ASHP- struggling to warm house in the cold weather

[AliG]

3 hours ago, PaulH said:

ASHP's are not gas boilers - they do not respond to timers - ie 3 hr's in a morning  and 5 hrs on an evening. they don't respond quick enough due to the low temperature output. I turn mine on in October and off in April - running costs vastly reduced, whole house a comfortable, constant weather compensated temperature. Undersizing as well as oversizing cause issues - there is no guess work involved in sizing a heat pump - it's a specialist area - DIY can work but it's usually 'luck' that gets it to work satisfactorily.

 

You are right Paul. I pointed out that it may be better depending on the COP at different temperatures to run the ASHP longer with a lower flow temperature. ASHP works best in a well insulated house where the required flow temperature is low and you can run it for long periods at a high COP. If we knew the flow temperature here it would help. The interaction of the COP at different output temperatures would tell you what the optimum way to run the system is.

 

This is why it probably also works best with UFH which reacts much more slowly than radiators. It is not clear if the system has been set up with this in mind here or set up like someone would set up a boiler and radiators to come on just before needed. It might be that in fact the ASHP is being asked to provide a high flow temperature for short periods which would not be the best thing to do at all.

 

The problem at the moment is it seems that the heat losses are higher than the output of the ASHP. You cannot optimise it at the moment. We don't know if it is already running at a high flow temperature or running at a low one that can be raised and we do not know if it actually can heat the house when it is below zero outside.

 

@Jude1234 if you have any information on this it would be useful. Normally there is a gauge on the manifold that shows the flow temperature. How have you had the heating set?

 

 

[joe90]

i do not know my lecky consumption yet, the house is still settling down but the last bill was higher than expected. I think I will follow @JSHarris and change to either E7 or E10 and time ASHP and DHW to run at cheap rate. The house is very stable in that temps takes a long time to change and it’s very noticeable that during mild weather the heating is needed very little.

[Jude1234]

So I have some technical information for you from the SAP and EPC that I have just received:

 

Predicted Energy Assessment Energy efficiency 80, CO2 rating 82

U values of upstairs 3G 0.80

U values of downstairs 2G 1.20

U value Bifold 1.40

Walls downstairs 0.24

Walls upstairs 0.14

Roof 0.11-0.18

Ground floor 017

100% LED lighting

MVHR 84% efficiency

ASHP flow temp <35C

 

House is 230 sqm

On the EPC it states that the annual space heating is 11,645 kWh and water heating is 2,169

 

Currently the UFH is zoned and each zone has its own thermostat and timer.  The majority are 'on' for 3 hours in the morning and approx 6 hours in the evening.  The rest of the time they are set at 16C so they are not left to go cold.

 

The house is feeling warmer since there was an adjustment to the heat curve (whatever that is) on Monday, but it is also milder externally so it is difficult to tell if it is just coincidence or not.

 

Looking at the manifold the temperature in is currently 32C and the return is about 28C.  Not sure where I would find out about the COP?  Is it on the technical spec of the ASHP?

 

 

[ProDave]

Why was the house designed with much better insulation upstairs?  Why not the same downstairs?

 

What are the on/off times set for the heating?  If only 3 hours in the morning I would want that 3 hours before you got up and turning off about the time you got up.  Likewise in the evening, turn on 3 hours before you get in from work and off an hour before you go to bed.

 

If the EPC says 11654KWh for heating, then assuming heating is on for 6 months of the year then that is an average of 447KWh per week, but of course it will be a lot higher when really cold and a lot lower in spring and autumn.

 

Again looking at averages, that is an average of 63KWh per day, and you are trying to deliver that in 9 hours so when the heating is on that is an average of 7KW to achieve that. Again more in the really cold periods.  Can the UFH even deliver 7Kw let alone more when needed?

 

I think you need to be looking at running the heating longer, or increasing the flow temperature to increase the output.

[newhome]

15 minutes ago, ProDave said:

What are the on/off times set for the heating?  If only 3 hours in the morning I would want that 3 hours before you got up and turning off about the time you got up.  Likewise in the evening, turn on 3 hours before you get in from work and off an hour before you go to bed.

