Most efficient way to run underfloor heating

matelodave

You can adjust your hot water temperature a bit if you find that its not hot enough, you can set it to whatever suits you but it's worth understanding how it affects your COP - we don't have a problem with 45 degrees as we've got a dishwasher and it's a good temperature for the shower etc.
It's hard to try and evaluate whether having the hot water hot enough to wash dishes is more economical than running the dishwasher. However we are quite lazy so the dishwasher wins whatever it costs. Its the same with washing clothes, we use a washing machine and tumble dryer rater than going down to the river or hanging it out on the line - even in the summer.

dllive

Im just reading the Ecodan manual that you sent, regards DHW. I now understand the difference between Eco and Normal modes:

Normal mode will heat the water in the DHW tank more quickly using the full power of the heat pump. Eco mode takes a little longer to heat the water in the DHW tank but the energy used is reduced. This is because heat pump operation is restricted using signals from the FTC based on measured DHW tank temperature.

I think Im going to get slightly obsessed with COP in the coming months! 

tim_p

matelodave said:

It's hard to try and evaluate whether having the hot water hot enough to wash dishes is more economical than running the dishwasher. However we are quite lazy so the dishwasher wins whatever it costs. Its the same with washing clothes, we use a washing machine and tumble dryer rater than going down to the river or hanging it out on the line - even in the summer.

My Bosch slimline dishwasher instructions show the power consumption varies between something like 1.2kWh and 1.7kWh depending on program (the 30 minute one is the 1.2 and the 2.5hr one is the 1.7) can’t imagine you’d use an amount of water that would be heated by that when doing it by hand but most things you read seem to suggest using a dishwasher is somehow more efficient/ greener, possibly based on water consumption. 

matelodave

No, there are lots of variables when it comes down to washing the dishes - how often you do it, how much water you use and how much water is wasted running the tap till its hot.

Every time I run off the cold water to get it to run hot it wastes half a gallon of water - I've measured it. That also means that half a gallon of valuable heated water then sits in the pipework to get cold until the next time you want hot water..If you've got longer pipe runs then it could be even more.
I've been trying to persuade my wife to not keep taking small draw offs of hot water just to rinse hands or, even worse,to rinse empty yoghurt pots because of the amount of both wasted water and energy.

As there are only two of us we run our dishwasher every couple of days which is roughly equivalent to 6-7  lots of washing up - we could run it at 50c if we really wanted to save money but we don't. 
Obviously others have a different regime but it's worked for us for the past ten years -  some days we use it a bit more often depending on how many dishes we've used or how many people are being catered for.

dllive

Hi @matelodave. I hope you had a relaxing - and warm - christmas!

Each monday Ive been taking my readings and calculating my COP. Ive made slight tweaks, but the COP has largely stayed the same. Basically, the COP for the DHW is ~2.86 and heating is ~1.9. Overall COP is ~2.02. 

So, less than ideal.

My current setup is thus:

- DMH is set at 47 degrees. The cylinder heats back up when it gets down to 40

- Legionella comes on every 14 days, and runs for 30 minutes

- Heating is on a compensation curve with a target of 47 at -3 outside, but Ive reduced the whole curve down by 5 degrees (using the 'big digit' on the Ecodan homescreen)

- I have UFH for 1 zone (kitchen) set at a constant 17. The other 2 zones (dining room and snug) Ive just left in frost prevention mode

- On the rads upstairs I have 12am - 9am @ 17 degrees; 9am - 4pm @ 19 degrees; 4pm - 12am @ 17 degrees. I keep the TRVs at around the 3 or 4 mark

The past few days have been very cold outside, and the heating has struggled. (The DHW has been fine).

What do you suggest? Should I either:

1) Increase the TRVs on the rads

2) Increase the temperatures on the upstairs stat

3) Increase the compensation curve by a couple of degrees

Also, do you think bringing the whole compensation curve down by 5 degrees (using the 'big digit' on the Ecodan screen, rather than editing the actual compensation curve screen) has also reduced the DHW by 5 degrees? (I dont think it has).

I can take photos of anything if Im not making myself clear.

Thanks

matelodave

You could try reducing the compensation curve down a bit more but you may find that unless you keep the heating on longer it'll take a lot longer to bring the place back up to temperature.
It has been pretty cold this last couple of weeks so the unit has to work harder to produce the same flow temperature.

Most heatpumps COP rating are given at 7 degrees outside with a flow temp of 35 = an uplift of 28 degrees (look at the spec it's usually shown as 7/35). Some also give you the COP at around 0 degrees = an uplift of 35 degrees, (shown as 0/35 in the spec) and you'll see that its lower than the 7/35 COP. because it has to work harder to raise the temp another 7 degrees. (
Looking at the Mitisubishi Ecodan specs they quote at A7/W35 and  A2/W35 and it appears to reduce the COP from around 4.5 to 3.5 (check the specs for your specific unit)

 However if you've got weather compensation turned on then with a target of 47 with -3 outside the pump has to work even harder to raise the temperature by near enough 50 degrees which has an even more adverse effect on your COP. However if you dial in say -5 on your weather compensation then  you highest temp will limit to 42 degrees but at an outside temp of -8 which, although is still an uplift of 50 degrees when its everso cold it means when its about 0, the flow should only be about 35 degrees

Some systems have whats called an "equilibrium point" that tell the heating unit to kick in the boos/backup heater when the outside temperature drop below a certain level (which could be about -3 to -5 degrees) so be careful that you don't end up with 6kw of immersion coming on if it's been set - that has a COP of 1.

