ASHP Electricity Usage

[SBMS]

Hi all

 

Wondered if those members that had installed ASHP would be interested, if they have the data, in posting their monthly kWh usage for their systems, for the past 12 months, along with construction type, Insulation levels, and sqm of property? I'm trying to do some analysis on the mean and median usage of pumps across different build types to assess the impact of a solar/battery installation and get some general data on real-world consumption?  I completely understand that the numbers will vary greatly based on consumption, usage patterns, type of build etc but I am going to try and normalise those out to try and look at real world usage.

 

If members are interested am happy to post back the analysis for any other self builders interested.

 

Construction Type
Insulation (walls/floor)
MVHR?
Total sqm
Achieved Airtightness
Month	kWh Consumption
January
February
March
April
May
June
July
August
September
October
November
December

[ProDave]

I don't have the patience to do the sums to convert to monthly, I log my usage weekly.

 

But it is easy to do a whole year, so last 12 months ASHP has consumed 1015kWh heating DHW and 1480kWh heating the house.

 

Well insulated to near passive house levels, good air tightness with MVHR, triple glazed windows, EPC A94 in the highlands to colder climate than most.

[TerryE]

@SBMS, this is really a "how long is a piece of string" type of survey.  I don't think that it's going to help you very much at all. Why? 

 

The heat demand is nothing to do with the heating technology and everything to do with the thermal design and actual performance of your build. So you need to start there, with the trade-offs between cost, design and construction risk and performance.  We have a very airtight, passive-class build with decent MVHR; just UFH in the floor slab, and no other heating for the two upper floors, apart from a couple of small oil filled electrical rads that my HA system turns on for a few hours each night in the worst 3 winter months.  I don't even have an ASHP because I can't make a payback case based on my (electric resistance heating for my water UFH loop) energy bills.

 

Next if you use electric heating for your DHW (SunAmps, cold-fill DW and WM), then you can get away will running your ASHP at something like 30°C output temp and you should get a CoP of 4-5.  If you use conventional rads then you will need an output temp nearer 60°C and then you'd be luck to get a CoP of 2½.  Just because Dave used 1 mW electricity doesn't mean you will achieve something similar 2× better or 4× worse.

 

My suggestion about not bothering about ASHP to heat your water has been covered on other topics.  If you heat your water using E7 rates and use some highly insulated PCM-based system such as SumAmp and a HEP20 manifold system, than parasitic the heat losses are so minimal that you really don't spend that much heating your water using electric resistance elements -- certainly not enough to merit investment in more complex DHW solutions.

 

Edited March 11, 2022 by TerryE

[SteamyTea]

Many years ago, some of us, over at the other place, logged our weekly energy usage, along with some basic details of our houses.

All it really showed was that some houses use less energy than others.

[SBMS]

3 minutes ago, TerryE said:

@SBMS, this is really a "how log is a piece of string" type of survey.  I don't think that it's going to help you very much at all. Why? 

 

The heat demand is nothing to do with the heating technology and everything to do with the thermal design and actual performance of your build. So you need to start there, with the trade-offs between cost, design and construction risk and performance.  We have a very airtight, passive-class build with decent MVHR; just UFH in the floor slab, and no other heating for the two upper floors, apart from a couple of small oil filled electrical rads that my HA system turns on for a few hours each night in the worst 3 winter months.  I don't even have an ASHP because I can't make a payback case based on my (electric resistance heating for my water UFH loop) energy bills.

 

Next if you use electric heating for your DHW (SunAmps, cold-fill DW and WM), then you can get away will running your ASHP at something like 30°C output temp and you should get a CoP of 4-5.  If you use conventional rads then you will need an output temp nearer 60°C and then you'd be luck to get a CoP of 2½.  Just because Dave used 1 mW electricity doesn't mean you will achieve something similar 2× better or 4× worse.

 

My suggestion about not bothering about ASHP to heat your water has been covered on other topics.  If you heat your water using E7 rates and use some highly insulated PCM-based system such as SumAmp and a HEP20 manifold system, than parasitic the heat losses are so minimal that you really don't spend that much heating your water using electric resistance elements -- certainly not enough to merit investment in more complex DHW solutions.

