no use at all, and certainly won't show defrost cycles, what happens during recovery after defrost etc. 

@batalto and @mjr - did you by any chance see my post on hooking domoticz up to the Midia controller - it might achieve the above (minute by minute data) - link to that post here <= link. It since occurred to me that just such a USB/wifi dongle/connection might be Freedom's 'incredibly simple' bit of hardware that they won't tell any of us mere punters about.     

Midea 14kW (for now...) ASHP heating both building and DHW


   
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(@andris)
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@cathoderay 

3 things also would affect consumption with an ASHP. Defrost, ambient temperature, and pumps electric consumption.

1.When I run the Heat pump during the day (not in the last few days when it never even went over 0C during the day) It uses warmer ambient temperature (3-7c) At night when it is low (0-3 C)

2. Less defrost during the day because of the warmer air outside.

3. The pumps. I have 3 pumps in in the ASHP 170watt/h two little pump controlling the two heating zone 40watt each. This will use 1kw energy over 4 hours. if I run it 24 hours it uses 6kw just the pumps alone.

I hope that makes sense.

Andris

16kw Samsung TDM ASHP. 8.4kw PV, power optimizers 20×420watt panels 6kw SolarEdge inverter.


   
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Mars
 Mars
(@editor)
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@kev-m, I fully agree, and I've discovered that when we did a similar experiment in the autumn, we used less electricity. The issue for us was that if it's really cold outside (3C or less) our coldest rooms in the house can take 12-24 hours to get back to target temperature if the heat pump's been off for a long time. For people with well insulated houses, I think turning the heat pump off for extended periods of time could save them money in the long run without affecting comfort. 

I'm also seriously beginning to question all these 'absolute' rules that exist within the heat pump world. A lot of these rules have been put into place to make the installer's life easier and have been branded as a gold standard, and which don't have the homeowner's best interests in mind when it comes to things like efficiency and running costs.

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cathodeRay
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Posted by: @editor

our coldest rooms in the house can take 12-24 hours to get back to target temperature if the heat pump's been off for a long time

Exactly, this is the problem I have, and it happens even with a small setback to say 16 degrees, but that has the effect of turning the heat pump off for several hours. Once the setback kicks in, the house slowly cools down over the next several hours, and may even be just a fraction above 16 degrees when the setback ends. The house then equally slowly recovers temperature. The result is a slow wave cycle of chilling and warming which is not at least to this human very comfortable. 

Posted by: @editor

I'm also seriously beginning to question all these 'absolute' rules that exist within the heat pump world. A lot of these rules have been put into place to make the installer's life easier and have been branded as a gold standard, and which don't have the homeowner's best interests in mind when it comes to things like efficiency and running costs.

Absolutely, and that is why this forum is such a valuable resource. Thanks for all your hard work, it really is worth it.

Midea 14kW (for now...) ASHP heating both building and DHW


   
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(@bob77)
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The other variable, which applies to me and I suspect quite a few other people, is variable electricity tariffs. My energy costs about 33% less overnight, which of course coincides with time I don’t need the house so warm. But should I leave the heating running through the cheap period so it’s not playing catch-up at peak rate? 


   
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 mjr
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Posted by: @bob77

My energy costs about 33% less overnight, which of course coincides with time I don’t need the house so warm. But should I leave the heating running through the cheap period so it’s not playing catch-up at peak rate? 

Interesting. My E7 is about 67% cheaper overnight at the moment. But there's still little point heating rooms as much while everyone is under duvets, so I don't leave the heating running through the cheap period, but I do heat some water up and start heating soon enough to catch up before peak rate (or at least that's the aim: was 30 minutes late catching up this morning!).

This is going to be a common variable and isn't yet generally understood how to include it in heat pump settings. Octopus are rumoured to be developing a heat-pump-user-specific tariff with two cheaper-rate periods (early-morning and early-afternoon) and a higher evening peak rate and I expect their variable-by-half-hour Agile will return one day, too.

