Most Efficient Use of ASHP

[J1mbo]

I have a Vaillant Arotherm Plus system heating through 17 radiators containing about 250 litres of water and dual-rate electric supply with a 3rd-party 'smart' control and weather compensator.

 

Q: Might it be cost effective (or at least cost neutral) to heat the building continuously? Obviously fabric heat loss would be higher, because the average temperature would be higher.

 

This would avoid heating the system from cold every day and enable a lower the flow temperature to be selected. Because it has a plate heat exchanger and buffer cylinder, the ASHP runs (significantly) hotter than the radiators when heating from cold. E.g. 50*C HP flow to get to 40*C radiator circuit flow - which levels out to a delta of maybe 2*C once up to temperature.

 

Will the higher building thermal loss be offset in terms of operating cost buy the lower flow temperatures and avoiding the cold start? As it would obviously be more comfortable to always have a warm house. Any experiences with similar?

[joth]

I think the key info is how often is the building occupied? If some one is in it 24/7 then yes heating constantly might be best. If someone only there every 5th day it would be very different

I know our own house (very poor insulation while we rebuild) heating is very different this winter Vs last as we now work from home rather than away from it for 10 hours every day

[SteamyTea]

Easiest way is to try it while take meter readings every day, and keep a record of the external temperature, sunshine/cloud cover and wind speed (you can get that from a local WeatherUnderground station)

 

You also need to know what the difference in price is, and when your dual price time windows happen.  Then calculate the amount of time you use cheap rate compared to day rate.

 

Use Alt 248 to get the ° rather than *

Edited December 3, 2020 by SteamyTea

[tonyshouse]

No, the total energy used is the area under the graph of the difference in temperature between indoors and outdoors approx = to total heat loss. The warmer they house the greater the heat loss the more energy consumed

 

it will be more comfortable 

 

I don’t have a heating system anymore but I advise different temperatures at different times of the day for those that do 

 

in terms of absolute efficiency heat pumps work best at lower output temperatures, exactly balancing over 24hhs may be more efficient but will also be more expensive/use more energy 

[joth]

Just now, tonyshouse said:

No, the total energy used is the area under the graph of the difference in temperature between indoors and outdoors approx = to total heat loss

ASHP confuse matter as the COP changes the faster you try and heat the space, so a little and often approach will work out more efficient than occasional big blast, for identical values of actual heat output

[SteamyTea]

1 minute ago, joth said:

ASHP confuse matter as the COP changes the faster you try and heat the space

Yes, unlike gas and oil burners that get better the closer they are to maximum rated output.

[J1mbo]

20 minutes ago, joth said:

ASHP confuse matter as the COP changes the faster you try and heat the space, so a little and often approach will work out more efficient than occasional big blast, for identical values of actual heat output

 

Yes this is my thinking... currently the system is slogging away at a COP of maybe 2.5 to heat the system... if I can save 10°C of flow so the COP goes up to like 3.5.

[J1mbo]

26 minutes ago, SteamyTea said:

Use Alt 248 to get the ° rather than *

 

Thanks, takes me back to the dot matrix days, where one could look up the 'special characters' in the printer manual!

Edited December 3, 2020 by J1mbo

[SteamyTea]

1 minute ago, J1mbo said:

special characters

I still use them so that I can swear on here.

[J1mbo]

So I did a 'fag packet' calc, based on observed performance so far. Continuous heating looks to increase thermal load by 35% but would increase cost by 20% if the COP improved by 0.3. Lots of assumptions here and doesn't account for sunshine...

 

[joe90]

Well I did no calculations at all ?, I basically copied Jeremy (two people, similar sized house, similar age), and run ours 24/7, although it very rarely comes on. Room stat set to 20’ and ASHP set to 48’ (only one water temp allowed with my controller unfortunately). The UFH is blended down to 24’. I have no idea what the COP is but I pay about a 1/4 for my electricity than a mate with a smaller bungalow and storage heaters (and his bungalow is colder!). I am thinking of economy 7 but would need to measure lecky/time to see if it would be beneficial.

[ProDave]

Mine is "on" all day from 6AM to 9PM.  It goes off at night for a silent night, though the only noise is a gentle low level hum from the water circulating pump that would only bother 1 other person i can think of.

 

The reality is after it has warmed up in the morning the room stats turn on and off just to maintain the lower level heat input to maintain the room temperature. e.g right now it is only the bathroom keeping it running just now all downstairs rooms have turned off.

[ash_scotland88]

3 minutes ago, ProDave said:

the only noise is a gentle low level hum from the water circulating pump that would only bother 1 other person i can think of.

???

 

I've noticed our gas boiler is quite noisey these days- cupboard off under the stairs in the hall, can hear it in the lounge through a brick wall.

