Is my COP rubbish?

[Lofty718]

Interesting, this is my first proper winter upcoming with UFH so I will test it out myself but I thought it was not ideal.

[SteamyTea]

Worth remembering that generally, a house gets warmer during the day with the extra human activity, maybe sime usable solar gain and warmer OAT.

This means that as the day progresses, there are less thermal losses.

A bit of extra warmth at 'getting up time' may be more useful.

I think with modern house design, we have to get out of the habit of thinking that when the heating goes off, the internal temperature plummets. Even a mild thermal renovation on any old house will improve matters no end.

[Beelbeebub]

21 hours ago, Bob77 said:

The 13,000 per year figure was our total usage, not just heating/DHW. Based on my figures above, we seem to be using around 400 kWh per month for other things, so you can probably take off 4000-5000/yr, leaving a heating/DHW demand of 8000-9000 kWh per year. Albeit that was with a lot less space to heat!

That seems to indicate you system under read the heat delivered, unless your heating demand for the last winter was drastically less than usual. 

 

If you did in fact use 8,000kwh last heating season your Cop would be a bit above 3, which would be much more plausible.

 

 

[AliG]

21 hours ago, SteamyTea said:

A bit of extra warmth at 'getting up time' may be more useful.

I have played around with the settings on my parents' ASHP and heating but won't know until this winter as they only just moved in at the end of last year and we were still getting things sorted out.

 

In the spring I set the heating to heat to 23C overnight, but be at 21.5-22c during the day. My thought process was to get it a bit hotter than I would have it in the morning (they like it hot TBH) so the AHSP would run more on cheap electricity during the night then only occasionally kick in during the day if the temp fell back below 21.5. The house will only cool down that much on a very cold day. On Intelligent Octopus electricity is 4x as expensive during the day as during the night so this makes a big difference to costs. I would therefore recommend a larger HW tank to anyone installing an ASHP so that they can heat water during the night and only run the ASHP for HW during the day if absolutely necessary. In a well insulated house HW costs can be as much or more than heating costs.

 

As I only set this up after the coldest weather it remains to be seen how it works when the temperature is below 0C as the house will need more constant heat input. The flow maxes out at 45C and -5C outside I think. Their CoP is around 3.5, but the flow temp is only 35C when the outside temp is 5c or above. If the systems as described needs a higher flow temp then CoP will suffer. This is what happens when we read horror stories of badly installed ASHPs. I wouldn't describe this as that bad, just not ideal.

 

An issue I find consistently in my house and now in their house is how sensitive people are to whether or not the heating is actually on.When the outside temp is around 12-14C, it can be 21C inside and the heating doesn't kick in. People will consistently complain it is cold. But if the temperature outside is 0C and the heating is on with an inside temp of 21C people are quite happy. I call it the "cusp" problem where it is neither too hot nor too cold outside. People like the feel of he radiant heat from the floor when the UFH is on irrespective of the actual air temperature. I think that people most feel this first thing in the morning and in the evening so it does impact how I set up the heating irrespective of just keeping a constant air temperature.

[DanDee]

2 minutes ago, AliG said:

An issue I find consistently in my house and now in their house is how sensitive people are to whether or not the heating is actually on.When the outside temp is around 12-14C, it can be 21C inside and the heating doesn't kick in. People will consistently complain it is cold. 

 

This is due to the rate of heat loss of the body, even tough the room temp is 21C, the radiating surfaces absorb the heat that leaves your body quicker than when the heating is on. As well when the heating is on, there not much heat flow from your body towards other surfaces.

This is the paradox of the "cold" radiators with heat pump, even tough you can keep a stable temp of 21, with radiators at 25C, the 30-35C will feel "warmer" in the same 21C room temp due to the radiating heat transfer(or lack of, from your body to the environment).

A metal spoon has the same temp as the wooden spoon, but when you touch them, one absorbs more heat(feels colder)

[JohnMo]

We have found with UFH in the heating season, because the floor isn't sucking so much heat away, the rooms actually feel warm with the thermostat set at 19.

