DHW Heat Loss

[AdyHoggs]

Hi again, I’m currently having an ASHP installed with a new hot water cylinder which I plan to heat to 50c, the heat loss is 1.22 kW/24hours, so was wondering what the temperature would be after 12 hours. I’d also like to know what the temperature of the water would be with my old cylinder after 12 hours if the heat loss was 3.16kW/24hours,  thanks for any help, Ady

[J1mbo]

You need to know the capacity of both.

[ProDave]

Well the new cylinder will be losing heat at about 1/3 the rate of the old one.  So temperature drop of the new one will be 1/3 that of the old one (if they are similar size)

 

It's a bit of an odd question.  I could understand if you were trying to heat the cylinder on say an overnight off peak cheap rate and hoping it would store enough heat to not need heating again until the next night, but assuming you are happy to use the ASHP at any time it becomes a non issue.

[AdyHoggs]

Hi, They are both 200 litres, and I am using economy 7, just working out my best options 

[ProDave]

 Given that the DHW is a lower temperature from an ASHP that you will be used to heating it with an immersion heater, I would use at least a 300L tank if you want to stand any chance of a tankfull heated overnight will last all day.

[AdyHoggs]

Tbf I was just looking for an answer from my question about heat loss temperatures, so I can work out all my options 

[SteamyTea]

[J1mbo]

The 1.22kW in 24 hour cylinder will lose less than 3K in 12 hours.

 

The issue with ASHP in my experience is more related to the DHW priority, which can leave extended periods of time with no heating in the mornings when everyone is using the shower. I’ve solved this personally by heating the cylinder via the “legionnaires” programme on E7 overnight - to 70+ degrees - then leaving DHW “off” until lunch when the ASHP is asked to reheat it to 48 degrees. Using the Bulb 5p overnight rate would make this even better but even with normal E7 the reduction in COP works out neutral cost wise and ensures I have heating in the morning. 

Edited November 10, 2021 by J1mbo

[Nickfromwales]

12 hours ago, J1mbo said:

The 1.22kW in 24 hour cylinder will lose less than 3K in 12 hours.

 

The issue with ASHP in my experience is more related to the DHW priority, which can leave extended periods of time with no heating in the mornings when everyone is using the shower. I’ve solved this personally by heating the cylinder via the “legionnaires” programme on E7 overnight - to 70+ degrees - then leaving DHW “off” until lunch when the ASHP is asked to reheat it to 48 degrees. Using the Bulb 5p overnight rate would make this even better but even with normal E7 the reduction in COP works out neutral cost wise and ensures I have heating in the morning. 

That strategy also extends the longevity of the HP, which is a strategy I adopt with my new clients, and also helps avoid any possible ‘freezing’ of the unit in the winter / adverse months.

Off peak rates ( 5p/kwh ) can now be utilised quite advantageously and using that plus PV during summer sees the ASHP having a very easy ( therefore long ) life. 
Ive just done a couple of ASHP’s ( PV + Octopus GO! ) where we’ve not bothered with the wet circuit to the UVC at all. Helps a huge amount as both have cooling in the summer and I did not want the ASHP doing 180 degree turnarounds from cooling to heating for no ‘good’ reason. 

[SteamyTea]

14 hours ago, J1mbo said:

Using the Bulb 5p overnight rate would make this even better but even with normal E7 the reduction in COP works out neutral cost wise and ensures I have heating in the morning. 

My E7 night rate is 12p+VAT now (I think).

Your strategy does nothing to reduce CO_2e emissions.

[J1mbo]

2 minutes ago, SteamyTea said:

My E7 night rate is 12p+VAT now (I think).

Your strategy does nothing to reduce CO_2e emissions.

 

I don't know that's necessarily true. The overnight CO2e for UK electricity is generally lowest between 2am and 4am (real-time data here: Carbon Intensity), obviously heating the water higher does have an impact on COP and I'm currently getting about 2.5 for DHW in this configuration.

[SteamyTea]

23 minutes ago, J1mbo said:

 

I don't know that's necessarily true. The overnight CO2e for UK electricity is generally lowest between 2am and 4am (real-time data here: Carbon Intensity), obviously heating the water higher does have an impact on COP and I'm currently getting about 2.5 for DHW in this configuration.

With a CoP of 2.5, you can reduce that low, overnight intensity, even more.

There is also the marginal increase. People forget that if a higher emitting power plant needs to be turned on, while the average intensity seems to be unaffected, the energy unit intensity vastly increased. This is why we are installing RE generation. Turning on unused RE does not increase the marginal tonnage emitted.

[epsilonGreedy]

2 hours ago, Nickfromwales said:

That strategy also extends the longevity of the HP, which is a strategy I adopt with my new clients, and also helps avoid any possible ‘freezing’ of the unit in the winter / adverse months.

Off peak rates ( 5p/kwh ) can now be utilised quite advantageously and using that plus PV during summer sees the ASHP having a very easy ( therefore long ) life. 
Ive just done a couple of ASHP’s ( PV + Octopus GO! ) where we’ve not bothered with the wet circuit to the UVC at all. Helps a huge amount as both have cooling in the summer and I did not want the ASHP doing 180 degree turnarounds from cooling to heating for no ‘good’ reason. 

 

 

A very interesting post, I might follow this advice.

