Keep your CoP up

[SteamyTea]

A comment in @Conorthread about heating water got me thinking.

https://forum.buildhub.org.uk/topic/10668-small-ashps-units-primarily-for-dhw/

 

5 hours ago, PeterW said:

 

Go with E7, heat the tank to 45c with ASHP to maintain CoP above 2.5, and then top to 62c with E7 immersions. 

 

Does heat pump constantly monitor the CoP, and if it does, it should be possible to only run it when it is over a set limit i.e 2.5 or 3.

If it does not constantly monitor itself, then it should not be too hard to make a monitor and switch to make it do so.

The reason for this as that it should avoid any icing issues, though it may reduce the overall energy delivered, just when you need it.

[joe90]

3 minutes ago, SteamyTea said:

A comment in @Conorthread about heating water got me thinking.

https://forum.buildhub.org.uk/topic/10668-small-ashps-units-primarily-for-dhw/

 

 

Does heat pump constantly monitor the CoP, and if it does, it should be possible to only run it when it is over a set limit i.e 2.5 or 3.

If it does not constantly monitor itself, then it should not be too hard to make a monitor and switch to make it do so.

The reason for this as that it should avoid any icing issues, though it may reduce the overall energy delivered, just when you need it.

 

Interesting, I spouted something similar some time ago with regard icing, saying I am sure it’s possible to “instruct” the ASHP to switch itself off if it was about to “defrost” (by sucking heat back out of the house). When trying to set my “kingspan” ASHP the programming was very confusing and in my opinion ambiguous, thanks to @JSHarris I got it working( but not sure I understand it yet)

 

 

[joe90]

8 minutes ago, SteamyTea said:

Go with E7, heat the tank to 45c with ASHP to maintain CoP above 2.5, and then top to 62c with E7 immersions. 

 

Not sure why you need to top up to 62’ (previous discussion here confirms no domestic legionella risk) I don’t, mine is 47’ and quite adequate fir all our DHW needs (plus DHW tank losses greatly reduced at this temp).

[Jeremy Harris]

12 minutes ago, SteamyTea said:

A comment in @Conorthread about heating water got me thinking.

https://forum.buildhub.org.uk/topic/10668-small-ashps-units-primarily-for-dhw/

 

 

Does heat pump constantly monitor the CoP, and if it does, it should be possible to only run it when it is over a set limit i.e 2.5 or 3.

If it does not constantly monitor itself, then it should not be too hard to make a monitor and switch to make it do so.

The reason for this as that it should avoid any icing issues, though it may reduce the overall energy delivered, just when you need it.

 

Not as far as I know, at least not directly.  Most seem to operate on a combination of target conditions.  Ours first tries to meet the target flow temperature, and will ramp up to it's maximum input power to try and get there.  As soon as it reaches this flow temperature it will usually start to throttle back the input power (compressor speed mainly) until the output power is just enough to meet the required flow temperature.  As the return temperature increases, the input power throttles back further, as output power is proportional to the ∆_T and water flow rate (water flow rate tends to be constant, though).

 

I measured the COP of ours by measuring the flow rate (which in our case stays constant all the time), monitoring the ∆_T between flow and return and measuring the input power level.  AFAIK, the ASHP itself cannot do this measurement directly, and seems to rely primarily on temperature sensors to adjust it's input power level.  It's complicated by the way the fan works, in that the fan speed seems to be controlled by the evaporator temperature, so ramps up and down semi-independently of the compressor speed.

[SteamyTea]

Really just wondering if there is any merit in the idea of sensing CoP and using that as a control.

Trouble is I think, that we want a fixed amount of energy out of an HP in as short a time as possible.

But that is often not the best way to run them.

[le-cerveau]

I keep my efficiency high by careful setting adjustment (fluke).  I have a Panasonic Aquarea Generation H 9kW split which heats 2 x SnnAmp Pre UniQ PCM38 cells, it also cools the floor direct.  I have a ΔT of 5^oC set on the flow temperature and a target of 45^oC, this is what the SunAmp requries to cease the call for heat.  It usually takes a couple of hours to re-heat the cells so difficult to remotely monitor the CoP: (this display shows a tank re-heat (shower) and evening cooling)

 

If the heating is running then it takes significantly longer as the majority of the heat will be taken into the slab and the system will run as long as the house is calling for heat.  Whilst it is doing this it sits at 38^oC for as long as the house is calling for heat as the UFH only takes water at 26^oC but is taking most of the capacity of the ASHP, the SunAmp cells are taking water at 38^oC, the PCM  temperature, so the cells are being re-charged incredibly slowly as the house heats, this gives a better performance history as shown below:

 

The first few hours variation are down to outside temperature, as the outside temperature rises the system drops down to a steady 1.2kW draw which gives a CoP of over 7, then at the end when the UFH has stopped calling for heat and the system finally ramps up to heat the cells (it cuts off around 43/44^oC) the CoP drops again.  I have numerous days when this can be seen.  It also ties in with the stated efficiencies:

 

In fact I am getting better than expected performance whilst heating to 38^oC as the system is drawing a lot less power than expected.

