Most efficient way to run underfloor heating

Rodders53

Reed_Richards said:.

You cannot put your hand on the floor and feel the hot areas where the water pipes/tubes are beneath and the cold areas in between,

I so wish that were true, but it is precisely why I don't rate the wet underfloor heating as installed in this house (and it's your toes that find the cold spots!).

matelodave

I think you have to be aware of the limitations of underfllor heating and hopefully it's only installed where appropriate - generally in newer builds with better insulation than say retrofit.

Not saying that retrofit is not possible because thats what we've done but you have to do the sums to ensure that you do get enough heat.

Bear in mind that most radiator systems are designed for a relatively quick heat up period of 1-2 hours rather than the long slow cook that you get with underfloor.

I've worked out my heating requirement for a reasonably well insulated 1986 bungalow - the lounge is 7m x 4m = 28m2 and the heat loss calculation works out at about 100w/m2 when it's -1 outside, so ideally we'd be running our u/f with a flow at around 50-60 degrees.

However it's not -1 or lower all that often and we are able to to run it at a much lower flow of between 30-40 degrees which keeps us toasty warm BUT it runs for a lot longer than a radiator system at 50-60 degrees would. The big advantage is that it's warm all over, not just near the radiators. Our place is pretty draught free but any that there are have been warmed by the floor.

 I could speed up the heating time by winding up the heatpump flow temperature but as we are at home all day we are happy with it idling away with just a couple of degrees of set-back overnight.

Interesting what Rodders says about cold spots - our system piping is spaced at 150mm and if I use an infra red thermometer I can tell where the pipes are as they are a degree or two warmer but generally our floor is around 21-23 degrees and we have piping all over except under the kitchen units, the built in wardrobe, shower tray and the toilet bowls (if you stand too close your toes can feel the cold bits).

Being an overlay system the piping is just under the floor covering rather than a couple of inches down in the slab.(the kitchen bathroom and utility rooms are tiled straight onto the grooved boards whereas the other areas with carpet have 6mm ply over the boards with low tog underlay and carpet. The hall has laminate flooring.

This is what it looks like in the hall - the multiple pipes across the hall feed the three bedrooms


  this is what it looks like now with the floor down and a cover to hide the manifold and heating controllers

Reed_Richards

matelodave said:

Bear in mind that most radiator systems are designed for a relatively quick heat up period of 1-2 hours rather than the long slow cook that you get with underfloor.

You'll have to enlighten me on how this is done. I guess you would have to deliberately over-size the radiators and then increase the heat capacity of whatever heats your hot water to match.  You can find radiator size calculators online for a conventional delta T but I am not aware that they include an over-size factor.  The penalty for over-sizing is that you pay more for the radiators and more for the boiler (or whatever) so if you are an installer wanting to give a competitive quote I cannot see you doing this, or not without explicitly discussing it with the client.

Nice looking hall, by the way.  

dllive

Hi @matelodave
I thought Id give you an update. (Im sure youve been eager!) 
Ive been diligently keeping weekly logs. Unfortunately nothing exciting has happened. It seems whatever I do, I only get a DMW COP of ~2.7 and a Heating COP of ~1.7. The overall COP being ~2 . (although a few weeks was a low as 1.4)

I was hoping - now the weather is starting to get a bit warmer - that the COPs would creep up. But not so far.

Ive currently changed the DMH from keeping the tank at a consistent 45 degrees, to scheduling it to come on for only 1 hour in the morning and another hour in the afternoon (target temp of 45 degrees). I thought that was quite a fundamental change, but it doesnt seemed to have made any difference.

The Heating I have put it back to Comp Curve and reduced by -5 degrees. (See below)

Ive also reduced the legionaire setting to come once per month.


Ive also reduced the stat by a few degrees and turned the TRVs back to about 2 or 3. (I find anything more than that and I get uncomfortably warm).

So Im not sure what else to do! Can I expect the COPs to creep up as we go into summer? Could there be something fundamentally wrong with the setup?

Thanks

matelodave

I'm not sure if you aren't getting a bit focussed about your COP and you should be trying to average it out over the whole year to see how it's performing. A couple or three poor months could easily be compensated by a few good ones.

I dunno about where you live but  we've had a lot of overnight frosts and very cold days for most of April and my energy consumption of 850kwh* was a lot higher than my ten year April average of 515kwh* (in fact the highest April in ten years is 650kwh*) so it's likely that the COP isn't going to be as good as it will be when it gets warmer. In fact a look at my records shows that we've used about  1000kwh* more this year than our average. TBH May isn't looking good either - we've aready used our average for the month.

*Caveat - the consumption figures I've given are my total consumption, so include washing, cooking and other stuff as I can't accurately record the heatpump separately to the main household supply but we haven't really done anything abnormal.