 

Re. the freezing, is this claim based on the fact that the most likely scenario for a freezing ASHP is early morning when the outside temp is lower and the ASHP is working hard to reheat the hot water cylinder?

[SteamyTea]

7 minutes ago, epsilonGreedy said:

 

Re. the freezing, is this claim based on the fact that the most likely scenario for a freezing ASHP is early morning when the outside temp is lower and the ASHP is working hard to reheat the hot water cylinder?

Not always.

You have to take into account the RH.

The important temperature is 277K, not 0°C.

Then you have to take into account how long the ASHP is on. The longer it is running for, the greater the chance it will get some frosting.

 

We still have a mentality that says 'turn the heating off at night, and heat up the water just before it is needed'.

This is not the best way to run an ASHP system.

[Marvin]

7 minutes ago, SteamyTea said:

We still have a mentality that says 'turn the heating off at night, and heat up the water just before it is needed'.

This is not the best way to run an ASHP system.

 

Yes ASHPs are different and this is one thing I have had to consciously change in behaviour:

 

If the hot water needs heating then heated at the warmest time of the day. 

House heating and occasionally cooling on 24 hours at low level.

 

Not like my old gas central heating at all!

 

[SteamyTea]

7 minutes ago, Marvin said:

 

Not like my old gas central heating at all!

Old has heating was like my first turbocharged car and my old Kawasaki KH500.

They did nothing for ages, then delivered full power, then stopped.

[epsilonGreedy]

12 minutes ago, SteamyTea said:

We still have a mentality that says 'turn the heating off at night, and heat up the water just before it is needed'.

This is not the best way to run an ASHP system.

 

 

That's true but I also imagine that the "off at night mentality" can be attributed to experiences of living in an EPC band C or worse home where the overnight loss of heat is appreciable with associated bills.

 

For example in my future house if I achieve a so-so band B EPC score and elect to turn the ASHP off between 9pm and 5am then I assume the ground floor UFH slab will drop significantly particularly if it is a thin-crust anhydrate pour of say 50mm.  The ASHP will then work hard between 5am and 10am to restore the temp of the slab, in this case extra DHW demand could tip the ASHP into frosting.

[ProDave]

Re DHW heating times.

 

We usually shower in the evening, not the morning (I can't understand why you would want to go to bed in need of a shower and not have it until the morning)  So there is usually not much demand for HW in the morning, so there is plenty left in the tank for that from the day before.  I have the ASHP set to turn on DHW heating at 11AM, and the logic behind that is by then the solar PV should be generating reasonably well so is a good way to self use a reasonable chunk of PV power.

 

So for the 6AM space heating start up, the ASHP is only heating the UFH and nothing else.

[SteamyTea]

3 minutes ago, epsilonGreedy said:

 

That's true but I also imagine that the "off at night mentality" can be attributed to experiences of living in an EPC band C or worse home where the overnight loss of heat is appreciable with associated bills.

 

For example in my future house if I achieve a so-so band B EPC score and elect to turn the ASHP off between 9pm and 5am then I assume the ground floor UFH slab will drop significantly particularly if it is a thin-crust anhydrate pour of say 50mm.  The ASHP will then work hard between 5am and 10am to restore the temp of the slab, in this case extra DHW demand could tip the ASHP into frosting.

Then design the floor correctly.

You can have the ASHP come in earlier, but heat the slab to a lower temperature, say 2 K below normal operational temperature.

If you insulate the DHW cylinder, then even at a relatively low temperature of 48°C, you should only have a drop of 1 or 2 K over 48 hours.

This does depend how much you draw form it. If you draw HW until it is too cold to bath in, the fit a larger system.

[SteamyTea]

4 minutes ago, ProDave said:

I can't understand why you would want to go to bed in need of a shower and not have it until the morning

I work in an industry that needs cleanliness.

If I see an unshaven chef, I usually don't eat there.

You can bath twice a day.

Edited November 11, 2021 by SteamyTea

[epsilonGreedy]

2 hours ago, ProDave said:

So for the 6AM space heating start up, the ASHP is only heating the UFH and nothing else.

 

 

I thought those blessed with Passiv house type thermal performance kept their ASHP ticking 24x7 and the UFH slab at a constant temp? What overnight temperature drop do you experience up to 6am in the winter? 

[epsilonGreedy]

2 hours ago, SteamyTea said:

Then design the floor correctly.

 

 

What design remedies are available for my scenario as outlined above?

[SteamyTea]

1 minute ago, epsilonGreedy said:

 

What design remedies are available for my scenario as outlined above?

More under slab insulation and decent perimeter insulation.

Don't you ever read what is said on here.

[epsilonGreedy]

1 minute ago, SteamyTea said:

More under slab insulation and decent perimeter insulation.

Don't you ever read what is said on here.

 

 

That won't achieve much if a mid EPC B home looses 4 degrees of heat overall overnight and so the UFH slab leaches energy into the air above it.

 

I will have 170mm of PIR below the screed or maybe a bit less PIR and 20mm of something more squashy as a base lay to even out the bumps.

 

Is 25mm of perimeter insulation enough for you?

 

 

[SteamyTea]

Just now, epsilonGreedy said:

Is 25mm of perimeter insulation enough for you?

Would it be enough in your loft?