 

But when the UFH cooling is on, even though the room can be 23/24 it actually feels nice and cool.

[AliG]

On 27/08/2023 at 16:05, Bob77 said:

Jan 2023: 1286 kWh, 635, 147, (504)

Feb 2023: 1094 kWh, 500, 136, (458)

Mar 2023: 1091 kWh, 435, 125, (531)

Apr 2023: 794 kWh, 220, 110, (464)

May 2023: 553 kWh, 53, 100 (400)

Jun 2023: 433 kWh, 5, 90 (338)

Jul 2023: 405 kWh, 0, 75 (330)

BTW this shows something that I have noted in our energy (gas) use, but not had specific data on. We use a lot of hot water, I think people tend to think of hot water costs as being a constant through the year. In fact it costs a lot more to heat hot water in the winter as the temperature of the water coming into the house will be closer to 0C compared to maybe 8C in the summer. On top of this with an ASHP it will be running with a lower CoP due to the colder outside temperature.

 

Based on these numbers I would guess you will use around 2900kWh a year for space heating and 1300kWh  for DHW and just over 5000kWh for everything else. This suggests that your heating energy(heat and DHW) has dropped from 8000kWh a year to 4200kWh. Assuming a 2.5 CoP for DHW, then space heating has dropped from 4750kWh to 2900kWh. The problem then is it is extremely difficult to guess what difference the extension has made to space heating costs. The house is 37% larger. If we assume 30% higher space heating use then this again gives a quite poor CoP estimate of around 2ish. But we also have to adjust for weather differences etc. Winter 22/23 was somewhat colder than 21/22. It looks like it was on average maybe 1-2C colder. This will both drive up space heating requirements and drive down the CoP. Anyway I find the CoP estimate from the ASHP quite believable based on this.

 

The thing with your weather compensation curve is that it becomes irrelevant with temperatures above say 14C. So you have a flow temp of 50C at -2 and 34C at 14C. At temperatures above this the heating is highly unlikely to be on. The average temperature in Hampshire where I think you are was 9C, 5C and 4C Jan-March. Your heating will have run more the more below average the temperature was, so the effective average temp your heating was running at was probably closer to 0C.  At 0C you have a flow temp of 48C and your CoP would be barely above 2. Periods of temperatures around 0C will probably account for almost half the year's heating demand. Once we get into next winter watch your ASHP energy use daily and compare cold days with warmer days. I did this at my parents' house it is really quite shocking. It comes out in the wash over time, but cold days are very expensive for heating. Looking at the chart below, I suspect that to get a CoP above 3 for the year you would need to get the flow temperature dow to the low 40s when the outside temp is just above 0C which is not likely feasible in your house, i.e. a CoP nearer to 2.5 when it is cold. The periods when your heating is running with a flow temp below 40C will mean an outside temp of above 8C and these just don't account for much of your heating requirement over the year. You can see this from the dramatic drop off in energy use in April then May when temps get above these levels.

 

 

What this kind of thinking is quite useful for is to tell you whether you might want to use gas or an ASHP for heating. If you were using electricity before then I suspect you don't have that option and an ASHP is much cheaper than direct electric heating.

 

However, if you do have the option, then you need to able to run your CoP into the 3s before an ASHP is cheaper to run than gas. During the day electricity under the new cap will be around 27p and gas 7p. Allowing for the efficiency of gas (already included in the CoP calculation for an ASHP, electricity is 3.25-3.5x more expensive than gas. However, if you get cheap overnight electricity then this costs roughly the same as gas, so the calculation depends on the average price of your electricity. If you can get 50% of your ASHP running during the cheap period then you don't need a CoP much above 2 for an ASHP to be cheaper than gas.

 

Therefore if there was a way to readily estimate CoP before a heat pump was installed you could actually figure out who should and should not install one. It would probably be dependent on the average U-value of a house including windows. Best results would come with a total house U-value of around 0.25. My parents are getting around 3.5 with an average U-value of around 0.3. I suspect that problems arise as the average U-value rises above 0.4 which is around the minimum current building regs spec (depends a bit on the percentage of windows).

