ASHP efficiency in hot weather

[ragg987]

I have a separate electric meter on the incoming supply to the heating system, and the ASHP reports output energy (which seems to be a calculation inside the firmware based on water flow and temperature delta, according to my read of the manual).

 

Some observations:

• in reality this is not COP, more a energy-out vs energy-in
• energy-in measures whole system - circulators, thermostats, valves as well as ASHP
• I noticed that the ASHP consumes about 50W even when everything is off
• it cannot measure instantaneous COP - only average over a period

Edited July 26, 2019 by ragg987

[Jeremy Harris]

OK, that makes sense.  I too have an energy meter on the ASHP power supply, that chucks out power, PF, frequency, voltage, current and accumulated energy, and that deals with the input side OK.  When off, our ASHP draws nothing at all that I can measure, it seems to completely shut down, although the controller draws about 1 W or so.

 

I found that most of the errors I was getting were with measuring the output, as it's not at all easy in practice, due to the lag in the response of temperature sensors.  The flow and return temperatures, together with the mass flow rate of water through the unit, need to be measured fairly accurately to get an indication of the COP, but even then I wasn't that confident that the results were accurate to better than maybe 10% or so.  Averaging over time doesn't work as well as I thought it would, as it takes ages for the heat to soak out of the pipes, so the output measurements are never that good.

[H F]

We have two meters that were installed near the boiler (for RHI). One's for electricity used for the heating, the other is the heating output. Is it as simple as dividing heat produced by electricity consumed? Our electricity used is 2184 Kw - heat produced 5216 kW, which works out to approx 2.3. If that's the case, that's not very good, is it, given it's only been in used since March (with a coldish May)?

[Jeremy Harris]

On 26/07/2019 at 17:56, Home Farm said:

We have two meters that were installed near the boiler (for RHI). One's for electricity used for the heating, the other is the heating output. Is it as simple as dividing heat produced by electricity consumed? Our electricity used is 2184 Kw - heat produced 5216 kW, which works out to approx 2.3. If that's the case, that's not very good, is it, given it's only been in used since March (with a coldish May)?

 

 

Yes, if you have RHI energy metering that is more or less the way that COP is measured.  There will be some errors, but it's probably near enough. 

 

The COP you're getting seems about right, perhaps a little low, given that I suspect your house isn't that energy efficient, so probably has a fairly high peak heating/hot water demand.  The key to getting a high COP, as mentioned earlier, is making sure that the ASHP isn't asked to produce a high temperature differential (between outside air temperature and water flow temperature).  Hot water provision is usually the main culprit for driving COP down, especially if it's set to a high temperature.  55°C is as high as is needed, and you can probably get away with setting it to 50°, or even a little lower, depending on how hot you like your showers.  I run our shower at 38°C, and have found that the thermostatic mixer valve needs about 45°C at the hot water input in order to work properly, so that's probably around the lower limit for DHW.

 

Do you know the flow temperatures that your ASHP is using for heating and hot water?

[H F]

I'll check later today about the flow temperatures.

 

Another related question - when the cold weather does come in, will it be more efficient to just leave the heating on all the time. We have UFH downstairs and rads upstairs. I'm guessing it'll be better to leave the UFH on at 21C, as opposed to heating when we're using the downstairs during the day, then turning off art night, and back on again in the morning. Why advice here?

[Jeremy Harris]

A lot depends on how quickly the house loses and gains heat. 

 

If your house is well insulated, with a long decrement delay, is pretty airtight and has MVHR, then the chances are that it will have a thermal time constant that exceeds the diurnal period and so will stay warm with just a bit of heat injected once a day.  Ours is like this, and in winter we have the UFH set to come on overnight (on the E7 rate) and that works well.  Much of the time the heating only comes on every second or third day, and it's only when it's really cold that it comes on every night.

 

On the other hand, if your house tends to cool down fairly quickly when the sun goes down, then you will need to heat it more often, and as UFH tends to respond a fair bit more slowly than radiators, you may find that the heating needs to be on all the time.

 

In terms of economy, then having the heating on all the time always costs more, especially with UFH which is inherently less efficient than radiators (because there is always some heat loss down to the underlying ground).  