 

I suspect it is down to setting the ΔT of 5^oC and not pushing it too hard, but letting it work relatively easily so as not to force the system.  I have not had any defrosting issues either.

 

 

[ProDave]

A lot suggest the concept of heating the DHW so say 45 degrees with the HP then topping up with the immersion heater.

 

That concept falls over for me as it would work fine on the first tank warm up.  But then you draw some water, or the tank cools by losses, and it will drop to the point of the immersion heater coming on but will rarely get low enough for the HP to turn on.  

 

As @joe90 says just use the HP to heat water to 47 or 48 degrees, that is all you need and well withing the parameters for a heat pump.

[dpmiller]

As I've never seen an ASHP with a COP of less than 1, what's the advantage of using the immersion anyway?

[joe90]

5 minutes ago, dpmiller said:

what's the advantage of using the immersion anyway?

 

Just a way of getting DHW to higher temps where the ASHP would probably have to defrost to get it to that temp which would ruin any decent COP. As @ProDave has said in our cases we heat water to 47/48’ and find it more than adequate fir our needs. I have asked before about whether an ASHP could heat water up to the point where it was about to defrost then simply stop, it may mean that in dryer warmer weather you could get hotter water without the downside of defrosting. However after using my ASHP for about 9 months I see no requirement fir water any hotter.

[Jeremy Harris]

16 minutes ago, dpmiller said:

As I've never seen an ASHP with a COP of less than 1, what's the advantage of using the immersion anyway?

 

Whole life cost, which is very dependent on the amount of hot water you use.

 

Say you buy an ASHP for DHW and it costs £2,000 installed (reasonable guess for a cheap one) and the alternative is an immersion heater at a cost of around £25. 

 

Used at 55°C the ASHP will probably run at an average COP over the year of between 2.5 and 3.  If you need 5 kWh per day of hot water (about what we use) then with a COP of 2.75 around 1.82 kWh/day of electricity would be used.  Around 3.18 kWh of electricity would be saved per day, over using an immersion heater.

 

A saving of 3.18 kWh/day is around 1,160 kWh/year, which is a financial saving of about £175 a year in electricity cost, over using an immersion heater.  For this use case the ASHP would take just over 11 years to recover the capital investment, not accounting for any interest that may have been accrued had that money been invested. 

 

The question then is whether or not an ASHP would last for about 11 years without needing repair or replacement.  If it would, then it might make financial sense,  It's a judgement call as to whether it's worth investing in an ASHP to supply just hot water like this, especially as there are other factors, like lost interest, variability in electricity cost (using E7 for the immersion and ASHP reduces the cost saving) and environmental concerns.

[dpmiller]

No, I mean for topping up the temperature.  @joe90 has probably hit the mark referencing defrosts but that's got to be somewhat dependant on your locality.

[ProDave]

13 minutes ago, dpmiller said:

No, I mean for topping up the temperature.  @joe90 has probably hit the mark referencing defrosts but that's got to be somewhat dependant on your locality.

Up here in the Highlands I probably get colder winters than most of you.

 

Our ASHP seems to work fine here.  It does not defrost as often as I feared it would, and it defrosts very quickly, taking about 2 or 3 minutes to complete a defrost cycle, so the amount of wasted heat is quite small.

 

And as others have said, the worst time for icing up is when close to 0 degrees.  When it's down to -10 outside the air is so dry it rarely needs to defrost.

 

I would not be buying an ASHP just to heat DHW.  Ours does heating as well, and in the winter it spends a lot more time heating the house than heating DHW, but because it heats the house with low temperature UFH it rarely needs to defrost while doing space heating, and although it runs for much longer periods doing that, the HP is not working anywhere near as hard as when heating DHW.

 

And very topical out HP can also do cooling.

[Ed Davies]

40 minutes ago, joe90 said:

Just a way of getting DHW to higher temps where the ASHP would probably have to defrost to get it to that temp which would ruin any decent COP.

 

But even with defrosts, would the COP go below 1? Even if it's sometimes down to 1.1 it's still a win over using an immersion.

[SteamyTea]

4 hours ago, Ed Davies said:

But even with defrosts, would the COP go below 1

If i just uses resistance heating then probably no, though there will be some parasitic losses.

Not so sure if it uses stored hot water.

There may be a rare scenario where it pumps warm water back though the system for too long, and if it has had a very low CoP just before, it may dip below 1.

I am starting to wonder if icing is a real problem.  It may just happen when trying to raise the temperature to much for too long i.e. an undersized system.

Edited July 26, 2019 by SteamyTea