 

If anyone could come up with an easy way to calculate this it would be extremely useful.

 

 

 

 

[Bob77]

Thanks @AliG for that detailed post. I suspect that my flow temperature can come down further and I will try that out once heating season starts. Because we didn’t have the house fully insulated/airtight and still had bare concrete floors etc during the coldest weather last winter I did have the heat pump working harder to make it more comfortable. 
 

And I certainly noticed the cost - when I started monitoring the ASHP usage in January (which was less cold than December) the peak daily cost for ASHP usage alone was over £16! But that was with a flow temp of probably 50C. 
I will see if the house is warm enough with a flow temp in the low 40s at zero outside. 
 

The way the system is set up is with a single flow temperature for space heating which is then mixed down to feed the UFH. I don’t know is if this is standard - the ASHP can do two loops, one for radiators and one for UFH, but it was installed as one loop with a separate mixing valve which takes the UFH temperature down to about 35C. 
 

So clearly any flow temperature above 35-38C ”should” be fine for downstairs, it’s the radiators upstairs that will need a higher temperature. Plenty to experiment with this winter anyway!

 

 

PS I just got back from a few days away (rest of the family were still at home) and my lowering the DHW temperature by a few degrees already seems to have boosted my COP. For the last week it is just under 2.9, and I only changed the temperature part way into that week 👍

[AliG]

Good that it is running better.

 

One of the things to do when installing an ASHP is to design the system for the lowest reasonable flow temps as this improves the CoP and reduces running costs. Larger radiators and closer spaced UFH pipes for example as well as insulation.

 

Mixing down the temp for UFH means that the system runs less efficiently than it should. At this point the cost of larger radiators upstairs so you can reduce the flow temp to them to be the same as for the UFH would probably be too high relative to the modest savings from a better CoP. This definitely will partly explain your worse than average CoP.

 

Hopefully the improved insulation since last year means that you can run lower temps on the radiators and have a better CoP.

[Big Jimbo]

@AliG So basically, nice large oversized rads upstairs, and loops at say 100mm apart downstairs ? The more surface area you have to let the heat out, the better ?

[AliG]

Yes. The installer should be looking at these depending on heat loss calculations. We have different pipe spacing in different rooms.

[JohnMo]

5 minutes ago, Big Jimbo said:

@AliG So basically, nice large oversized rads upstairs, and loops at say 100mm apart downstairs ? The more surface area you have to let the heat out, the better ?

Not really that necessary for 100mm centres if well insulated, I am running 300mm centres, max required flow temp is 35 at -9 outside.

 

Really if you match the rad size and the floor centres so they all have the same flow temp on paper, you can then balance the flow rates to get the room temps where you need them. Then you are running at the lowest flow temp. Then you start questioning the need for any mixers or additional pumps.

[Beelbeebub]

As an aside, several HPs have a zones function.

 

Clukd you have the DHW on it's own "zone" with a specific flow temp stratetgy as now, UFH on zone 1 set at a WC curve specific for that and the rads on zone 2, set on a higher WC curve for rads? Then effefrciely time slice between thr UFH zone and Rads, say every 10mins.

 

Defrost cycles would open both zones to maximizer system volume availble.

 

When the outside temps are getting to the point where the system would cycle unacceptably on only one zone, both zones could be connected to help. 

 

The extra cost woukd be fitting an extra zone valve effectively an S+ type plan. 

 

Just an idle thought 

[JamesPa]

6 minutes ago, JohnMo said:

Really if you match the rad size and the floor centres so they all have the same flow temp on paper, you can then balance the flow rates to get the room temps where you need them. Then you are running at the lowest flow temp. Then you start questioning the need for any mixers or additional pumps

At last somebody has suggested this.  I never fully understood the logic of combining ufh with radiators that need a higher flow temperature given that there is no good way, with a boiler or a hp, to achieve two different flow temps simultaneously. 

 

Of course I realise that it's the pragmatic solution in some retrofit scenarios, but I can't see how it makes sense in a new build.

