Most efficient way to run underfloor heating

dllive

Hi
I had underfloor heating (running from an air source heat pump) installed last year. It covers 3 zones (lounge, kitchen, snug) downstairs, so is quite a big floor area.
Ive had it on a couple of times. What the most economical way to do it? Should I leave it on all the time at ~20 degrees? Or only when its needed. It takes about 40 minutes to start heating up, so by that time Ive usually finished whatever it is Im doing downstairs.
Perhaps someone else has a similar setup and knows the most efficient way of running it?
Thanks

matelodave

As you have discovered U/F heating is very slow to respond, both heating and cooling. In fact it works more like a storage heater so hoping that it will respond like a normal boiler will not be everso effective.

The ideal way, especially if you've got a heatpump is to set a background temperature of around 17 degrees and just increase the temp during the times that you want it higher taking into account the lag between getting up to temp and cooling down again.

Letting the place get stone cold overnight or during the day and expecting it to recover within a hour or so doesn't work.

What is your flow temperature - our flow is set to between 30-40c (weather compensated) and the programmable room stats (we've got eight) are set to the desired working temps for the times that the rooms are in use eg, lounge 0800-2200, bedroom, bathroom 0700-0900 and 2000-2200, study 0800-1700 (taking the lag into account, although the stats have optimum start which take this into account). We use around 19-20 as our room stat setting because the heat comes up through the floor we dont need it set as high as if we had radiators - even the draughts are warm.
They then set back to 17 degrees for the rest of the time. The hall and spare room are set to 17.

This allows the heating to do it's own thing including coming on overnight if necessary - we dont turn it off just down.. If we let the place get stone cold it can take 24-36 hours for the temperature to recover. We could increase the heatpump flow temps but the efficiency drops and the consumption increases so the lower you can keep the flow temp the less it costs to run (its reckoned that for every degree about 35c flow temperature increases the consumption by 2.5%.)

Heatpumps dont heat above 50-55 degrees, so to get your hot water or flow above that will need the the back-up/boost immersion heater which will cost you dearly.

It took me some time to tweak the temperatures to get it just right - leaving it a couple of days between adjustments because of the slow response. It's also worthwhile taking lots of leccy meter readings (ideally daily, when you get up and when you go to bed) for a few weeks so you can get a feel of how much energy you are using and how any adjustments affect your consumption as well as your comfort.
Lastly, make sure that you are on a decent tariff, IMO and e7 tariff is not ideal as you are using energy all the time, - we pay 12.25p/kwh and get through an annual total of 7200kwh of which about 3500 is for heating & hot water via our heatpump.

dllive

Hmm. Starting from a room temp of 14 degrees, Ive set the target at 19 degrees. That was 1.5 hours ago and is currently showing 15 degrees!

I dont know what the flow temp is set at. Would this be on the main wall unit? I have a mistubshi. I think the engineer left it at the default settings, he said it was fine for the majority of cases.

I feel a bit let-down by the installers. I got no users manual. The only thing they gave me was a manual with a load of cicruit board diagrams in it!! Ive had a play with the main unit, but I dont really understand what Im doing or what the most efficient way to use it. The reason why Ive only had it on a couple of times is becuase the electricity bill was massive! I bought the bloody thing thinking it was fuel efficient!

Youve written a very helpful post with some great tips. (I wish the installers had told me that!). Im going to have a look at excatly what system I have and what the flow temp is, and then I may come back and ask a few more questions if thats ok.

The water is fine. We get hot water 24/7. And the radiators upstairs are all working fine. I just turn them on/off as I need them. Its just the UFH that I have no idea about.

Thanks

dllive

Ive just looked-out all the gubbins I was given. I have a Mitsubishi Ecodan. Ive got the installers manual (hence all the circuit diagrams!) and the MCS certificate.

2 hours on. Current temp is showing 15 degrees (still). 

matelodave

Bear in mind at this time of year you will use a lot more leccy than in the summer. Mine uses around 2kwh a day for our hot water in the summer (we have it set to 45 degrees) but in the winter the system can ramp up to quite a lot (the worst I can remember was about 60kwh in one day) The worst month was January 2013 when we used a total of 1500kwh in one month (thats an average of 30kwh a day) - look at the graph of my consumption over the past 10 years. My system is very sensitive to outside temperature. the really low months are when we've been on holiday But we have to set the stat sto holiday mode to ensure the system comes back up at least 24 hours before we come home otherwise it can be very cold 


Have a shufti on the internet to download a user manual - see whats here that might help https://library.mitsubishielectric.co.uk/pdf/directory/heating/sales_literature/homeowner_guides  
I suspect that if you've got lashings of hot water and the upstairs rads are quite hot then you might be using the back-up heaters which is a very very expensive way to run it. Which Ecodan system have you got (model no) Mines a Daikin and I've been quite happy with it for the past ten years.