And pretty sure I can hear the water moving around in the "master" bedroom too.

[ProDave]

7 minutes ago, ash_scotland88 said:

???

 

I've noticed our gas boiler is quite noisey these days- cupboard off under the stairs in the hall, can hear it in the lounge through a brick wall.

And pretty sure I can hear the water moving around in the "master" bedroom too.

Bleed the radiators, you probably have air in the system, you will need to top up the system pressure as you do that.

[SteamyTea]

15 minutes ago, ProDave said:

Mine is "on" all day from 6AM to 9PM.  It goes off at night for a silent night, though the only noise is a gentle low level hum from the water circulating pump that would only bother 1 other person i can think of.

I think there needs to be a distinction between being 'on' and 'running'.

You can have it on, but if no call for heat, then it is not running, but you cannot have it running and off.

Also, daytime, or more correctly daylight, hours are generally warmer than nightimes.  So that has to be factored in.

 

56 minutes ago, joe90 said:

I am thinking of economy 7 but would need to measure lecky/time to see if it would be beneficial.

How long you been saying that, just adjust the time for between 1AM and 7 AM (just had letter from EDF about changes to times, but not till March 2023), and see if you have enough for a day.  Bet you do.

[joe90]

10 minutes ago, SteamyTea said:

daylight, hours are generally warmer than nightimes. 

I did ask about whether an ASHP was more economical to run during the day because the air temp was generally warmer (more heat available in the air?) but was told the difference in night and day air temps did not affect efficiency. Was/is this correct?

 

13 minutes ago, SteamyTea said:

How long you been saying that,

Since I installed it ?

[J1mbo]

The COP does reduce as the temperature delta increases, but lowering the system low temperature generally provides greater benefit.

[SteamyTea]

Just now, joe90 said:

Since I installed it

Before that I seem to remember, you asked about it about 2011, over at the other place.

[joe90]

Just now, SteamyTea said:

Before that I seem to remember, you asked about it about 2011, over at the other place.

can’t rush into these things ?

[SteamyTea]

8 minutes ago, J1mbo said:

The COP does reduce as the temperature delta increases,

There are two temperature differences to take into account.

The difference between the outside air temperature (and the enthalpy) and to cold side of the ASHP, then between the return temperature and the HP output temperature on the hot side.

CoP is a rather crude method of combining the two as it only takes into account the low entropy electricity and the high entropy output.

 

Oh look, enthalpy and entropy in the same paragraph, I never remember which is which.

Edited December 3, 2020 by SteamyTea

[ProDave]

Yes accepted that "on" does not mean "running"  Also, like now, when it is just feeding the bathroom UFH the ASHP compressor does not start up very often.

 

I do time the DHW to not come on until late morning to ensure that is running when it is likely there will be good PV generation to help with maximum self use.

 

I have been put off off peak tariffs due to the much higher daytime rates and standing charges.

 

I am sure COP will be worse at night, here it was -4 last night and a hard frost.  It might just get up to 0 if the sun stays out today. But the issue is not so much a drop in COP but if you work the ASHP too hard when it's cold, icing may be an issue and it may need to defrost.  It is probably okay today as being so cold humidity will be low. That is more an issue when the temperature is stuck around 0 and it's snowing / sleeting.

[SteamyTea]

3 minutes ago, ProDave said:

and a hard frost

Works in an ASHPs favour, the humidity is already reduced.

 

Remember it is the mean of the temperature differences where maximum efficiency is delivered.

Edited December 3, 2020 by SteamyTea

[J1mbo]

10 minutes ago, SteamyTea said:

Works in an ASHPs favour, the humidity is already reduced.

 

Remember it is the mean of the temperature differences where maximum efficiency is delivered.

 

@SteamyTea, what would be the ideal temperature drop across the ASHP wet circuit? 

[joe90]

7 minutes ago, ProDave said:

I have been put off off peak tariffs due to the much higher daytime rates and standing charges.

I am with Octopus energy and the standing charges are the same fir both “normal” and economy7, normal rate is 16.09 and eco 7 is day rate 18.58 and night time 11.11. I would need to see usage charts with time of day separating ASHP from the rest to make an informed choice, not sure how I could do that??? @ProDave,s comment about possible lower COP at night might make a difference as well. I will probably think about it fir a few more years ???.

[SteamyTea]

15 minutes ago, J1mbo said:

what would be the ideal temperature drop across the ASHP wet circuit

Half way between what goes into the radiators/UFH/thermal store/UVC/Cylinder and what comes out.

This is why I am starting to think that buffer tanks should always be fitted to heat pump systems.

 

In the same vein, maximum power into the room will be the mean temperature between the slab/radiator and the desired temperature.

 

Note this is power, and not energy.