Edited September 1, 2023 by JamesPa

[Beelbeebub]

1 minute ago, JamesPa said:

At last somebody has suggested this.  I never fully understood the logic of combining ufh with radiators that need a higher flow temperature given that there is no good way, with a boiler or a hp, to achieve two different flow temps simultaneously. 

 

Of course I realise that it's the pragmatic solution in some retrofit scenarios, but I can't see how it makes sense in a new build.

Except the conventional wisdom is that UFH is hard to do upstairs.

 

There is some truth to that as UFH upstairs makes routing services between the ceilings and upper floors much harder, or at least less sensible. 

 

That said, in a new build the upstairs rads can be spec'd so thry can run at the same temp as thr UFH. 

 

 

[HughF]

1 hour ago, JamesPa said:

At last somebody has suggested this.  I never fully understood the logic of combining ufh with radiators that need a higher flow temperature given that there is no good way, with a boiler or a hp, to achieve two different flow temps simultaneously. 

 

Of course I realise that it's the pragmatic solution in some retrofit scenarios, but I can't see how it makes sense in a new build.

A decent mixer (ivar) and pump with hydraulic separation at the mixer inlet… 

[JohnMo]

1 hour ago, Beelbeebub said:

Except the conventional wisdom is that UFH is hard to do upstairs.

 

There is some truth to that as UFH upstairs makes routing services between the ceilings and upper floors much harder, or at least less sensible. 

 

That said, in a new build the upstairs rads can be spec'd so thry can run at the same temp as thr UFH. 

 

 

UFH in bedrooms is just wrong, we have it, basically just switch it off (well circuit is actually on, but heat output minimal) as either to warm or not warm enough, cannot control it as the lag time is massive.  For the times you need heat, an hour in the morning a couple at night, a panel heater may be the best solution, with a timer and thermostat. Wet rooms upstairs dual fuel the towel rail, and include UFH if you like.

 

You then keep the heat pump installation really simple, one flow temp, one zone.

1 hour ago, Beelbeebub said:

DHW on it's own "zone" with a specific flow temp stratetgy

DHW is treated differently from heating zones by a heat pump already. If it gets a signal for DHW heating, the complete operation strategy changes on the heat pump. Circulation pump runs at max fixed speed, it does not modulate on mine, mine will not defrost in DHW mode, for best CoP temp rise is slow from the heat pump.

[JohnMo]

3 minutes ago, HughF said:

A decent mixer (ivar) and pump with hydraulic separation at the mixer inlet… 

I had a Ivar mixer (tried a couple of different ones), you cannot close the feed of returns water to the feed water, no matter what alterations you make to the settings.  So if your UFH flow temp at the manifold is 30, your feed to the mixer could be in 35+ range. So you instantly get a CoP hit.

[Bob77]

2 hours ago, AliG said:

Good that it is running better.

 

One of the things to do when installing an ASHP is to design the system for the lowest reasonable flow temps as this improves the CoP and reduces running costs. Larger radiators and closer spaced UFH pipes for example as well as insulation.

 

Mixing down the temp for UFH means that the system runs less efficiently than it should. At this point the cost of larger radiators upstairs so you can reduce the flow temp to them to be the same as for the UFH would probably be too high relative to the modest savings from a better CoP. This definitely will partly explain your worse than average CoP.

 

Hopefully the improved insulation since last year means that you can run lower temps on the radiators and have a better CoP.

 

Since last winter I have added another 170mm of insulation to the loft (and boarded above that) so the upstairs bedrooms should be warmer. I think enough heat should travel up from downstairs to keep the bedrooms comfortably warm with a lower flow temperature, so maybe before the start of the heating season I will lower the WC curve by a decent amount and wait to see if there are any complaints regarding the indoor temperature!
 

[HughF]

51 minutes ago, JohnMo said:

I had a Ivar mixer (tried a couple of different ones), you cannot close the feed of returns water to the feed water, no matter what alterations you make to the settings.  So if your UFH flow temp at the manifold is 30, your feed to the mixer could be in 35+ range. So you instantly get a CoP hit.

