wierd overnight power draw of LG Therma V R32 monobloc 9kW AWHP

[chris47]

I appreciate that the overnight power draw is not wasted but is heating the DHW tank, but my concern is the relatively 'short cycling' damaging the CoP and/or compressor life.

[Dillsue]

If all your scheduling is done within the Honeywell programmer can you turn that off overnight and see if the HP continues starting up? That will rule out the programmer. 

 

If you think it could be the HP topping up the buffer then can you turn the buffer set point temp right down before you go to bed? That should stop the buffer being topped up.

[JamesPa]

8 hours ago, chris47 said:

I appreciate that the overnight power draw is not wasted but is heating the DHW tank,

Is it, or is it heating up the buffer?

 

Given you are in Bristol its most unlikely to be frost protection because its (presumably) not yet cold enough.  You may be able to adjust the frost protection settings to find out for sure, but its equally likely you cant, given that the freezing point of water is the same worldwide and frost protection is a safety measure to protect the equipment.  Certainly my Vaillant, and several other heat pumps the manuals of which I have read, have no provision for adjusting the frost protection settings (neither did my boiler), for the simple reason that there is nothing really that it makes sense to adjust.

 

Im still thinking its most likely the heat pump satisfying the buffer because it has no way to know that there isnt a call for heat.  @Dillsue is suggesting ruling out the programmer, but I think you have already done this.  He is also suggesting turning down the buffer target temperature to detect whether its the heat pump satisfying the buffer.  If you can do that he is right it is a good way to confirm.  Alternatively trace the route from the receiver end of the wireless thermostat.  Does it only switch the secondary pump, in which case what I suspect is the cause is guaranteed to happen, or does it also switch the call for heat contact on the heat pump?  Or just ask your installer!

 

Although you are right to be concerned about the short cycling, probably more significantly long term is your mode of operation.  In most houses heat pumps are most efficient run 24*7 on weather compensation and without a buffer, with all or most of the TRVs and thermostats turned up a couple of degrees above the desired set point.  Depending on what your flow temperature is, the buffer alone will increase your running costs by 15%.  Running at a flow temperature 5 degrees too high and bouncing off thermostats could easily be another 15%.  And, depending on your house characteristics part time running another 15% (unless your heat pump is well oversized in which case you may be incurring the penalty anyway).  Really your installer should come back, replumb the buffer as a 2 port volumiser in the flow, attach the Honeywell to the call for heat from the heat pump, operate it 24*7 then turn up all the TRVs/Thermostats and adjust the WC so it just heats the house.  That's generally a good starting point from which to tweak.  Of course there are always exceptions to the general rule, but in a well insulated house (you say yours is reasonably well insulated) with a reasonably right-sized heat pump the general rule is almost always the right starting point.

 

 

 

 

Edited Saturday at 07:43 by JamesPa

[chris47]

On 13/09/2025 at 08:40, JamesPa said:

Is it, or is it heating up the buffer?

 

Given you are in Bristol its most unlikely to be frost protection because its (presumably) not yet cold enough.  You may be able to adjust the frost protection settings to find out for sure, but its equally likely you cant, given that the freezing point of water is the same worldwide and frost protection is a safety measure to protect the equipment.  Certainly my Vaillant, and several other heat pumps the manuals of which I have read, have no provision for adjusting the frost protection settings (neither did my boiler), for the simple reason that there is nothing really that it makes sense to adjust.

 

Im still thinking its most likely the heat pump satisfying the buffer because it has no way to know that there isnt a call for heat.  @Dillsue is suggesting ruling out the programmer, but I think you have already done this.  He is also suggesting turning down the buffer target temperature to detect whether its the heat pump satisfying the buffer.  If you can do that he is right it is a good way to confirm.  Alternatively trace the route from the receiver end of the wireless thermostat.  Does it only switch the secondary pump, in which case what I suspect is the cause is guaranteed to happen, or does it also switch the call for heat contact on the heat pump?  Or just ask your installer!

