Air source heat pump costing loads in electricity.

Hey!teacher! · 12 February 2017 at 11:36AM

I'm hoping someone here might have some ideas for us before we're broke or frozen!
We moved into a house in June last year, it's a 1930s semi but with a new extension making it a 4 bed. 1700sq ft.
Its got an air source heat pump and underfloor heating throughout and it's been great all summer with solar panels for the water giving us plenty of hot water.
But we've just had our bill from September until January and we've used £800 in electricity! It's around 60kwh per day.
I've turned it down to what we can bear, which is on the chilly side, we had open fires last night to take the edge off but without them it was colder than I'd like. (It's been snowing though so has been cold) Its gone down to 46 units a day now but with two loads of washing, a heated airer and about 2 dishwasher loads in 24 hrs. Only tv on during the day, with the odd phone charger etc. Nothing else that could be using that amount.

I just wonder if we can get this down further and be a bit more comfortable. We do have thermostats but the previous owners took them off, the heating engineers
told us most people wouldn't use them with this type of system. I've been told different things about leaving the floor on or off during the day when we're at work for about 10 hours, as it would take a long time to heat up.

We know the house needs better insulation in places plus we've got three old double glazed windows which need replacing soon which will make a difference but I'm just wondering if anyone has any ideas of anything we can try now and if it's worth setting thermostats/controllers back on the individual pipes for each room?

Cardew · 12 February 2017 at 12:53PM

Whilst all properties have different characteristics, as a general rule, with a heat pump supplying under-floor heating(UFH), it should be left on 24/7 in winter.

The lower the water temperature, the higher the efficiency of a heat pump and 30C-35C is typical for UFL.

If your Sept to January figure is for a full 5 months, IMO £800 is not excessive for a 1,700f2 1930's semi with questionable insulation.

matelodave · 12 February 2017 at 1:06PM

We have an airsource heatpump in a 1350 square foot detached bungalow out in fenland. It's got underfloor heating as well and yes, you'll find that the energy bill leaps by quite a bit in the winter but you have to look at it over the whole year.

Our place was built in 1986, has no underfloor insulation, 300mm in the loft, 15 year old double g;lazing and indeterminate insulation in the cavity walls (we know there's some but don't know how good it is)

This is our 7th winter with it and the first couple took us a bit of tweaking to get it working to our satifsaction.

Firstly you really need to understand that it does not work like a normal boiler fed system - to get maximum efficiency you need to get the flow temperature down to as low as possible and then run it a lot longer - ours runs almost continuously. If you try turning it on twice a day then you'll be cold and it will cost a lot to run.

Ours has weather compensation where the flow temperature adjusts to the outside temperature - it's around 40 when it's 0 degrees outside and and about 30 when it's 10 degrees outside. We've also got a a programmable thermostat for each room (probably overkill) and we never shut it down (we did one year and it took two days to reheat the place).

The stats are set to 17 degrees and increase to 19-20 degrees for the rooms when we are using them. (ie Lounge & study, all day, bathroom & bedroom for a couple of hours in the morning and evening at getting up and bed time) Kitchen for mornings, lunch and meal time.

We only heat our hot water to 45 degrees for two hours a day, except once a week when it increase to 60 to sterilise the tank. We have enough hot water for the both of us to to have a decent shower each and for all our other requirements. The tank is well insulated and the water will stay hot enough for a couple of days, so we can actually have a shower on the second day if we want although te last man standing might find it gets quite cool.

What system have you got Ours is a Daikin 11kw Split unit). Has is got an electric booster heater, at what temperature does it kick in. I've tweaked ours so it wont come on at all.

We are all electric and we use around 7000-7500kwh a year, nearly 5500 of which between November and March - this year it might be a bit more because its been a lot colder than than last year which is being reflected in our consumption.

Hey!teacher! · 12 February 2017 at 1:25PM

OK that's great thanks. Its from 11th sept till end of jan. I think we got a bit of a shock as we moved from a 2 bed with gas central heating. I think the ufh temp was set at 55 which I brought down to 25 as I say though this is feeling really cold underfoot at the moment!
Maybe I'll try it slightly higher then. Plus look at new windows...will be about a years worth of electric at this point for the ones we need!

Hey!teacher! · 12 February 2017 at 2:00PM

Thanks that's really useful info. We never switch it off as they told us to just let it run and do its thing. It's a nibe fighter system, as you say it should equal out over the year a bit more but it's just this bill was a shocker! I think we used 7500 approx in 5 months. With a couple more months of cold weather to go potentially that's a worry but as it's a bigger house maybe that's about right. We are out most of the day and have 2 young kids. I've set the heat curve to lower than it was so I'm hoping that will help. I think that's similar to what you've described, it offsets the outside temp. Weve got thermostats but they aren't connected.