I’ve already got a CoP hit as my rads are sized for 45 flow at -2

[Beelbeebub]

2 hours ago, JohnMo said:

UFH in bedrooms is just wrong, we have it, basically just switch it off (well circuit is actually on, but heat output minimal) as either to warm or not warm enough, cannot control it as the lag time is massive.  For the times you need heat, an hour in the morning a couple at night, a panel heater may be the best solution, with a timer and thermostat. Wet rooms upstairs dual fuel the towel rail, and include UFH if you like.

 

You then keep the heat pump installation really simple, one flow temp, one zone.

It's a personal preference thing. Our bedrooms are UFH, but it's a bungalow with UFH in the slab. The UFH just keeps the slab at a constant temp, sort of crude weather comp. 

 

But you make a good point and I think over zoning is a bad thing for heatpumps.

2 hours ago, JohnMo said:

DHW is treated differently from heating zones by a heat pump already. If it gets a signal for DHW heating, the complete operation strategy changes on the heat pump. Circulation pump runs at max fixed speed, it does not modulate on mine, mine will not defrost in DHW mode, for best CoP temp rise is slow from the heat pump.

That is what I meant. At the end of the day it's all "zones" to the heat pump. 

 

It just gets told "if input X is closed do Y" 

 

Given modern microcontroller the only limit is how many inputs and output relays you have. The microprocessor is easily able to cope with the multiple control strategies. 

[Bob77]

Well now the heating has been running a bit more, my COP still seems to be averaging about 2.7, even though the weather hasn’t been cold enough to get the flow temperature above 40C. 

 

I suppose the real test will be how warm the house stays with a lower flow temperature, and what the usage is like if we get a spell as cold as last December (when it was below zero day and night for a whole week here). Peak daily usage was over 35kWh at one stage last winter, but I hadn’t got the settings optimised then, and I think there was lots of defrosting going on. 

Edited November 9, 2023 by Bob77

[sharpener]

On 01/09/2023 at 08:48, Bob77 said:

The way the system is set up is with a single flow temperature for space heating which is then mixed down to feed the UFH. I don’t know is if this is standard - the ASHP can do two loops, one for radiators and one for UFH, but it was installed as one loop with a separate mixing valve which takes the UFH temperature down to about 35C. 

 

 

On 01/09/2023 at 11:38, Beelbeebub said:

Clukd you have the DHW on it's own "zone" with a specific flow temp stratetgy as now, UFH on zone 1 set at a WC curve specific for that and the rads on zone 2, set on a higher WC curve for rads? Then effefrciely time slice between thr UFH zone and Rads, say every 10mins.

 

Did you ever explore the possibility of splitting the rads from the UFH? I would have thought this could give a worthwhile improvement. You could then preheat the slab at night rate until say 0600 then switch to heating the rads to get the bedrooms warm.

 

Maybe changing every 10 mins as @Beelbeebub suggests is a bit too frequent though, there will be a lot of water still at the higher temp pushed into the UFH at the changeovers which will reduce the savings. And it would maybe require an external controller.

[Bob77]

22 hours ago, sharpener said:

 

 

 

Did you ever explore the possibility of splitting the rads from the UFH? I would have thought this could give a worthwhile improvement. You could then preheat the slab at night rate until say 0600 then switch to heating the rads to get the bedrooms warm.

 

 

I haven't. To be honest as we still haven't finished the renovations after more than a year the last thing I want to do is start messing with plumbing again.

Just to clarify, the radiators are on a separate thermostat and pump circuit - they can and do come on independently of the downstairs UFH. So I do as you describe and have the UFH heating up overnight before the bedroom rads come on.

What I don't have is a separate LWT for the UFH and the rads. The central heating water is just one temperature, set by the weather curve.

[sharpener]

1 hour ago, Bob77 said:

The central heating water is just one temperature, set by the weather curve.

1 hour ago, Bob77 said:

So I do as you describe and have the UFH heating up overnight before the bedroom rads come on.

 

 

Assuming the flow temp is high enough for the rads you could maybe set the temp back during the UFH phase, to avoid the mixing valve having to temper it down again which will be costing you in terms of CoP. If the controls will allow it when using WC.