Working out ASHP min COP to beat lpg

[johny_99]

Hi

 

I realise this might not be the best forum for this type of question but I'm not sure where to ask it.

 

I'm trying to work out if an ASHP is right for my family and our property.

 

Main points to consider:

 

• We're currently using LPG with a 20 year old boiler (75% efficiency I think)
• Our LPG costs us 38ppl (flogas like changing the price a lot though and this is the cheapest it's been for a while)
• Electric is costing us 13.3p kWh
• LPG is costing us 5.5p kWh
• Required flow temp needed is 50 degrees as it would be used with rads

 

So as I know these variables, to work out the min COP, is it as easy as dividing the electric cost by the lpg cost (2.4~)?

 

But then I need to factor in running times somehow? e.g. if the lpg boiler heats the property sufficiently in 4 hours, due to the ASHP's lower temp it might take 12 hours to heat to the same level

 

Surely I don't just multiply the min COP by 3 as it's going to run for 3 times as long?

 

Thanks!

 

 

[Jeremy Harris]

The first point is that 50 deg C from an ASHP is going to hit the COP and take it well below 3, probably down below 2 or even below 1 on a damp, cool day.  The COP will vary a great deal, depending mainly on two factors, the flow temperature being demanded and the relative humidity outside.  Outside air temperature has a relatively small effect.  The COP when an ASHP is run at a high flow temperature, like 50 deg C, will be dominated by icing and defrost cycles, something that do not feature in the official COP figures.  On a day when the temperature is down around 4 to 6 deg C and the air contains a lot of water vapour (cold and foggy/misty days, for example) the ASHP will probably have to defrost two or three times an hour, for around 10 minutes each time.  Every time it defrosts it runs in reverse, taking heat out of the house to warm up the heat exchanger to melt the ice that's formed.  Clearly there is a negative COP during defrost cycles, plus a loss of operating time, which further reduces the net COP.

 

The best solution at the moment if you already have an LPG installation, is to look at using a hybrid heat pump.  These work by running all the heating using the heat pump, with a high COP because the heating flow temperature can be reduced right down, as long as you have UFH or over-sized radiators.  The ASHP will easily run at a COP of over 3 all the time when run like this.  When you need hot water, a hybrid ASHP will take some of the preheated water (normally at around 35 to 40 deg C) and instantly boost it to 50 to 55 deg C to use for showers, baths, etc.  This dramatically reduces the gas consumption, as LPG is inly being used on demand, when hot water is needed.  It also means that the ASHP never needs to try and deliver a high flow temperature so doesn't ice up and need to defrost.

 

Daikin are probably the best known hybrids, with the Altherma range, but there are others.

 

Finally, the COP divided into the unit electricity price does give you the running cost per kWh.  In your case, with a COP of 3 and a unit price of 13.3p, you would be paying around 4.43p per kWh for heat from the ASHP.

Edited November 23, 2017 by JSHarris

[johny_99]

Thanks so much for that information, JSHarris.

 

I questioned the installer about the high flow temp needed and I was advised the heat pump in question (Nibe f2040-12) "can still maintain a COP of 3.8 at the design external temperature of -2.2" (his words exactly). I did not know about the defrost cycles, thank you!

 

If I understand you correctly, I don't think the hybrid heat pump would be an option. When getting the quote for the heat pump, part of the job was to oversize the radiators in every room (singles to doubles mostly and even adding an additional radiator in one room). The 50 degree flow temp was the required temp after doing this.

 

46 minutes ago, JSHarris said:

Finally, the COP divided into the unit electricity price does give you the running cost per kWh.  In your case, with a COP of 3 and a unit price of 13.3p, you would be paying around 4.43p per kWh for heat from the ASHP.

 

See, I was also confused about this part (I feel a bit stupid). If I was paying 4.43p per kWh for the pump, and 5.5p for LPG, it's only cheaper if the pump heats the property in around the same time?

[ProDave]

To me the saving sounds too small for the capital outlay.

 

Might a new more eficcient condensing boiler make more sense?

