Best time to heat DHW with ASHP

[Blooda]

First of many ASHP questions.

 

So currently we have DHW set to heat to 42 degrees between 3 and 5 in the morning, this gives us enough hot water for the day. From the smart meter app we generally use between 1.5 and 2.0 kWh and it gets to temperature in around half an hour. 

The initial thinking was that at this time of day it would not interfere with the heating, but now I have turned the heating off for summer, I was thinking it may be slightly more efficient to run it when the air temperature is higher in the day time ?

 

Thoughts please.

Edited June 5, 2023 by Blooda

[DanDee]

  On 05/06/2023 at 14:59, Blooda said:

First of many ASHP questions.

 

So currently we have DHW set to heat to 42 degrees between 3 and 5 in the morning, this gives us enough hot water for the day. From the smart meter app we generally use between 1.5 and 2.0 kWh and it gets to temperature in around half an hour. 

The initial thinking was that at this time of day it would not interfere with the heating, but now I have turned the heating off for summer, I was thinking it may be slightly more efficient to run it when the air temperature is higher in the day time ?

 

Thoughts please.

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science speaks for itself, 20% better in the afternoon vs early morning

 

[SteamyTea]

  On 05/06/2023 at 15:20, DanDee said:

Science speaks for itself, 20% better in the afternoon vs early morning

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But only if you use the correct temperature scale.

When calculating only the kelvin scale should be used, or you get the ridiculously saying 'it is twice as hot as yesterday' when it may only be 10⁰C warmer at 20⁰C.

[SteamyTea]

  On 05/06/2023 at 14:59, Blooda said:

So currently we have DHW set to heat to 42 degrees between 3 and 5 in the morning, this gives us enough hot water for the day. From the smart meter app we generally use between 1.5 and 2.0 kWh and it gets to temperature in around half an hour

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How much are you paying to get power at that time if day?

 

[DanDee]

  On 05/06/2023 at 15:24, SteamyTea said:

But only if you use the correct temperature scale.

When calculating only the kelvin scale should be used, or you get the ridiculously saying 'it is twice as hot as yesterday' when it may only be 10⁰C warmer at 20⁰C.

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how do you calculate?

[DanDee]

[SteamyTea]

  On 05/06/2023 at 15:29, DanDee said:

how do you calculate

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With the kelvin scale if you must show temperature increase as a percentage.

 

But if you want to know the improvement in CoP the you look at the difference between the two electrical energy uses.

Remember that temperature is not energy.

[DanDee]

  On 05/06/2023 at 15:35, SteamyTea said:

With the kelvin scale if you must show temperature increase as a percentage.

 

But if you want to know the improvement in CoP the you look at the difference between the two electrical energy uses.

Remember that temperature is not energy.

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can you give an example of calculation?

[SteamyTea]

  On 05/06/2023 at 15:37, DanDee said:

can you give an example of calculation?

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283 [K] ÷ 293 [K] ×100 = 96.59%

 

10 [⁰C] ÷ 20 [⁰C] ×100 = 50%

 

This gives the size of the numerator, with respect to the denominator, expressed as a percentage.

 

The mean free path speed of the molecules, in an identical material, will be the same in both instances.

[JohnMo]

If you have a cheap overnight tariff then heat at night, if you have solar heat when sun on the panels. Other than that when it's most likely to be warmest.

[joth]

  On 05/06/2023 at 16:39, SteamyTea said:

283 [K] ÷ 293 [K] ×100 = 96.59%

 

10 [⁰C] ÷ 20 [⁰C] ×100 = 50%

 

This gives the size of the numerator, with respect to the denominator, expressed as a percentage.

 

The mean free path speed of the molecules, in an identical material, will be the same in both instances.

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This is of course just the ratio between the two absolute temperatures. It does not equate to the % difference in efficiency at those respective outdoor temperatures. That will depend on other factors too. Best just to look it up in the heatpump datasheet.

[SteamyTea]

  On 05/06/2023 at 17:37, joth said:

Best just to look it up in the heatpump datasheet.