Has your manifold got it's own pump and bypass and are there manifold actuators fitted.What does the thermostat operate - the heating unit or just the manifold pump and actuators. Understanding how it all goes together makes a lot of differnce in working out the best way to set it up and tweak it (it took my a whole winter to get it running to my satisfaction)

matelodave

have a shufti at this https://www.youtube.com/watch?v=5Im3-LlqmHk there are lots of videos and other info on t'interweb to help you understand how to tweak the heatpump and what some of the setting mean
Here's quite a good one on the theory https://www.youtube.com/watch?v=m2-_x0XZUSM

dllive

Great - thankyou @matelodave ! I shall take a closer look at your posts tomorrow.
UPDATE: Another 2 hours on, current temp showing as 15.5 degrees. Getting there! 

Mickey666

Sounds like you’re learning how your particular UFH systems behaves.  They can vary a fair bit depending on the depth of screed and the flow temperature, but as pointed out they are basically a large storage heater and will always have a significant time-lag compared to a conventional radiator system.
I allow 4 hours for our UFH to work to do it’s thing, meaning I have it timed to come on at 4am so the place is comfortable by 8am.  Conversely, it is timed to go off at 6pm yet the place remains warm up to around midnight.
But that’s just what suits me - everyone will have their own specific requirements; the main thing is to factor in the time-lag.

Mouldymeat3

We have the smallest 5kW Ecodan installed 2013. There are 6 zones. The control module, now, is demanding 36C in Eco mode with an outside temp of 7C. The commissioning engineer said 'I Know you will experiment but don't treat it like a boiler'. In the end, after much chasing the thermostats, I let it do it's own thing. It's a bit annoying when a bit of solar gain brings the temp up in the lounge only for it to fall again so that t'stat is set at 24C just to keep the flow up and 22C in the kitchen, 18C in the bedrooms. Even with a low tog carpet, it is very slow to respond.

Even a slightly warm floor gives a perception of heat, a bit like the ceiling heating we had in our first house. I should add that our tiny bungalow, 64m^2, was designed around air-source, it was a condition of our planning consent. We chose not to use it for hot water; we have an under-sink heater and a dishwasher. A 4 minute instant shower is about ten pence each.

Transfer of heat is is related to the temp difference so a warm room will drain less heat from the floor. Our annual consumption is around 3800 kWh, nearly £500 on our current tariff.

Also be aware that the garden unit will have a crankcase heater of a couple of 100 watts in standby mode, adding 2 or 3 kWh a day when idle.

dllive

Ive just been downstairs. The current temp is showing 16.5 degrees, but the target temp I set yesterday at 4pm.
So, nearly 12 hours later, it still hasnt reached its target temp. Perhaps because its been a cold night?

matelodave

I think you need to find out where your thermostat or sensor is - are you just using the the controller on the heatpump or is there a separate room stat.
It is also worth checking what the timers are doing is the unit allowed to run continuously or is is it timed, if so what timings are set just in case it shut down overnight and so you haven't actually been heating all night.

What size heatpump have you got, I found that when I was researching ours there was a tendency for the Ecodan suppliers to offer a smaller unit than I though was appropriate (ours is an 11kw Daikin but I did get two quotes for 8kw Ecodan and even one for 5kw) My own heat calculation reckon we needed about 10kw for our 140m2 bungalow.

Look at the balancing as well, if upstairs is toasty warm and down stsirs isn't, then it might be worthwhile turning the upstairs TRVs down a bit just in case they are diissipating more heat than the heat pump is delivering and starving the underfloor system downstairs. Does the floor seem warm to the touch. If you've got carpets, have you got low tog carpet and underlay.

Also check on your hot water timer - most heat pumps dont heat water and supply heating simultaneously so ensure that water is being heated outside the main heating period  - we only heat ours for an hour or so a day and get more than sufficient hot water for our needs - there's usually no need to have the hot water set to continuous.

Our u/f heating isn't buried in concrete, its an overlay system which is  a lot more responsive but, as I said earlier, ours can take 24-36 hours to get the place up to temp if it's left to get stone cold and still takes a fair while to increase from 17-19. That's why we don't let the place get cold. The bathroom and kitchen are a bit more responsive as they are tiled but the other rooms are carpeted which slows it down