 

Best bet will be to see how the house responds.  If you can "charge" the floor up with heat overnight on the off-peak rate that probably helps, but if that tends to make the house too warm first thing in the morning, and you're not in during the day, then any cost saving may be wasted.

[H F]

I'm getting a bit apprehensive about the efficiency  - I really hope it costs us a lot less to run than the old oil boiler we had.

[Jeremy Harris]

17 minutes ago, Home Farm said:

I'm getting a bit apprehensive about the efficiency  - I really hope it costs us a lot less to run than the old oil boiler we had.

 

Our house isn't typical, as the heating requirement is very low, but in another thread I did work out the relative cost of heating it for a year, using different fuels:

 

Quote

I recently did a running cost comparison, for a typical year for our underfloor heating with different fuels and came up with these figures, which may be of interest (costs include standing charge/LPG tank rental, etc) :

 

LPG fired boiler running UFH = £336.49

 

Peak rate electric boiler running UFH = £329.05

 

Oil fired boiler running UFH = £218.91

 

Off peak (E7) electric boiler running UFH = £206.43

 

Mains gas boiler running UFH = £143.76

 

ASHP at peak peak rate running UFH = £102.87

 

ASHP at off-peak (E7) rate running UFH = £89.25

 

Our ASHP tends to run with a fairly high COP, though, as it never runs at a flow temperature above 40°C, so much of the time the ∆_T between the outside air temperature and the flow temperature was around 30°C or so.

[H F]

Are those figures annual?!

 

Not wanting to pry, but why are your heating requirements low? Houses in the uk require loads of heating.

[ProDave]

2 minutes ago, Home Farm said:

Are those figures annual?!

 

Not wanting to pry, but why are your heating requirements low? Houses in the uk require loads of heating.

Not if you build them properly.

 

My house in the Highlands , not as well insulated as @JSHarris cost £234 to heat last winter with an air source heat pump.

[H F]

2 minutes ago, ProDave said:

Not if you build them properly.

 

My house in the Highlands , not as well insulated as @JSHarris cost £234 to heat last winter with an air source heat pump.

Wow.  We’re in a renovated Victorian place, which is relatively well insulated... would love to pay a lot less for heating. Oil last year winter was a financial killer.

[ProDave]

2 minutes ago, Home Farm said:

Wow.  We’re in a renovated Victorian place, which is relatively well insulated... would love to pay a lot less for heating. Oil last year winter was a financial killer.

There are two things needed to get a house to be low energy.  Insulation, and air tightness.

 

Insulation is relatively easy and even in an older building can be added to some extent.

 

Air tightness is quite easy to get right in a new build, it is mostly a matter of attention to detail, rather than anything particularly expensive.  That is very much harder to sort out in an old existing house.

 

Once you get good insulation and good air tightness then you fit an MVHR system (mechanical ventilation with heat recovery) and that ventilates the house while removing most of the (otherwise wasted) heat in the exhaust air and puts that back into warming the fresh air going in.

 

Then you can get very low heating bills.

 

And it needn't add much to the overall cost of a new build.

 

Unfortunately the mass market house builders have not caught up and in general they just build what they can scrape through building regs.  It is only a minority of self builders that strive to build something better.  Ours meets many of the parameters for it being a passive house but we did not even bother trying to get it certified as such.  The low running cost is all that matters to me.

[Jeremy Harris]

36 minutes ago, Home Farm said:

Are those figures annual?!

 

Not wanting to pry, but why are your heating requirements low? Houses in the uk require loads of heating.

 

Yes, the heating figure is annual.

 

The house was designed to only need a tiny amount of heating, it's a passive house in principle, although it looks pretty traditional in most respects.   Over the course of a year we generate more energy from the solar panels on the roof than the house needs, so we don't actually pay anything for energy, as the revenue from electricity exports plus the feed in tariff payments significantly exceeds our annual electricity bill.  We don't have gas, oil, solid fuel or whatever, the house is wholly electric. 