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Yes. Then find there is not a lot in it.

 

In reality, if you know the temperature of the refrigerant on the cold and hot sides, you can calculate the efficiency.

But as both if those will be varying with OAT and delivery temperature, it does not help much.

Using the kelvin scale is the better way to work out the potential energy available in the outside air, but then humidity levels can screw it up no end because around 273 K, water will stop condensing and start to change phase, and that release a different amount of energy. Also that water is at its most dense at 277 K. Confusing it more.

 

It would be useful if there was easily accessible data, through a range of OAT and RHs, while delivering a range of temperatures and power.

Kind of think that once written a spreadsheet is great for.

[AliG]

If you take this as a typical COP chart then you will get a better COP running the ASHP when it is warmer.

 

We have had days recently when it was 5C during the night and 15C during the day so, heating DHW to 50C, you would get a COP of 2.8 at night and 3.2 during the day. The big difference would come if the night time temp was between 0 and 5 and the daytime temp was 10Cish as you might get in the spring or early winter. Then the COP might be 2.2 vs 3. So it could be 15-30% cheaper to heat DHW during the day.

 

But the real issue is what do you pay for electricity. If you have free PV or cheap overnight electricity then that will make a way bigger difference than the small change in COP at different times of the day.

 

As an aside when I monitor our energy usage we use quite a bit more during winter because the water coming into the house to be heated is sitting closer to 0C whereas in the summer it is close to 10C. So you need roughly 1/3 more energy to heat it, increasing the temperature by 40C in winter and 30C in summer.

 

[SteamyTea]

  On 05/06/2023 at 18:05, AliG said:

As an aside when I monitor our energy usage we use quite a bit more during winter because the water coming into the house to be heated is sitting closer to 0C whereas in the summer it is close to 10C

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I find the same with my vented DHW. The loft F&E tank gets to over 20⁰C. 

[DanDee]

  On 05/06/2023 at 16:39, SteamyTea said:

283 [K] ÷ 293 [K] ×100 = 96.59%

 

10 [⁰C] ÷ 20 [⁰C] ×100 = 50%

 

This gives the size of the numerator, with respect to the denominator, expressed as a percentage.

 

The mean free path speed of the molecules, in an identical material, will be the same in both instances.

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With all due respect, why overcomplicate the things? My reference to the science was purely in the form of the general knowledge that a heat pump uses/needs more energy if the outdoor air is lower than when it's warmer. The easiest way to find an answer would be to look at some data sheet that shows the COP at different outside temperatures.

 

@JamesPaThis mention of 2% improvement with every degree difference as rule of thumb, in the context of the topic, it kinda applies both external temperature runnigs and LWT.

[Marvin]

Hi @Blooda

 

A question that raises questions!

 

My thoughts are:

 

Ignoring all other variables the smaller the difference between the outside air temperature and the water temperature exiting the ASHP the more efficently the ASHP runs.

 

However, if your real aim was to spend less when producing a suitable amout of hot water, then things become more fun!

 

Adding variables such as the heat loss of the tank over the peroid of time between the tank water being heated and used may make the result different.

 

As you seem to be able to measure every kWh used, you can try your ideas out and see what is best fror you.

 

By testing this way, for us, with our ASHP and hot water setup and water use, we found the afternoon was a better time to heat our water.

 

During the winter, using hourly air temp readings, we noted that the coldest part of a 24 day could be anytime and that extreme temperatures were greater with sunny days with cloudless nights.  

 

We now use a PV excess electricity diverter with ASHP as back up.  

 

I wait with bated breath for your questions on heating using an ASHP!

 

Good luck

 

Marvin

 

Edited June 6, 2023 by Marvin
Clarification, hopefully.

[Dave Jones]

what if you fixed a radiator in front of the heat pump and sent hot water from the heat pump into the radiatior so the heat pump was allways nice and warm.

 

This would heat up the air and give free heat permanently.

 

patent pending.

[Dave Jones]

v2.

 

build a box around the heat pump with 200mm celotex, fully air sealed.