 

Although you are right to be concerned about the short cycling, probably more significantly long term is your mode of operation.  In most houses heat pumps are most efficient run 24*7 on weather compensation and without a buffer, with all or most of the TRVs and thermostats turned up a couple of degrees above the desired set point.  Depending on what your flow temperature is, the buffer alone will increase your running costs by 15%.  Running at a flow temperature 5 degrees too high and bouncing off thermostats could easily be another 15%.  And, depending on your house characteristics part time running another 15% (unless your heat pump is well oversized in which case you may be incurring the penalty anyway).  Really your installer should come back, replumb the buffer as a 2 port volumiser in the flow, attach the Honeywell to the call for heat from the heat pump, operate it 24*7 then turn up all the TRVs/Thermostats and adjust the WC so it just heats the house.  That's generally a good starting point from which to tweak.  Of course there are always exceptions to the general rule, but in a well insulated house (you say yours is reasonably well insulated) with a reasonably right-sized heat pump the general rule is almost always the right starting point.

 

 

 

 

Dillsue said  'If all your scheduling is done within the Honeywell programmer can you turn that off overnight and see if the HP continues starting up........'

The Honeywell programmer is for CH only; DHW is controlled by the LG programmer.

 

Rather than running 24/7 I'm timing my start/stops for DHW to match Octopus Cosy tariff that gives me 3 daily cheap rate slots totalling 8 hours @ 12.9p/kWh. The other rates are 3 day rate slots totalling 13 hours @

26.31p/kWh & a 3 hour slot @ 39.46p/kWh. I'm timing my start/stops for CH using all the above slots but avoiding the expensive slot of 39.46p/kWh

During my 'first' winter coming up using an AWHP I will balance the radiator temps using the lockshields (TRV's fully open) & then adjust the TRV's as you suggested.

I also have a Give Energy battery backup which I can charge with solar PV & Octopus Cosy cheap rate to help reduce the running costs of the AWHP.

I think this coming winter is going to be a lot of 'trial & error' & a steep learning curve.

ps

I turned OFF the main AWHP isolator switch Friday night between 01:00 & 07:00 & all the power draw spikes disappeared; proving it was the AWHP & or an internal AWHP electrical heater running that caused these 'spikes'.

I noticed from the installation manual that their is a reference to 2 DIP switches that control an 'electrical heater' but it doesn't say whether it's the buffer tank heater (which I don't have) or some other heater (like the Willis heater referred to earlier in this post).

I shall be installing a wireless temperature sensor in the DHW tank to help determine whether CH or DHW heating is being supplied by the AWHP.

I've sent the initial data from the start of this post to LG support & will report back their suggestions/comments if any!

 

Thanks to everyone for you input & I will post again when I have something to report.

[JamesPa]

@chris47

 

All above makes sense.  I didn't realise you had cosy which does make the picture of how best to operate the heat pump a tad more complex (unless you also have a battery of course)

 

Have you discounted the possibility that the ashp is simply satisfying the buffer then?

 

There is another thread on this forum discussing alternative ways to wire controls when a buffer is present, and the installer in question is advocating (he claims for efficiency) doing it in the way that is guaranteed to cause the symptoms you observe.

 

Given that one installer favours this it's likely that others do, so quite possible yours did.

Edited Sunday at 17:43 by JamesPa

[chris47]

4 hours ago, JamesPa said:

@chris47

 

All above makes sense.  I didn't realise you had cosy which does make the picture of how best to operate the heat pump a tad more complex (unless you also have a battery of course)

 

Have you discounted the possibility that the ashp is simply satisfying the buffer then?

 

There is another thread on this forum discussing alternative ways to wire controls when a buffer is present, and the installer in question is advocating (he claims for efficiency) doing it in the way that is guaranteed to cause the symptoms you observe.

 

Given that one installer favours this it's likely that others do, so quite possible yours did.

I do have a battery - Give Energy 9.6kWh with 3.1kW AC coupled inverter:)

I haven't discounted the possibility that the AWHP is simply satisfying the buffer; I intend fitting a wireless temp sensor to the buffer feed pipe so I can determine if indeed the heating of the buffer is the cause of the spikes.