[Jeremy Harris]

Heat pump sales people are very fond of ignoring the laws of physics, sadly, not helped by the official test conditions being related to temperature only, not humidity, and it's humidity that REALLY hits performance hard.  The specs for our ASHP show that it has a high COP when delivery a flow temperature of 50 deg C, but read the small print and you find that was done in dry air conditions and the impact of defrost cycling was ignored.  Realistically, I've found out, by trial and error, that keeping the flow temperature down around 40 deg C means the heat pump never needs to defrost, at least in our installation, and always seems to run at a COP above 3, sometimes over 4.

 

As Dave says, an efficient condensing boiler may well be a better option for you, cheaper capital outlay and reduced running costs over an older non-condensing, or only partially condensing, boiler.  Also, a condensing boiler run at 50 deg C will be very efficient, they stop condensing at around 56 deg C usually, so lowering the flow temperature just that little bit can make a very significant improvement to performance (there's a big efficiency boost as soon as the boiler is running in fully condensing mode).

[Nickfromwales]

Plus if you stick with LPG then no need to pay towards the radiator alterations. Work out what that would have cost and see how much gas that would pay for . 

[johny_99]

Thank you all.

 

It appears that the correct decision is a new condensing boiler.

 

Perhaps I can revisit ASHP's when/if the efficiency improves, or LPG becomes ridiculously expensive.

 

2 hours ago, Nickfromwales said:

Plus if you stick with LPG then no need to pay towards the radiator alterations. Work out what that would have cost and see how much gas that would pay for . 

 

Probably over a years supply of gas

 

2 hours ago, JSHarris said:

Also, a condensing boiler run at 50 deg C will be very efficient, they stop condensing at around 56 deg C usually, so lowering the flow temperature just that little bit can make a very significant improvement to performance (there's a big efficiency boost as soon as the boiler is running in fully condensing mode).

 

I also did not know this. Thanks!

 

However, this would require new radiators in every room as this was the temperature I was adviced to run a heat pump at.

 

Are the savings that substantial in condensing mode that it's sensible to upgrade radiators?

 

Thanks! 

[Nickfromwales]

4 minutes ago, johny_99 said:

Are the savings that substantial in condensing mode that it's sensible to upgrade radiators?

 

Thanks! 

How long do you intend to be in the property? 

[johny_99]

This is our first house of our own and we like it

 

I can't see us leaving anytime soon so I think 10+ years

[A_L]

@johny_99 - to get condensing you only need to keep the return (to boiler) below 56/57°C, the flow can be higher. Any reasonable amount of condensing will get 10-15% more heat from the same amount of gas. Depending on the design of the system you could get a flow temp of 65°C and still be deeply condensing. I would aim for a return around 50°C. A conventional radiator at 60°C will give out about 40% more heat than at 50°C.

[Nickfromwales]

1 hour ago, johny_99 said:

Perhaps I can revisit ASHP's when/if the efficiency improves

As Jeremy would say, "the laws of physics won't be changing any time soon"

 

[billt]

This looks like a case where a weather-compensating controller would be worthwhile. They modulate the boiler flow temperature according to the outdoor temperature.

 

Most of the time they will keep the water temperature well below 50°C even with a system designed for high water temperatures, keeping the boiler operating in the condensing range. The flow temperature will only be high for the relatively small time when the outside temperature drops towards 0°C.

 

My system has used weather compensating controllers for the last 25 years, initially with an oil boiler, now with a big thermal store and LPG boiler. They both worked very well.

 

Of course, if you have a near pssivhaus then the benefit is negligible.

Edited November 25, 2017 by Nickfromwales
Typo : LPG for log

[SteamyTea]

Rather than pay out for a new boiler/ASHP, have a look at reducing your heating load first.

Ideally you should be doing this anyway.  So look at adding extra loft insulation and fixing any air leaks around windows and doors.

I spend less than £200 on a few very basic improvements and have, over the years, reduced my annual usage from just under 12 MWh to just under 4 MWh.

 

[johny_99]

Thanks all.

 

All good info I will take on board.

 

I will definitely be looking at improving our insulation.

[Nickfromwales]

18 hours ago, billt said:

This looks like a case where a weather-compensating controller would be worthwhile. They modulate the boiler flow temperature according to the outdoor temperature.

 

I tried to get the all-singing-all-dancing Vaillant W-Comp control to work and it was so uneconomical on time and effort by the the time I'd have actually got it fine tuned, the transmitter in EXACTLY the right position outside and blah blah blah it would have soaked up about 2 years worth of savings getting it to work properly. 