 

Smaller rad inside this time.

 

Put  a couple of single room MVHR units into the box with heat recovery to provide air flow.

 

patent pending.

[JohnMo]

  On 06/06/2023 at 05:47, Dave Jones said:

what if you fixed a radiator in front of the heat pump and sent hot water from the heat pump into the radiatior so the heat pump was allways nice and warm.

 

This would heat up the air and give free heat permanently.

 

patent pending.

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Seen the idea on another forum also.

 

If the heat was coming from somewhere else other than the heat pump, that may work.  Such as from solar thermal.  If the heat, to heat radiator came from the heat pump, you would have to make more kW out of the heat pump to support the radiator.  So the result would be likely be more energy usage, not less.  You can't magic energy or efficiency from the same heat source.

 

Have been mulling the idea of a radiator heated by solar thermal, heating the air being pulled in by the ASHP fan.  Issues I have thought about, would need to run a small pump to circulate through solar panel and some control logic. Weather compensation would require temperature probe moving as it would be influenced by radiator heat.

 

Days where it would work best are, cold sunny winter days and if you tied in the logic to work with DHW heating all year.  But would need to be off when in cooling mode.

 

Have everything needed in the garage, may be worth rigging up if I have a day spare.

[Blooda]

Thanks for all the information. phew!  We don't have Solar and we are on a standard tariff so no difference in price.

Will set it to come on at 1:00pm.

We have limitations - the ASHP records at a resolution of 0.5kWh per hour, so COP calculations are all over the place, using less than 2 per day on DHW.  But the smart meter measures at ¹/₂ hour intervals to 0.01kWh. so should be able to see some change there.  

 

 

[SteamyTea]

  On 06/06/2023 at 07:38, JohnMo said:

You can't magic energy

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That's entropy man.

 

 

[AliG]

  On 06/06/2023 at 08:27, Blooda said:

 

Thanks for all the information. phew!  We don't have Solar and we are on a standard tariff so no difference in price.

Will set it to come on at 1:00pm.

 

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The best way to save money then is probably to move to a low priced overnight tariff.

 

I have moved my parents to Intelligent Octopus(they just got an EV). I then reprogrammed their DHW to heat overnight and set the UFH to slightly overheat the house at night (I set the thermostats to 23C at the end of  the cheap period then turn them down to 21-22 during the day). Thus the vast amount of ASHP running is on cheap rate. Depending on the rate you can get you will likely have more than half your electricity use on the lower rate. They use so much electricity overnight it’s not worth installing solar.

[Dave Jones]

solar for the summer and shoulder months would mean a much reduced consumption even over night. Free air con when the weather is like it is now is worth having!

[AliG]

Solar would reduce consumption, the problem is that overnight electricity is so cheap that the ROI on solar becomes poor depending on your usage.

 

Their consumption looks like the chart below since they moved to IO in March.

 

They use around 0.5kWh per hour outside IO hours.

 

They have room for a 2.7kW array which judging by my solar production would generate around 1500kWh a year. Slightly less than optimal as they have a flat roof.

 

Considering weather, spikes in usage, the fact that power is concentrated between 9 and 4pm looking at my system and so on, they wouldn't be able to self consume much more than 50% of the production, maybe even less. I get a reasonably good idea from the data on my system.

 

So say they utilise 750kWh of PV that is £225 a year at the July price cap. I have consistently argued that people should not make PV investment decisions based on recent inflated electricity prices. Wholesale prices continue to fall and if and when the Ukraine war ends they will likely fall considerably. Using say 20p per kWh, still well above pre war levels you get £150 a year of self consumption, maybe £200-250 including SEG payments. As they were quoted £5k for the array, it wasn't really worth the bother (Installers massively bumped up prices when electricity prices went up).

 

[JohnMo]

Just had a look on money saving expert and they did a writeup on the benefits or otherwise of E7. Unless you are using a min 40% of your all electric on cheap rate, you are loosing money by having E7.

 

https://www.moneysavingexpert.com/utilities/economy-7/