My suggestion would be to try turning off the weather compensation and set the flow to say 40 degrees for a few days and if it improves and you aren't too cold take it down another degrees or so but be aware the lower the flow temp means it will take a lot longer to reheat the place to the point where if it gets everso cold outside the heat loss will be higher than the heat input into the building so it wont actually heat up. In the end weather compensation is there to try and optimise the lowest flow temperature to offset the heat losses to get the best efficiency so it's a balancing act

Dont get hung up about a low COP at this time of the year. Days when the temp are below zero don't happen all that often and some systems are sized to use a certain amount of boost heater (I've shut mine off) and if it gets everso cold I just tweak the compensation up a bit, but again, if you go above about 50 degrees (say +3 on your compensation) or so then the system might try using the boost heater to achieve the higher flow temp which is what you need to avoid. 

I expect to use lots of leccy at this time of the year, more when its everso cold and less when its not so cold. This is what my energy profile looks like over the past 10 years - you can see when it was cold and when it wasn't (this is my whole house consumption - the heavy consumption in Oct&Nov 2020 was before I had the system set up properly and you can see that Dec 2013 and Jan 2013 were very cold months as was March 2013)

matelodave

BTW I have tried running mine at a fixed 30, 35 and 40 as well as with weather compensation and 30 isn't hot enough when its cold outside so it never turns off but can be ok when its above 10 degrees, 35 is ok when its around 5-10 degrees and although 40 is better it costs more to run it so I'd rather it didn't keep running at that temperature continuously if it doesn't need to.

Weather compensation adjusts the flow temperature so it should be running at about optimum when its set up correctly.

My unit is running at 40 at the moment as its just about zero outside but as the day warms up (assuming that it will) the flow temp will decrease to around 35 and even down to not much over 30 if the outside temp reaches double figures and then wind up again when the sun goes down and it gets cold overnight.

The unit is kicking in an out all night at the moment but it's only using an average of just over 1kw an hour and then comes up again early in the morning when the stats wind up and the water starts to heat - we are using between 40-50kwh a day at the moment and actually got through 60kwh last Thursday when the outside temp didn't rise above -4 all day.

Although 40-50 kwh a day sounds horrendous I'd like to know what the equivalent (kwh) gas or oil consumption would be or even how much leccy an equivalent amount of storage or wall heaters would be using. Also bear in mind that we don't usually use anything like 50kwh a day, even in the winter unless the temperatures are are below or around freezing., Our annual average is 20kwh/day, 8-10 in the summer months and a lot more in the winter (look at the graph above).
We presently pay 12.5p/kwh and so our annual leccy bill is still less than £1000 a year and that's for a 140m2, 3 bedroom detached bungalow which is occupied most of the time (we are both retired) and my wife whinges and moans if she's cold, so I cant get away with really low temperatures (day or night)

dllive

This week Im going to take it off Compensation and set the flow temp to 40 (as you suggest) and see how it affect the COP. Then next monday perhaps reduce by a degree or two.

"... although 40 is better it costs more to run it so I'd rather it didn't keep running at that temperature continuously if it doesn't need to.

Weather compensation adjusts the flow temperature so it should be running at about optimum when its set up correctly."
This is why Ive been reluctant to take it off compensation. But happy to test for a week and see what the COP is if I have the flow temp at 40.
One thing that did cross my mind is to edit the compensation curve; not the whole curve, but just the target temp. So it would be 41 degrees when its -3 outside.

This is the legend on the side of my unit:

It shows the COP figures/stats exactly as youve stated in your post.

Im probably being simplistic here, but perhaps I could set the compensation curve so that when its 2 degrees outside, the flow temp is 35 degrees? (which - in theory - would give me a COP of 3.34). Or, even better, when its 7 outside have the flow temp at 35.

matelodave

Yes, you could try that but you may find that when it's 2 degrees outside and a flow temperature of 35 the heat loss from your house is higher than the heat input but it's well worth trying to get closer to the optimum . It could take several days to see what effect it has but bear in mind it's also affected by the wildly varying outside temperatures which could be between +10 during the day and -5 overnight and something completely different a day or so later.
A lower temp will slow down the heating recovery time in the building so be careful you don't lower it to the point where it can take two days to get the room temperature back up (that why people get upset about heatpumps and underfloor heating, they have a very slow response rate

You need a slight understanding of heat loss and heat transfer - have a look at the following table which may help. It's based on the heat output of a radiator (it doesn't matter what sort or radiator) which give an emission factor based on the temperature difference (called deltaT) between the rad temp and the surrounding air. Most radiator outs are quoted at a deltaT of 50 degrees (which assumes that the rad is at 70c and the room temp is 20c = difference of 50c).
If you run cooler water through the rads then the deltaT reduces so you need to de-rate the radiator output at the lower temp. That's why heatpump systems have much larger radiators than a boiler system - the lower flow temp means that you get less heat radiated for the same sized rad (it looks like about 10% less per 5 degree change) - if your your flow temp to is 45 degrees, then your deltaT will be 25 degrees which will reduce the heat output of a standard rad to 40% of its rated output. Dropping it to 35, your deltaT is now only 15 degrees (20% down)
This may be enough over a very long period or when the outside temp is around 7-10 degrees but could be insufficient at 0 or even 2 degrees if the house insulation isn't everso good and it loses heat faster than you are replacing it. As I said, it's a case of trial and error and it's what suits your system.

Delta T (°C)	Correction factor
5	0.050
10	0.123
15	0.209
20	0.304
25	0.406
30	0.515
35	0.629
40	0.748
45	0.872
50	1.000
55	1.132
60	1.267
65	1.406
70	1.549
75	1.694

matelodave

Have a look at this - the first graph shows the approximate COP changes for flow temps for the Ecodan heatpump - https://heatpumps.co.uk/2014/03/06/getting-the-best-from-underfloor-heating/
This guy has been into heatpumps for at least twenty years so there's lots of good info which might help your understanding