Single increment changes on the hub saw either too hot or too cold flow temp fluctuations and in the end even Vaillant admitted I was pissing in the wind.

I have no experience of other manufacturers W-Comp controllers so maybe someone could give feedback from a real life case study of a PROVEN setup that works accordingly?

[Nickfromwales]

On 23 November 2017 at 16:49, billt said:

My system has used weather compensating controllers for the last 25 years, initially with an oil boiler, now with a big thermal store and LPG boiler. They both worked very well.

I'm unsure how you achevied that as oil boilers don't modulate ? Did you use digitally controlled electromechanical blenders off the store? I know Vaillant do such a thing but it was off the chart complicated from an end consumers POV and wouldn't control two different temp zones ( a mix of high temp rads and UFH ) so I didn't pursue that any further. 

[dpmiller]

older weather comp controllers for oil aren't much more than an Optimum Start device AFAIK.

[Nickfromwales]

Vaillant do a complex setup as I describe above, basically it allows an uvc to be heated at high flow temp whilst the heating is separated from the S plan and is then blended down to suit the required ( reduced ) flow temp for the times that the two are running simultaneously. 

Afaic just fitting a W-plan ( DHW priority ) arrangement would do that with ease. ?

[billt]

4 hours ago, dpmiller said:

older weather comp controllers for oil aren't much more than an Optimum Start device AFAIK.

 

I can only speak for the one I used (Honeywell AQ6000), but it was a lot more sophisticated than that. Yes it did optimum start, but it modulated the water temperature, which it did by cycling the burner. It also controlled the burn length and number of cycles to maximise the boiler efficiency; it was an excellent controller, much better than anything else readily available at the time.

 

The system that I use now is controlled by an Acaso Automix 30, which drives a mixing valve on the return to the thermal store.

 

They were both a bit fiddly to setup as you have to match the slope and offset of the temperature curves to match the characteristics of the house - something that's going to be a lot easier if you're living with it and can tweak a bit at a time over the heating season. I can see that it would be tricky to set up if you are installing for a living and don't have time for trial and error.

[billt]

Just to demonstrate that it does work here are graphs showing outside and water temperatures for the last week, a week in June and the coldest week last winter.

[SteamyTea]

The CH1 bottom floor temperatures seem to swing quite a bit.  Can you plot floor temp against external temp to see the correlation better.

[billt]

The yellow line is the outside temperature. Compare the top of the blue line to the yellow line and you should see in inverse relationship. It isn't perfect, partly as there's an internal temperature sensor which modifies the offset.

 

The large swings are due to the heating being timed (like most domestic systems). High flow temperatures are when normal room temperatures are being demanded, low flow temperatures are during set back periods. As the house has a long thermal time constant the internal temperature doesn't usually drop to the set back temperature so the flow temperature will drop towards (internal) ambient.

[SteamyTea]

So is your air temperature set to around 18°C.

On November 29^th, 30^th and December 1^st the flow temperature went up to over 60°C.  That implies quite a high temperature difference between the slab and the air temperature, do you know how hot the floor is actually getting, seems to be around 35°C (from an eyesight average).  If that is about right, and you are happy with the temperature stability, then you can safely run the ASHP up to 40°C and get a good CoP.

Have you got any raw data rather than charts?

[billt]

I fail to see how you can deduce anything about the air temperature from those graphs.

 

The system uses radiators, not UFH and a lpg boiler, not an ASHP.

 

The OP has a radiator system at the moment and it has been suggested that his best option is to keep the system as is and use a condensing boiler. I was merely suggesting that using a weather compensating controller will keep the boiler in condensing mode for as long as possible and maximise the efficiency of the system. The following discussion was about the operation of weather compensating controllers.

 

Yes, I've got lots more data but it's not going to be any use to anyone else.

[SteamyTea]

It is amazing what can be deduced from data, why we have statistics

I had forgotten that it was radiators, but they are just convection devices anyway, so it is possible to compare systems.  It is the air temperature that you are trying to keep stable, not the rest of the house, that is a bit of an irrelevance.

I just don't see a decent correlation between the two datasets, though there may well be a weak one, but without looking at the data properly it is hard to tell.