 

All useful stuff but reason for asking is no good datasets for average ASHP usage exists that I can find. They exist for LPG, oil, mains gas, storage heaters etc but none that I can find for ASHP. Just as each of these could be deemed irrelevant it’s often actually very useful. Of course there are anomalies and I doubt I can get a large enough dataset on here to get anything hugely useful but some data is better than no data - especially for those trying to make informed data-driven decisions about what to invest in. That, and I just like data. 

[TerryE]

Just now, SBMS said:

but none that I can find for ASHP

 

That's for the reasons that I discussed. The Q just isn't meaningful as phrased.  Very east to ask, but you are expecting a lot of time and effort from other members, and any answers taken out of context will be useless. 

 

Your design / construction and heating approach can impact your ASHP bills by a range of 5-8x.  It's a bit like asking: what is the average MPG for a vehicle; well it all depends on the type of vehicle: bike, motor bike, eBike, Tesla, conventional compact ICE, Diesel, gas guzzler, ...

 

IMO, you really need to do your own research instead of expecting members to give you lots of detail on meaning of life Qs.

[SBMS]

4 minutes ago, TerryE said:

 

That's for the reasons that I discussed. The Q just isn't meaningful as phrased.  Very east to ask, but you are expecting a lot of time and effort from other members, and any answers taken out of context will be useless. 

 

Your design / construction and heating approach can impact your ASHP bills by a range of 5-8x.  It's a bit like asking: what is the average MPG for a vehicle; well it all depends on the type of vehicle: bike, motor bike, eBike, Tesla, conventional compact ICE, Diesel, gas guzzler, ...

 

IMO, you really need to do your own research instead of expecting members to give you lots of detail on meaning of life Qs.

I think you’re pre supposing I have a conclusion - I don’t! I was simply asking (not ‘expecting’) for any insight others might like to share. I suspect there is value and correlation in the data but who knows? Who knows what it might illustrate? You might be 100% right and it shows nothing.

[SteamyTea]

A negative result is a result, but you do have to collect suitable data.

[Iceverge]

Given that buildings heat demand varies so greatly it would be perhaps a little meaningless to draw and any specifics about an ASHP ( or any other CH system) even if you had a good return of surveys. 

 

Perhaps the question you might have hoped to ask was what was the real world COP of ASHP with regard to space heating and DHW. 

 

Perhaps the variables could be .

 

1. Annual, monthly, daily building heat demand + DHW.

2. Annual, monthly, daily breakdown of an ASHPs acutal kWh usage. 

3. Annual monthly, daily breakdown of external temperatue + humidiy. 

4. Flow temp of central heating and Temp of DHW. 

5. Size of ASHP vs peak heat demand. 

 

 

This data would be very handy to arrive at some optimum design and setup's of an ASHP. Unfortunatly the level of survilance required would be impractical without a huge sample size considering the variations in heating and DHW and heating preferences , even within similar houses.

 

 

In general however for the best COP...............(corrections welcome of course)

 

1.Size the ASHP suitably so that its not too large and constantly cycles on and off. ( a buffer may help)

2.Is big enought that it can run without defrosting
3.Couple it to as low temp a CH demand system as possible (UFH in a thick slab)

4.Heat a large volume of DHW to as low a temp as possible. ( better than less water to a high temp)

5. Site the outdoor unit in an area where it will not struggle for airflow.

6. Use sensible large diameter pipes excellently insulated with as short a distance as possible to bring heat inside. 

 

 

It would be nice to have real world data to help optimise for purchase cost vs performance rater than rules of thumb I agree. 

 

 

 

 

 

 

 

[Marvin]

Hi @SBMS

 

Sorry to say I've gone down this road before. The amount of variables are so many that extrapolating any information from combined results will not give you anything except the range of results, not the reason for the results.

 

There are endless reasons for the same results from different setups and endless reasons for different results from the same setups. 

 

Probably 20 or 30 basic reasons.

 

 

[Marvin]

Hi @SBMS

 

However the opposite way would be interesting:

 

Given all the details one could calculate what the energy use should be, however there could be 40 questions to answer.

