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What causes a geothermal heat pump electricity usage to skyrocket?

  • 08-01-2022 11:29pm
    Registered Users Posts: 1,004 ✭✭✭ poker--addict

    The internet is scarce on information. Pump currently turned off this evening, without intervention my bill is going to hit 1k.

    Supposed to be rated 3.7kw, seems to use about 4.5 when it is not completely losing the plot this week, but last few days I am seeing 6-7, constantly running even with water up to temp. I have banged out a few days of 8-100kw (20e/day) which i can see on a live monitor.

    What settings or issues could drive huge amps or KW usage?

    I am not long in the house so not familiar with system or history. But the usage is clearly abnormal.


  • Registered Users Posts: 11,333 ✭✭✭✭ KCross

    What you have described sounds like an air-2-water heat pump running defrost cycles in cold weather but you’ve said it’s geothermal so it shouldn’t really be affected by cold weather that much.

    is this your first winter in the house?

    at 6-7kW it looks like it can’t keep up with the demand and it’s running the immersion element to make up the shortfall. It’s not really cold enough yet for that

    any errors showing? Is the compressor actually running? Maybe it’s running entirely on immersion?

    ultimately you likely need someone to run diagnostics as its not normal, as you’ve said.

  • Registered Users Posts: 2,971 ✭✭✭ graememk

    Is there a control panel for it, maybe where errors can be shown?

    Do you have a manual or even the model number to look up the manual?

    But for sure somethings not right, Get it looked at, I assume its not a new system then?

  • Registered Users Posts: 1,004 ✭✭✭ poker--addict


    have a manual. Tbh it isn’t much help.

    no errors at moment

  • Registered Users Posts: 11,333 ✭✭✭✭ KCross

    What make/model is it?

    Can you hear the compressor cutting in/out?

  • Registered Users Posts: 1,004 ✭✭✭ poker--addict

    Dimplex, WPM controller, underfloor.

    The logic if i understand it rightly is there is a weather curve which generally drives a temperature of around 29/30 degree water in UFH if the outdoor temp is about zero. The pump heats the water to 2 degrees above the target, say 32 degrees, and pump goes off. Water slowly decreases to 2 degrees the other way, with heat kicking back in at 28. On the main screen you use red arrow or blue arrow to nudge that target up or down. From the automatic target. I have it nudged up 4 squares, so target is about 32.

    Yesterday it ran all day, I am thinking now it couldn't get itself to 2 above target. I saw it above target but it might not have ever hit a full 2 degrees. Most unusual there was decent frost on some of the pipework as a result of the machine while this was going on. It melted super quickly once the pump stopped.

    Today two things happened which mgiht explain why it is now working normally - albeit still drawing 5.7-6+ KW/hour.

    The first one was I played with the curve end point and drove it up to something much higher 35 or 40, and pushed the red arrow up, trying to command as much heat as possible. No idea if that is a good or bad idea, but had to try something. That did seem to drive the water up another bit and then I put the curve back down to normal, and sure enough the target temp returned to normal and the pump turned off when it realised it was now 2 degrees above. This might all be irrelevant though;

    The other thing that happened is the outdoor temp increased from 2 or 3 to 7 or 8, tbh I think this might have been what got the water running a bit hotter(?) and the pump to eventually get itself over the line. So I am wondering if the pump is just struggling at colder temps.

    None of that really explains it drawing 5.9 kw/hour when it is running "normally"?

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  • Registered Users Posts: 2,971 ✭✭✭ graememk

    Also when we say immersion, we mean an immersion for heating your water to heat the house,

    Pulling 5-6 kw could mean both the compressor and immersion running to try and get the heat up like you said.

    I wonder has the compressor lost the gas it uses, (it's as it's Aircon in reverse). So the compressor is doing little to nothing.. and all your heat is coming from a resistive element.

    Well spotted tho. Better finding it now than when the bill comes in.