I dont kow how accurate the generation figures are that are being produced by your heatpump and what is being powered through the heatpump kwh meter (does it include controls, pumps and other odds & sods) Does your heatpump have a crankcase heater (many do and they can be usig around 100-120w continuously). It wont contribute much to the heat output but will contribute to the overal power consumption and make the COP look worse. Make sure that the backup/boost or immersion heater isn't kicking in - I can actually isolate both so there's no chance of it happening. 

Here are a some FAQ's which really just confirm that what you are doing is about right https://heatpumps.co.uk/heat-pump-resources/frequently-asked-questions-heat-pumps/

I'm assuming that you've got an MCS certificate and EPC if you are claiming RHI - what sort of figures are being use to calculate the RHI - dont forget that they are annual averages rather than narrow snapshots in the coldest months of the year.

Thanks for keeping us updated, hopefully the more info that we share the more confident we can get at tweaking and optimising our systems.

dllive

Thanks @matelodave . The pipework and Ecodan are in the garage. As Ive said before, its always fairly warm in there. SO perhaps the crankcase heater is always on or something.
Ive got a seperate reading for the immersion heater. I can see it only comes on once a month when the legionnaire purges.

Ill persist! Ill report back if I have anything definitive to add.
Many thanks

matelodave

Thinking about it, running it less wont do much to improve the COP and in fact could make it worse if it's got to work harder to get back to where it was - it's running it at lower flow temperatures and with higher outside temperatures that will have the most effect on the COP.

dllive

Hi @matelodave
Im only keeping fortnightly logs now. Ive had the heating on 'pause' since 31st May; and the water heats up to 45 degrees for 2 hours per day (once in morning, again in afternoon). I may change the DHW back to what I did have it (keeping the tank at a constant 45 degrees) becuase it doesnt seem to have made much difference.

Im using about 80kwh per fortnight (this includes ALL electricity the house is using), but the COP is still only around 1.8

Ive just checked the logs, and its saying - over the past fortnight - its consumed 28kwh for heating; and delivered 0kwh.

Why would this be? I have the heating on pause! The immersion hasnt been on. Is there a reason?

Thanks

PS
I copied your idea of having a radiator cover over the pipework! Mine is under the stairs, so I didnt have to do it, but its certainly made the understairs a bit tidier, and I now I have a handy shelf!

shinytop

@dllive, that's only 2kWh/day.  That could easily be the background consumption for the system.  There are a lot of pumps, CPUs and other stuff that will consume power all the time.  The metering on the system may include all background consumption in the heating number, even when it isn't delivering any heat.  My heating COP in June is 2.75 but in April (which was much colder) it was just over 3.  That makes sense because the heat delivered in June is much less but the background usage is still there.    

You mentioned the ASHP is in the garage - is that correct?  I would have thought that would mean a cooler garage and a less efficient ASHP? 
   

matelodave

A lot (if not all) air sourceheatpumps have a crankcase heater which stops the refrigerant from freezing or the oil separating out of the refrigerant.

This heater seems to be on all the time - if you've use 28kwh in 14 days, thats about 2kwh a day or 80-100watts for the crankcase heater.(which seems quite good as mine appears to be around 120w = nearly 3kwh/day). The only way I've found to turn it off is to shut the whole heatpump off). I guess that the power consumed by the heater is calclated in the in SCOP but probably not in the raw COP and wont be taken into account in the heatmeter so you'll always use a bit more input energy than the heat output suggests.

Try doing your sums but discounting 2kwh/day for the crankcase heater plus a bit more for any pumps or stuff is powered through the heating kwh meter - My guess is that you could easily find that between 2-3kwh a day of leccy isn't actually being used to generate heat but to run pumps, controls and the crankcase heater.

I'd not get too stressed about the calculated COP, unless you are using a calibrated heatmeter I'd take the heatpump reading as a bit of a random guess rather than gospel..

A heatmeter calculates the amount of heat produced by meauring the input water temp, the output water temp and the volume of water flowing between the two sensors, however most units are set to measure clean water and if your system has inhibitor or antifreeze incorporated then that will introduce errors unless the heatmeter has been addjusted to compensate for it.

My heat pump is set to heat my tank for two hours but it only runs for between 1-1.5 hours and consumes anywhere between 1.5-2kwh.

We dont use a great deal of hot water as we've got an eco-shower head (about 6lpm) and neither of us spend more than 3-4 minutes in the shower (I can do it in less than 2). Dishes, except pots and pans get washed in the dishwasher. The rest gets wasted by my wife rinsing her hands and recyclable pots under the hot tap. She seems to be totally incapable of using cold.