Edited March 11, 2022 by Marvin

[J1mbo]

Orientation, glazed area, occupants, background load, how much you cook, how much washing is done, how many teenagers you have that leave windows and doors open continually. It’s very difficult to isolate the space heating requirement from everything else IMHO

 

FWIW. Arotherm plus 12kW heating 1970s house, 200m2 or so, consumes 5MWh pa for space heating.

Edited March 15, 2022 by J1mbo

[SteamyTea]

2 minutes ago, J1mbo said:

It’s very difficult to isolate the space heating requirement from everything else IMHO

If you plot energy usage against heating degree days, that gives you a fair estimate.

[J1mbo]

Of your own use, yes, but a neighbour with the exact same house could have very different numbers.

[SteamyTea]

2 minutes ago, J1mbo said:

Of your own use, yes, but a neighbour with the exact same house could have very different numbers.

Yes.  But it can be standardised to a set temperature i.e. 15.5°C or 12°C.

What you will get is a distribution, that can be useful.

[Marvin]

Ha! Ha! Ha! Ha! People please! what about the other 100 reasons for different results!

 

Some of the biggest are the installation details, right from the thickness of the external insulation to how long the pipes between the ASHP and the Buffer Tank and or hot water tank which is either direct or gravity, possibly different sizes being additionally heated with either a 

mains or solar excess power system. How insulated the tanks are.  Are the tanks in side the thermal envelope or outside. 

 

And what about solar gain? How much glass what orientation.

Location location location.  Here we get about 180 hours more sunlight a year than London. 1 hour every other day on average.

Cat flap? Letterbox?

Smokers?

Cooking habits?

Tumble dryer?

Elevation?

Is the building in shadow?

Typical wind direction?

In a valley?

Wind funnelling caused by other building positions?

What about what temperature the inhabitants prefer?

Wood burner?

MVHR? or other mechanical ventilation?  What rate of air change?  and on boost? and how long on boost?

Internet?

TV's running

All electrical items inside the thermal envelope (I think they almost all or all produce heat)

Internal gym?

Internal swimming pool?

The way the ASHP is run: Constant same temperature 24 hours a day? On and off different times? Weather compensation?

Batteries being charged produce heat. From the PV?

UFH? Radiators? Water coils?

 

And as for the airtightness and Insulation well..

 

Structural elements like steels altering the average thermal resistance.

You end up with a scatter graph and 5 dots near each other that can be for 100 different reasons.

Glazing

 

We have thermal blinds and curtains and these definitely make a thermal difference.

 

The efficiency of the ASHP?

 

The type of buffer tank with or with out coil? size of the tank.

 

Yes lots of these are little details, but add those small differences together and...

 

Here I have changed a little detail (that I can) one at a time to come up with what works best here with what we have.

 

Good luck!

 

M

[SBMS]

14 hours ago, Marvin said:

Internet?

Thanks Marvin for the reply - my original question was purely looking at consumption of the ASHP in relation to space heating and DHW demand, not total household electricity consumption, so not sure why the internet or TVs would have any bearing on ASHP electricity consumption? MIght have misunderstood though.

[Marvin]

5 minutes ago, SBMS said:

Thanks Marvin for the reply - my original question was purely looking at consumption of the ASHP in relation to space heating and DHW demand, not total household electricity consumption, so not sure why the internet or TVs would have any bearing on ASHP electricity consumption? MIght have misunderstood though.

Hi @SBMS

 

Yes it took me a while to work this one out. Almost everything that uses power creates heat. Put you hand on your WIFI box for instance mine reads about 40 centigrade. Touch the back of the freezer when cooling. I have a temp gun which tells me these things. And as for cooking almost all energy is used for heating and where does the heat go? If you can't stand the heat stay out of the kitchen....

 

The point is, generally the more power used in the home the more heat supplied other than by the ASHP. 

 

Another one is the ratio of people to the house:  2 people here in 100m2. Some 2 people to 300m2. heat from 2 people about 150Watts or 200 when discussing politics or religion. so could be as much as 3.6kWh heat per day. 