  • Registered Users Posts: 1,004 ✭✭✭ poker--addict

    I should be able to rule out the pump immersion though by turning the “aux. Pump heating” off? Which I have done in a bid to rule that out.

  • Registered Users Posts: 168 ✭✭ Curiousness99


    Ive a NIBE heat pump geothermal with underfloor heating and have eye watering bills too; (unfortunately I don’t have a great handle on my usage as I’ve only recently being tracking and my high bills aren’t all down to the heat pump

    As to the heat pump it’s in Summer mode now which is supposed to mean it only heats water and I’m seeing 6-8kwh bursts of usage, presumably just to heat the water;

    the water tank which has a capacity of approximately 500litres is warm to touch in a few places which makes me think it’s degraded over the years, it’s about 20 years old now, so that probably increases the electricity usage materially

  • Registered Users Posts: 2,971 ✭✭✭ graememk

    6-8kwh bursts? That's a decent chunk of power, but to heat a 500l tank, which would be about right to keep a 500L tank topped up with an immersion anyway.

    As its a common mixup is do you mean spikes of 6-8kW (power) or actual 6-8kWh(energy)

  • Registered Users Posts: 1,004 ✭✭✭ poker--addict

    My pump is/was set to heat the water to about 41/42 Celsius. Heated to 60+ once a week with immersion.

    It ”burns” about 6kw/hour when heating the water, peaking higher at 7-8kw/hour when water over 40 degrees.

    I figured the KWs burned wasn’t much different to the immersion, so I am currently using the immersion to limit stress on my temperamental heat pump. tank 300 odd litres and probably 10 kw per day to supply us with hot water.

    Pump is off for the summer. Average summer load looks to be 25kw/day.

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  • Registered Users Posts: 1,621 ✭✭✭ bullit_dodger

    I figured the KWs burned wasn’t much different to the immersion, so I am currently using the immersion to limit stress on my temperamental heat pump. tank 300 odd litres and probably 10 kw per day to supply us with hot water.

    If your heat pump is working (assuming it's not lost coolant) and it's 15C+ outside, then heat pumps are waaaay better than a direct immersion. 3-4x times better. If it was me, I'd use the Heat pump to get you up to 40 deg, and then use the immersion to get you the rest of the way to your desired temp.

    Could do it something like heat pump from midnight to 5am. Stop. then have a timer for the immersion to heat from 5:15am to (say) 7am for when people are getting up?

  • Registered Users Posts: 168 ✭✭ Curiousness99

  • Registered Users Posts: 2,971 ✭✭✭ graememk

    A brief overview

    kW : kiloWatts - Measure of power. Eg an immersion will use 3kW when running

    kWh : kiloWatt hour - Measure of energy, or unit of electricity, a 3kW immersion running for an hour uses 3 kWh of electricity (heat can also be measured in kWh)

    kWp: kiloWatt Panel - The number of kilowatts of solar panels you have, eg you could have 7kw of panels on a 6kw inverter.

  • Registered Users Posts: 1,004 ✭✭✭ poker--addict

    You might be right for summer setup where ground is warmer. I haven’t thought about that. My approach makes sense for winter?

  • Registered Users Posts: 1,621 ✭✭✭ bullit_dodger

    Generally heat pumps are more efficient than direct immersion heaters (aka your electric immersion element). Even in winter a well functioning heat pump should get 2.5x the heat energy for a unit of electrical energy. What we don't know of course is if yours is working well or not, and your excessive bills would point to "something" not right, but without more data I wouldn't jump to any conclusions.

    Someone back in the tread suggested taking a note of the meter reading every day - that's a good idea. Then you can try both methods

    a) using the heat pump and "topping off" with the immersion

    b) using only the immersion.

    Take note of the readings every days and you'll have to do it for a week or so on each and then average them out. Yeah, it's a bit of a pain to do/remember every day, but could save you a ton of money a week, so worthwhile