 

I only have mine coming on once a day in the evening. I have the large room downstairs switching on cira 3 hours before I am due to get home and the other, smaller rooms 2 hours before. I did have everything coming on 2 hours before but I found that the largest room needed to come on earlier or it wasn't at the desired temperature when I got home. Everything switches off a couple of hours before I go to bed and the temperature hasn't dropped at all in that time. I don't bother with heating in the morning mainly because it only drops a degree or so overnight, the TS still contains enough hot water to use the shower, and I'm generally rushing around getting ready for work and don't need extra heat. I don't have an ASHP but that method works for me to keep electric bills manageable. My set temperature is also set at 16C but I've never know the temperature drop below that during the day so it just switches on again as normal in the evening. At weekends I have different times set and do have it on in the morning. 

 

[ProDave]

In the days when I had a proper 9-5 (well 8-4) job, there was 25 minutes between the alarm going off and me leaving the door,  Morning heating was irrelevant.

 

Now we both work part time and variable hours it just makes so much more sense to heat the house all day, but then it is so well insulated it hardly cools down much when off at night.

[AliG]

I found this table that gives the heat output depending on your flow temperature and the distance between the UFH pipes, your flow temperature is lower than the lowest quoted figure.

 

https://www.tradingdepot.co.uk/info/plumbing/polypipe/underfloor-heating-heat-output-tables/

 

The formula to calculate heat output is here, it depends on the temperature differential between the floor and the room.

 

http://heating.danfoss.com/pcmpdf/handbook_planning_vgctc202.pdf

 

At a rough guess you are putting out only 20-25W/Sq Metre. Assuming that you have 160 square metres of heated floor, your heat output is between 3 and 4 kW per hour.

 

With your heating on 9 hours per day, you would get around 30-35 kWh of output. Admittedly you should also be using a lot less electricity than you are using if this is the case also.

 

This is not enough to heat your house according to the SAP calculation and according to figures I input into the heat loss calculator.

 

The SAP calculation is 12000kWh per year, I suspect it may be 14000kWh with higher air leakage.

 

Assuming 6 months of heating, that is around 70kWh a day and probably up to 100kWh when it is cold outside. You would need to run the heating 24 hours a day to get close to this output. You also need to be able to overcome the heat loss at any one time which is likely much higher than your current output.

 

My guess is that running the heating from 430am to 930pm each day with a flow temperature of around 38C would be needed to heat your house when it is below 0, that would put out up to around 100kWh a day. If it was warner outside the thermostats would kick in and you would use less electricity. I am making an enormous number of assumptions here on floor coverings, area of UFH, pipe spacing etc.

 

Turning the temperature down to 16C is too low as a set back number, 21C during the day and 18C at night might be better.

 

The weird thing is that running the heating this much should use no more electricity than you are using at the moment. There could be some weird inefficiency coming in that I am missing or it is something else that is affecting your consumption.

 

Your insulation figures aren't that great nowadays, particularly the downstairs and window figures. Still though it is probably the air leakage that you can reduce. Halving the air leakage would cut your electricity usage by around 20% I would guess.

 

 

 

 

 

 

 

 

 

 

[Jeremy Harris]

I put together a spreadsheet to calculate UFH heat output that I've posted here a few times:  Floor heat loss and UFH calculator.txt

 

(save the file then edit the suffix to .xls - the forum software doesn't allow spreadsheet files as attachments)

 

 

[AliG]

Thanks @JSHarris. That is pretty consistent with my guesstimates. I was looking for about around 37 W/m Sq to get 100kWh per day output running for 17 hours a day. This would be the maximum output of the system and then if less is needed the thermostats should kick in and switch it off. This is confirmed by your spreadsheet as requiring a flow temperature just below 25C.

 

I reckon that would keep the house at 21C down to about -5C outside.

 

To be more accurate I would need the area of floor with UFH in it, I only know that the total floor area is 230sq m and that downstairs is larger than upstairs so I have taken a guess at 160 sq metres of UFH.

 

Using the table I attached, to get 37W/m would need 39C flow with 200mm pipe spacing and 37C flow with 100mm spacing, roughly 38C with 150mm spacing. I don't know the spacing figure.

 

The key figure is 100kWh per day of maximum output. You can then work back from the length of time the heating is switched on, UFH heated area and pipe spacing to get the flow temperature.

 

I have ignored the issue of no heating upstairs. It may be that you require putting out more heating than the calculations suggest to heat upstairs also. TBH with your insulation levels I suspect you will need extra heating upstairs sometimes, but let's see what happens when you have enough heat downstairs to heat the whole house. I have one unheated room that is open to a heated room below and it tends to sit at around 19c with the UFH running hard in the lower room. Partly this is due to it being almost 3x the size of the heated area in the room below.

