Air Source Heat Pump question - please help!

[thebeefchief]

Hi there,

I'm due to start renting a new build house this week and have been allowed in for the past few days so we can move our stuff in. It's a four bedroom detached house with an air source heat pump system, which I have absolutely no prior knowledge of whatsoever. There's also solar for the hot water, although I don't think that's working properly, but that's another issue.

Over a 24 hour period between 8am yesterday and 8am today, with nothing on but the heating (which had been set to stay on all day) the house used 75 units of electricity. The wee red light on the meter was going crazy. Both the heating and hot water controls were set to "on" throughout that entire period, as I say.

Today I did an experiment and put the controls onto timed, setting them both to go off at 8:30 and stay off until 5pm today. I checked the meter and the light was blinking once every 30 seconds, as opposed to 2 times a second as it was with the pump going.

I swung back at 1:30 to see what was happening and the light was blinking like mad again. I checked the usage and the meter had gone through 13 units - which averages out at pretty much exactly what it was using yesterday when the heating was on.

I checked the pump unit out the back and sure enough it was humming away with the blades spinning, yet both heating and hot water were set to be off.

So my question is - does the pump keep pumping regardless of what heat the controls are set to output, or should the pump only kick in when the timer does? I checked the control panel in the garage and the pump wasn't in defrost mode, so I have no clue as to why it was running when the heating and hot water aren't set to come on until 5:30pm.

Anyone have any ideas? I really can't afford to pay 75 units of electricity a day before even switching on a single light.

 

[stem]

My somewhat limited experience of air source heat pumps (we have them at work for heating the offices, and I am responsible for their maintenance and control) tells me that it shouldn't be running if it's set to off. The outdoor units should be kept powered up as I believe they have an exercise routine or need to keep some components warm or something, but ours definitely sit quietly when they are off.

Some of the controllers have several zones. If so, could one of the zones still be on? also, it's possible as it's a new build that the controls have been incorrectly installed or commissioned.

Air source heat pumps don't produce the sort of heat associated with a boiler, so they are normally installed in extremely well insulated properties. If the building has been empty for sometime, then it may take several days of continual operation to while to warm the fabric of the building up. This could explain the usage at the moment.

Is the air emitted from the outdoor unit icy cold? It should be if the indoor units are supplying heat.

Also, air source heat pumps get less efficient as it gets colder outside. Obviously there is less heat in the air for it to collect. So I was surprised that they are used in Scotland. Ours (in the Midlands) has to be left running 24/7 if it gets below -5 degrees otherwise we can't maintain 21 degrees inside. So you may have supplimentary electric heating elements to assist.

 

[thebeefchief]

Thanks for the reply. It's an air to wet system, which I probably should have mentioned, so going to some big radiators in all the rooms and underfloor heating in the bathrooms.

The heating has been on constantly for the past few weeks (glad I'm not paying the bill) and the house feels warm when you walk in. The house is well insulated, so it should keep the heat well when the heaters are off - but there seems no point in switching them off if the pump outside is going to continue working and costing the same amount of money anyway.

The air from the outdoor unit is definitely icy cold - there's a thin layer of frost built up on the vents around it.

It may well be that there's a problem with how the system has been put in. I'm going to check the final figure for electric usage over the full 24 hour period with the heating on timed, and see what the result is.

 

[muggles]

Do you have a buffer vessel somewhere (looks like a hot water cylinder, but probably smaller)? If so your ASHP may continue to run to heat this back up if it's not up to temperature. If there's an adjustable thermostat on the side try turning it down to see if the pump goes off

 

[thebeefchief]

Do you have a buffer vessel somewhere (looks like a hot water cylinder, but probably smaller)? If so your ASHP may continue to run to heat this back up if it's not up to temperature. If there's an adjustable thermostat on the side try turning it down to see if the pump goes off

There is indeed. The stat is currently set to 40deg. Just had a look and the pump only outputs 8.73kW. Is that low for a 4 bed house, albeit a well insulated one?

 

[muggles]

Depends how well insulated really, it's potentially quite adequate provided the insulation is really very good indeed

 

[dcawkwell]

The exact air source heat pump would help.
Be careful about what temperature they are set to.
Most heat pumps can only get as high as about 50-55 degrees if set higher
they use a normal immersion heater to boost the temperature up.

Check the temperature setting of the hot water tank. It should be set no higher than about 45-50 degrees as the air source will never get it higher.

If there is an immersion heater for the hot water tank this needs to be switched off.

Air source heat pumps should be as economical as a condensing gas boiler.

 

[thebeefchief]

It's a Grant Aerona HPAW85, full user manual available as a pdf here: http://www.grantuk.com/downloads/Grant-Air-Source-Heat-Pump-manual-DOC87-Rev05-January-2011.pdf

I've been playing around a bit tonight and here's what happened.

Came in and light on meter was blinking at 2 times a second. Nothing plugged in and no lights on, so all coming from heating pump.

I went up to the airing cupboard where all the controls are and both heating and hot water were showing as being on (set by timer). I switched these both to "off".

Came downstairs and meter was flashing at about 1 blink every 10-15 seconds. Pump unit out in the garden was silent.

About twenty minutes later I noticed light was blinking like mad again. Went outside and sure enough, pump was working away. Heating and hot water both set to off on the control panel, and I even turned the stat on the side of the storage cylinder right down. It gave a click around 30degrees.

Came downstairs, lights still blinking like crazy, pump still working away outside.

Just to check I flipped the main power switch to the pump off, then back on again. Pump didn't kick back in, and light on meter back to blinking once every 10-15 seconds (with lights on).

Maybe it's just that I need to get my head around how the system works. Why was it pumping when there were - as far as I could tell - no demands being made on the system? That's the bit I don't get.

Can someone explain to me how it's supposed to work? The pump heats the water in the cylinder upstairs, then uses that to heat the radiators and provide hot water - is that correct?

 

[dcawkwell]

The pump should work just like a boiler.

If there is demand it should run and heat it up until
the demand is satisfied.
Make sure you have hot water and central heating both off
at the time.
Set the hot water on the pump should run until the tank is hot it should then stop. There may be a few cycles on and off before the tank is finally hot.

Similarly with the central heating is should run on and off until
the wall thermostat reaches temperature.

Anything else it is wired wrong.

 

[thebeefchief]

The pump has kicked back in twice since I powered it down earlier. Both times as it has kicked in there has been a tiny dimming of the lights. Barely noticeable, but enough to tell me the pump is back on. Sure enough, when I go to check it has fired up again.

Looks like I'd better call the landlord and get him to look at it. There's no way I'm paying for 75 units of electricity a day to run a heating system that's switched off.

 

[SilentDancer]

I think it sounds like the heat pump is undersized. 8.75kW sounds small for a 4 bed house. Expect 10 to 12 kW for a 4 bed detached insulated house unless it has been insulated to the highest standards. If this is the case then the backup immersion will kick in whenever the weather goes cold and the electricity consumption you are experiencing is 'normal' for cold weather conditions.

 

[Norcon]

There's no way I'm paying for 75 units of electricity a day to run a heating system that's switched off.

Heat pumps. Don't you just love em!

 

[SimonH2]

Just a wild idea, but have you asked the landlord ? Though having seen the typical absence of documentation for heating systems I suspect it won't help.

Mind you, the last tenant to move into my flat failed to notice the instruction sheet I'd done and stuffed it into a drawer with the rest of the paperwork. And then wondered why the system didn't match an old boiler manual he'd found