 


   
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cathodeRay
(@cathoderay)
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Despite what I said earlier, there is still one thing that bothers me. Consider boiling and then simmering a saucepan of water: to get the water to boil, you have the cooker ring set high, but once it is boiling, you turn it down to keep the water simmering. You need more energy to raise the temperature than you do to keep as a fixed temperature. the same must apply to a building: it takes more energy to raise its temperature than it does to keep it at a fixed temperature. During the reheat phase after a setback period, it will take more energy during that reheat period than if it had been in a steady state throughout, but what about the total period, say 24 hours. I think the deal using made up numbers is that you save say 10kWh over the setback period, but then use say 4kWh extra during the reheat period than you would have used had you been in steady state throughout. The reheat period looks bad, higher usage, but if you look at the total 24 hour period, you have still used less total energy, 10 - 4 = 6kWh saved.    

Midea 14kW (for now...) ASHP heating both building and DHW


   
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(@oswiu)
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Posted by: @cathoderay
You need more energy to raise the temperature than you do to keep as a fixed temperature. the same must apply to a building: it takes more energy to raise its temperature than it does to keep it at a fixed temperature.

That's true, but it doesn't necessarily mean you need a hotter flow temperature (or stove top). Given enough time at the normal weather compensated flow temp, the temperature of the house will return to its designed temperature since as long as it's below its normal room temperature, the heat losses are lower than the energy the heat pump brings into the house meaning the temperature inside will increase until its heat losses reach equilibrium with the energy provided by the heat pump. The same is true on a gas stove (or in my case induction), although normally you would put it on a high flame to get it to boil, if you left it on a low flame which you know would allow it to simmer, the water would eventually start to simmer, it would just take a long time.

What I mean with that is that as long as

  1. You don't mind it being a tiny bit colder in the morning
  2. You don't leave the heat pump off too long
  3. You don't increase the flow temp from its normal weather compensated curve

Then you surely must save energy by doing this. If point 1. is a problem, then even leaving the heat pump off or lower for one hour at midnight will save an hour's worth of energy (~4%) at what I suspect to be no noticeable comfort loss since it would have mostly recovered by the time you wake up.

That's my hypothesis anyway, and I would be interested to hear counter points. I've only just got my heat pump working, so I now need to test it out!

Edit: I've just thought that there's not normally a way to modulate the power used to heat the water in the system up whilst it gets to target flow temp meaning the compressor might work at full tilt whilst it heats up the system water. Hence, I suppose, the complications around this. If only we could limit the power itself.

 


   
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(@kev-m)
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Posted by: @editor

I'm also seriously beginning to question all these 'absolute' rules that exist within the heat pump world. A lot of these rules have been put into place to make the installer's life easier and have been branded as a gold standard, and which don't have the homeowner's best interests in mind when it comes to things like efficiency and running costs.

I agree; I did this slightly tongue in cheek but there are some hard and fast 'golden rules' quoted by so called experts that are definitely not the best for everyone in all cases. 

My next truth or myth will be for cycling so watch this space. 🙂  

 

 


   
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(@kev-m)
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Posted by: @mjr

Posted by: @bob77

My energy costs about 33% less overnight, which of course coincides with time I don’t need the house so warm. But should I leave the heating running through the cheap period so it’s not playing catch-up at peak rate? 

Interesting. My E7 is about 67% cheaper overnight at the moment. But there's still little point heating rooms as much while everyone is under duvets, so I don't leave the heating running through the cheap period, but I do heat some water up and start heating soon enough to catch up before peak rate (or at least that's the aim: was 30 minutes late catching up this morning!).

This is going to be a common variable and isn't yet generally understood how to include it in heat pump settings. Octopus are rumoured to be developing a heat-pump-user-specific tariff with two cheaper-rate periods (early-morning and early-afternoon) and a higher evening peak rate and I expect their variable-by-half-hour Agile will return one day, too.

 

@mjr is this the Octopus tariff you mean?

https://octopus.energy/smart/cosy-octopus?fbclid=IwAR0B_KhYcBSGnI50qIhS3Kr_T4fGXpqu77t9HR3U8J70KGtR36dDr8SY9a4

 

 


   
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