 

@Jude1234 mentioned that it seemed better recently. At 7C outside the required heat output is closer to 60kWh per day which is 22W/m of output. 32W flow is going to give 18-21w/m of output. Along with incidental gains the system could keep the house warm at 7C outside temperature.

 

This actually explains why turning the heating up should not use much more electricity. You had enough heat output for temperatures down to about 7C, but it would not be enough below this. You would only use more electricity if the outside temperature falls below this level. 

 

I had just left our heating running until the last few weeks when the temperature finally got below 0. Now I have been fine tuning it as the high temperatures we had were not putting enough pressure on the system to know how well it was running. I had one manifold set at 32C, somewhat lower than the others. It would heat the room but was taking 6 hours to raise the temperature 1C, not helped by the wooden floor in the room. We have a gas boiler and don't run the heating all the time so I turned up the flow temperature. to the high 30s. I have also found a draught into the room that has to be fixed.

 

You can fine tune these numbers with the correct inputs. Basically then turn the flow temperature up to the calculated temperature and see what happens. You can fine tune it up or down a little from there. But don't turn it down until you have a cold snap and know it can cope.

 

You will use a little more electricity as the COP will be worse at a higher flow temperature, but I think the first thing is to make sure you can be comfortable in your house.

 

Then you can work on fixing air leaks to get you heating use down. I suspect that you could also help by turning the MVHR down to the minimum level as you have lots of air coming in anyway. Again you would have to see how that works out. If you could halve your air leakage you could turn the flow temperature back down by around 4C.

 

Someone could maybe help me with this one. The ACH from a blower test is much higher than real life air changes per hour. I am guessing that from your test plus running the MVHR you are maybe sitting at 2 changes per hour and could get this down to one if you fix all the obvious leaks. My guess is that your SAP calculation was done using a figure of one and the UFH was set up based on this too low figure.

 

 

 

 

[Jude1234]

On 08/02/2019 at 19:22, ProDave said:

Why was the house designed with much better insulation upstairs?  Why not the same downstairs?

The better insulation is primarily for sound insulation rather than heat (on a flight path)

 

Thank you all for the guidance, I think I have understood.  We need to increase the UFH input temperature and run for longer, and have the lower temp down only to 18C rather than 16C.  Is that correct?

 

From the manifold the pipe spacing is 150mm.  Tonight the input temperature is actually 40C (not changed anything, must just be that on a weekend the heating is on all day)

[AliG]

It may be that the builder changing the “curve” was increasing the flow temperature. 

 

Hwating on all day wouldn’t affect the temperature but it does have to run for a while to get up to temperature. 

 

I’d also get checking for air leaks. I keep finding new little ones in my place.

 

It’s also worth monitoring electricity usage more closely for a little while. I would expect you to use about 20% more when the temperature is freezing than when it is 7C. Checking it daily for a while will give you a better idea of what is driving consumption. 

Edited February 10, 2019 by AliG

[dpmiller]

14 minutes ago, AliG said:

It may be that the builder changing the “curve” was increasing the flow temperature.

 

flow relative to outside temp, likely.

[Jeremy Harris]

22 minutes ago, dpmiller said:

 

flow relative to outside temp, likely.

 

 

I agree, and that creates a problem if you have a house that responds slowly, as you really need the flow temperature to predict the required temperature in a few hours time.  It's one reason I binned the heat curve on our ASHP and just set a fixed temperature of 40°C.  It so happens that the ASHP works very efficiently at that flow temperature too.

[joe90]

I too have a fixed temp of 40’ from ASHP into a buffer tank running permenantly (in winter) controlled by tank stat, the  manifold mixes temp down to 25’ for the UFH, the manifold pump is controlled by a room stat in the hallway. I have a small temp reader in the lounge and it stays at 21’ + or - 1’. I am considering E7 or E10 like @JSHarris to lower electric cost.

[Ferdinand]

9 hours ago, Jude1234 said:

Thank you all for the guidance, I think I have understood.  We need to increase the UFH input temperature and run for longer, and have the lower temp down only to 18C rather than 16C.  Is that correct?

 

I work on longish hours for the ufh to be on, and have boosted the flow temperature for the winter, and also sometimes switch it to always-on in the cold snap.

 

Ferdinand

 

Edited February 11, 2019 by Ferdinand

[Ed Davies]

11 hours ago, JSHarris said:

and just set a fixed temperature of 40°C.