 

I'm far from being alone, many others here have similar very low energy homes.  The problem is that the mass house builders in the UK tend not to build well-insulated, airtight and low energy homes, so it's mainly self-builders, like those on this forum, who are leading the way.

[H F]

JS, I was thinking of doing a review on our ASHP - other than COP and the flow rate, what other things do you think I should mention/cover?

[SteamyTea]

8 minutes ago, Home Farm said:

was thinking of doing a review on our ASHP

Noise. You should be able to get a dB meter on your phone for this. Need to see how noisy it is at full chat as well. Worth looking at the MCS guidence.

Then there is start up current. There is a bit of a myth that they still draw a lot.

Rather than flow rate, worth looking at temperatures. Compare current and noise at different output temperatures.

Then there is general sturdiness, can kids lean bikes in it without a brake lever hitting the collector coils.

Size and mass are useful too.

Easy of setting up would be very helpful. Are the default setting good, do you need to buy a command unit separately, can you access data via a laptop or phone.

Going to be quite a challenge to do a meaningful report on one. Good luck.

[jack]

Simplicity of controls would be a big one. @ProDave, for example, has found his controls to be really user unfriendly. Ours are pretty poor too.

 

Some brands have more expensive controllers available, which are allegedly easier to use. You could look at the pricing of those and report on what additional functionality they provide.

 

Some recent ASHPs can be controlled via an app. If yours is one of those, that'd be worth reporting on.

 

[H F]

Fab ideas guys. Awesome! 

 

Our controls are not easy too.

[Ferdinand]

On 30/07/2019 at 21:39, Home Farm said:

Wow.  We’re in a renovated Victorian place, which is relatively well insulated... would love to pay a lot less for heating. Oil last year winter was a financial killer.

 

At some stage it would be interesting to hear details of your renovation standards eg by comparison with the Building Regs requirements, and the rest of the story.

 

I live in a converted 1940s bungalow - converted about 2007-8 with an extra storey by the previous owner, and our bills for 200 sqm run at about £1000 a year for 2 of us - for all energy heat light and water, before the money coming back from the solar. Solar we generate about 5-6 MWh per year.

 

My day job is partly as a landlord, and I still find people offering me tarted up properties with minimal work to the underlying fabric. I also see newbuilds at Building Regs Minimum standard described as ‘meet the latest eco’ standards (which is a lie in such a case). 

 

Ferdinand

Edited August 3, 2019 by Ferdinand

[H F]

On 03/08/2019 at 17:40, Ferdinand said:

Solar we generate about 5-6 MWh per year.

 

How big is your array?

[puntloos]

Can I interject with one small question - people seem to speak about ASHP without ever mentioning brands etc.

Are there any particular ones that are "clearly the best" according to certain metrics?

 

Obviously I want the quietest, powerfullest, cheapest..... (I know, I know.. pick 2..)

[joth]

2 hours ago, puntloos said:

Can I interject with one small question - people seem to speak about ASHP without ever mentioning brands etc.

Are there any particular ones that are "clearly the best" according to certain metrics?

 

Obviously I want the quietest, powerfullest, cheapest..... (I know, I know.. pick 2..)

 

Try this thread 

 

There's a few others on specific makes in the ASHP topic too

 

https://forum.buildhub.org.uk/forum/119-air-source-heat-pumps-ashp/

[Ferdinand]

8 hours ago, Home Farm said:

 

How big is your array?

 

9.98kWp.

 

E and W facing, Shaded, Solaredge.

Edited August 9, 2019 by Ferdinand

[SteamyTea]

1 hour ago, Ferdinand said:

Shaded

How badly shaded is it?

It only takes a small bit of shading to have a huge impact on performance.

Does your inverter/inverters have the ability to log long term performance data, text files are best as that can be easily manipulated in a spreadsheet.

[Ed Davies]

13 minutes ago, SteamyTea said:

It only takes a small bit of shading to have a huge impact on performance.

 

Umm...

 

1 hour ago, Ferdinand said:

Solaredge

 

[SteamyTea]

27 minutes ago, Ed Davies said:

Umm...

Even with optimisers you still get a hit that is out of proportion to the optimal.

I think we have done this conversation before.