 

Good luck

 

Marvin

[SBMS]

4 minutes ago, Marvin said:

Hi @SBMS

 

Yes it took me a while to work this one out. Almost everything that uses power creates heat. Put you hand on your WIFI box for instance mine reads about 40 centigrade. Touch the back of the freezer when cooling. I have a temp gun which tells me these things. And as for cooking almost all energy is used for heating and where does the heat go? If you can't stand the heat stay out of the kitchen....

 

The point is, generally the more power used in the home the more heat supplied other than by the ASHP. 

 

Another one is the ratio of people to the house:  2 people here in 100m2. Some 2 people to 300m2. heat from 2 people about 150Watts or 200 when discussing politics or religion. so could be as much as 3.6kWh heat per day. 

 

Good luck

 

Marvin

I see, makes sense.  I guess on a cold evening if the ASHP is struggling I can get the kids to gather around the Wifi router!!  I read this (https://link.springer.com/article/10.1007/s12053-019-09791-1) and think the standard deviation of consumption was so high that maybe my original question was a bit of a fool's errand - especially with a data set that would be pretty small (this forum).

[Marvin]

Hi @SBMS

 

Another challenge is splitting the hot water and heating (and in our case cooling). There are several factors regarding trying to calculate the splits.

1 minute ago, SBMS said:

I see, makes sense.  I guess on a cold evening if the ASHP is struggling I can get the kids to gather around the Wifi router!!  I read this (https://link.springer.com/article/10.1007/s12053-019-09791-1) and think the standard deviation of consumption was so high that maybe my original question was a bit of a fool's errand - especially with a data set that would be pretty small (this forum).

Not as foolish as some of my questions!  But one day we will ask the best question ever. We just have to keep asking. 

[ProDave]

4 minutes ago, Marvin said:

Hi @SBMS

 

Another challenge is splitting the hot water and heating (and in our case cooling). There are several factors regarding trying to calculate the splits.

Not as foolish as some of my questions!  But one day we will ask the best question ever. We just have to keep asking. 

I solved that.

 

I bought a second hand old school dual rate electricity meter.  The power feed to the ASHP passes through this meter, and the pilot wire (that switches the meter from day to night rate) is switched by the feed to the hot water motorised valve.

 

So one dial on the meter measures it's usage in heating mode, and the other dial measures it's usage in DHW mode.

 

Adding cooling mode to that would be tricky.......

[SteamyTea]

4 minutes ago, ProDave said:

Adding cooling mode to that would be tricky.......

Just get a third meter that switches on when cooling.

[Marvin]

Hi @ProDave

 

Yes that's the direct input and as @SteamyTea says add one for the cooling, however there are quite a few things missing...

 

1. During the night the heat from the hot water tank dissipates from the tank to heat the room. ( In our case we super insulated the tank to reduce this)  
2. If the hot water tank is very hot (in our case due to solar excess power being used by the immersion) the temperature dissipates into the central heating water. 
3. When changing the ASHP from hot water to heating, the buffer tank can jump up by 10C when the hotter water remaining in the main 28mm pipes and ASHP flows in to the buffer.

I can feel the results becoming less and less reliable....

Edited March 16, 2022 by Marvin
spelling

[George]

I'll share what I can but I have to agree with the others, there are way too many vairables to get a decent answer. However I am also looking at battery install (already have PV) so have been trying to work it out for my house. 

 

 

Construction Type	Solid wall 90%, cavity wall 10%
Insulation (walls/floor)	60mm woodfibre 75%, 100mm PIR 25%, minimal in floor, 300mm rockwool in roof
MVHR?	Yes
Total sqm	172
Achieved Airtightness	?

 

 

Like others, I haven't kept a running total and I don't trust the onboard energy meter (actually that unfair, the consumed one I trust, but not the generated one).

 

Average annual kWh used by the entire ASHP system - 4872kWh.

 

In winter I used about 18kWh/day, currently about 3kWh/day (have two small children who like to bath then shower!), in summer it'd be less than that. 

 

For a worse case, add an additional 10kWh/day due to other heat sources in the house for fairness. 

 

My conclusion was, if I could get on a car charging tariff, (without an EV) I'd make 100% daily use of a battery so the pay back time will be minimised. It's one of those 'if you don't use it, then it's a waste' scenarios. It's people who don't use much electricity when a battery is a marginal decision.