 

11 hours ago, joe90 said:

the  manifold mixes temp down to 25’ for the UFH

 

What's the point of running the ASHP more than a few degrees above the UFH temperature? Say 30° rather than 40° which would, I suppose, improve the CoP at least a bit.

 

I realise that @JSHarris also uses the buffer tank to pre-warm DHW before raising it to full DHW temperature via his Sunamp so a slightly higher temperature in the buffer helps there but is there any other reason? Just to keep the buffer tank small without cycling the ASHP too much?

[joe90]

4 minutes ago, Ed Davies said:

 

 

What's the point of running the ASHP more than a few degrees above the UFH temperature? Say 30° rather than 40° which would, I suppose, improve the CoP at least a bit.

 

I realise that @JSHarris also uses the buffer tank to pre-warm DHW before raising it to full DHW temperature via his Sunamp so a slightly higher temperature in the buffer helps there but is there any other reason? Just to keep the buffer tank small without cycling the ASHP too much?

Fair point, it’s just that @JSHarris ran his at 40’ with no defrosting, perhaps I will wind the tank stat down to 30k but keep the ASHP running at 40’ so any heat up time required is minimal.

[Jeremy Harris]

I run it at 40°C because the DHW pre-heat is useful (cheaper than using more grid energy to charge the Sunamp) and because there seems to be no noticeable performance difference between having the ASHP running at 30°C and 40°C.  At 30°C the ASHP would tend to cycle on and off, as the heat output was below its lowest modulation capability, whereas at 40°C it stays on.  There seems to be a very slight advantage in having it run for longer periods, as when it starts up it ramps to full output over a period of around 30s to 1m, then gradually modulates down to a speed where it can maintain the set flow temperature.

[joe90]

5 minutes ago, JSHarris said:

I run it at 40°C because the DHW pre-heat is useful (cheaper than using more grid energy to charge the Sunamp) and because there seems to be no noticeable performance difference between having the ASHP running at 30°C and 40°C.  At 30°C the ASHP would tend to cycle on and off, as the heat output was below its lowest modulation capability, whereas at 40°C it stays on.  There seems to be a very slight advantage in having it run for longer periods, as when it starts up it ramps to full output over a period of around 30s to 1m, then gradually modulates down to a speed where it can maintain the set flow temperature.

 

Right, I run my ASHP at 40’ so is there any advantage of turning down the buffer tank stat to say 30’?

[Jeremy Harris]

I think you'd have to try it and see, TBH.  I have an energy/power meter on the ASHP supply, so I could measure the energy/power going in and estimate what was coming out, and after some experiments with different settings I concluded that, for our set up, there was no benefit in reducing the ASHP flow temperature down to 30°C. 

 

I should qualify that by saying that I only tested the different settings over a modest range of weather conditions, and didn't try turning the flow temperature down in very cold weather, to see if there was any performance improvement.  As we don't get many days a year here when it's sub-zero, I'm not really that bothered if it is a bit less efficient under those conditions, particularly as I can't see any obvious dip in performance from the energy use data; that seems to track the outside temperature reasonably well, albeit with a bit of lag.

[Ed Davies]

7 hours ago, JSHarris said:

At 30°C the ASHP would tend to cycle on and off, as the heat output was below its lowest modulation capability,

 

Thanks. That seems like the key point.

 

I guess to some extent it's a matter of the hysteresis on the buffer tank thermostat. If there's a wide hysteresis then the return temperature, at least for most of the “burn” will be lower allowing the HP to put more power into the water. But then, if you have, say, a 10 °C hysteresis when you want a minimum of 30 °C to mix accurately into the UFH you finish up at 40 °C anyway.

[AliG]

Hi @Jude1234

 

How have you got on with sorting this out?

 

I suspect since it has been much warmer it hasn't been an issue.

 

 

 

 

 

[Jude1234]

Hi @AliGas you say it has warmed up outside so the temperature inside is fine. An experienced heat engineer did come to assess and verbally told me that there should have been some sort of heating installed upstairs as the ground floor heating wasn't set up to heat the entire house. I am just waiting for this in writing then I will discuss with the contractors who originally advised me that no heating was required upstairs. I will need to have some electric heaters fitted in the bedrooms at some point before next winter when money allows. In parallel I have been going round sealing up any leaks ie where the main electricity comes in. 

[AliG]

It looks like @Jude1234 hasn't been on in over a year, but if they see this I wonder if they go this sorted?