Hey!teacher! · 12 February 2017 at 2:07PM

Meant to say I don't know where the electric heating kicks in, need to check as this might be set high. Weve got solar panels for water heating which never seems to run out so in theory in summer we should only be using electric to run appliances etc which should bring it right down.

matelodave · 12 February 2017 at 2:12PM

The optimum flow temp should be around 35 degrees at 7 degrees outside.

It's reckoned that it costs around 2.5% per degree above that so thats a temperature uplift of around 30 degrees. I dunno if yours has weather compensation but our system was designed with a flow temp of 40 degrees at 0 outside with the back-up heater coming on at -5.

As I said I've disabled the back-up heater. Our system definition is a booster heater which is a 3kwh immersion heater in the hot water tank, which comes on on Saturdays to increase the tank temp to 60 degrees.

The back-up heater is a 6kw immersion in the flow circuit to supplement the heatpump when it can't manage the heating load or if it's kaput. That's the one I've disabled. The output will drop as it gets colder so the unit has to be sized correctly to provide enough heat for the load at around -5 degrees that's why the back-up heater kicks in (its called the equilibrium point) and can usually be tweaked if you know how to drive it.

Most heatpumps will be running flat out with a 55 degree flow temperature and not very efficiently - that's where a lot of systems are designed incorrectly, especially those that have just been tacked onto an existing boiler fed radiator system, The rads are not large enough to heat the place at low flow temperatures.

Most systems won't acheive more than around 50-55 degrees, so the back-up heater will lick in, likewise if you have your hot water higher than that then either the back-up or immersion heater will cover the additional temperature.

The Heat pump will mostly do better than a COP of 2-3 and when running at optimum it should acheive 3.5-4 or possible better, whereas the backup or immersion has a COP of one so limiting the amount of time either of them are in operation has to be a good thing

I'd be incline to set the flow to around 35-40 for this time of the year and then tweak it up an down a bit to get it right. Ours has a +/- 5 degree setting which means we can give it a bit of a boost especially on wet windy days when the temperature is relatively high but the heat loss from fro wet brick is higher

Hey!teacher! · 12 February 2017 at 2:26PM

I really need to read the manual again as but it's so complicated, we weren't told anything about the system really but have had two visits out to look at it and they just said leave the settings alone.
I'm going on look again at what the flow is on ours, the display doesnt give much info really.

Cardew · 12 February 2017 at 3:08PM

I don't like to sound a note of caution, but people have posted on MSE that they have exprienced lots of problems with NIBE heat pumps. Do a search and you will find a lot more information.

matelodave · 12 February 2017 at 3:32PM

Dunno which version of the NIBE Fighter you've got but does this help you understand how it works http://www.nibe.co.uk/nibedocuments/8085/031944-1.pdf.

If not have a trawl of the internet to find the manual/set instuctions for yours. Dont forget, small tweak of one thing at a time will give you a much better idea of what has the most effect, rather than faffing about with everything and then wondering what you did.

Check and record your electricty consumption to see what happens - we have an energy monitor checking and monitoring and recording our instantaeous consumption - see here http://www.energyhive.com/dashboard/dave. It's monitoring the whole house so does pick up lights, cooker dishwasher etc but it doesn't take long to understand what's happening and when - consumption overnight is mainly the heating as it usually sits at around 200-250 watts depending on whats left on overnight. Try looking at it in a couple of days when it's a bit warmer and you'll see the difference

I'm guessing that by disconnecting the thermostats then the heating unit may not be optimised. Ours just tried to run contiuously and mainatin a flow temperature until we had the stats installed and configured. It now only runs when a room calls for heat rather than the machine just doing it's own thing.

lovesgshp · 12 February 2017 at 8:48PM

I do not know much about the Nibe units either, but the 55c you stated was way too high for underfloor heating, unless it was around -35c outside.
Your temperature changes could take between 24-48hrs to come into effect on a increase.
Just as a example from a GSHP simulator set at its maximum temp flow for UFH then these figures come up:
Out 15 - 23.2 flow
" 10 - 26.4
" 5 - 29.6
" 0 - 32.8
" -5 - 36.0
" -10 - 39.2
" -15 - 42.4
Obviously, there may have been some other adjustments made in the temperature settings, but the figures above are basics.

Air source heat pump costing loads in electricity.

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