Do you know the link to 'ways to wire controls when a buffer is present' you referred to above?

[HughF]

Remember, the LG doesn’t have a dedicated buffer sensor, it just uses an internal temp sensor in the outdoor unit to sense flow. Same as the Samsung.

 

On an LG there’s no way to separate the two sides of a buffer system from a call-for-heat perspective (unless you’ve done a terrible job installing it) - the 3rd party room thermostat wires back to the outdoor unit. They’re literally designed to have a room stat by default.

 

Good heat pump, complex and can do lots of things. But a good unit.

 

p.s. On the gen4 there isn’t a heater in the outdoor unit.

Edited Monday at 06:23 by HughF

[Dillsue]

1 hour ago, HughF said:

On an LG there’s no way to separate the two sides of a buffer system from a call-for-heat perspective (unless you’ve done a terrible job installing it) - the 3rd party room thermostat wires back to the outdoor unit. They’re literally designed to have a room stat by default.

 

Isnt the default set up to use the room temp sensor built into the remote controller?? An external/3rd party thermostat, if fitted, has to be enabled with a DIP switch

[ProDave]

21 minutes ago, Dillsue said:

Isnt the default set up to use the room temp sensor built into the remote controller?? An external/3rd party thermostat, if fitted, has to be enabled with a DIP switch

Yes.  I have mine set for external room thermostat input which is actually the call for heat contact from the UFH manifold controller.

 

Mine is an early one and the supplied controller is the most complicated, non intuitive thing I have met, I did not want to be using that to schedule on and off times etc, so I have a perfectly ordinary central heating controller for that, something which most people understand, and the heat pump is purely under control of the room thermostat input.

 

Doing the same for DHW was more of a challenge.  There is no official way to remotely turn DHW on or off.  So my hack was use the same central heating controller so switch a relay.  The relay contact then either connects the thermistor temperature probe to the controller (DHW on) or switches to a fixed resistor chosen to mimic the thermistor value at a high temperature.  So fixed resistor in circuit = DHW off because it always thinks the water temperature is way above the set point.

 

I am still waiting for some clever person to post details of the frost protection settings they found.

 

We all know water freezes at 0C buy my experience is mine starts it's water circulation thing when the water temperature gets down to just 10C  which I think is way too soon and I would love to be able to adjust it to say 5C

[JamesPa]

10 hours ago, chris47 said:

Do you know the link to 'ways to wire controls when a buffer is present' you referred to above?

 

Point 3 on the post by d438a1.  Interestingly triggered a disagreement between MCS inspector and installer apparently
 

 

2 hours ago, HughF said:

On an LG there’s no way to separate the two sides of a buffer system from a call-for-heat perspective (unless you’ve done a terrible job installing it) - the 3rd party room thermostat wires back to the outdoor unit. They’re literally designed to have a room stat by default.

Surely you can just leave out the room stat as far as the LG is concerned, it will then satisfy the buffer based on return temp and its WC curve.  Im not saying this is a good thing to do, just saying that the plumber may have done it if, like the one in the reference above, they prefer to do it this way.

Edited Monday at 08:28 by JamesPa

[Dillsue]

26 minutes ago, JamesPa said:

Surely you can just leave out the room stat as far as the LG is concerned, it will then satisfy the buffer based on return temp and its WC curve. 

That's my understanding of how things can be configured. I'm in the throws of setting ours up and currently have it configured to run and stop on a volt free contact which is the same as an on/off stat. Once all the hydraulics are sorted I'll be moving onto WC(AI in LGs world) with the volt free contact acting as a temperature limiter or to disable the HP for other reasons. I could leave the volt free contact/stat disabled with a dip switch and the HP would just pump water as required by WC/AI

[Dillsue]

16 hours ago, chris47 said:

During my 'first' winter coming up using an AWHP I will balance the radiator temps using the lockshields (TRV's fully open) & then adjust the TRV's as you suggested.

 

Balancing and WC setup can be done now. I wouldn't wait til winter to set things up in case you hit problems.....I couldn't balance the whole house and have had to shut the heating down to add a pump and control for it. Potentially could have been without heat for a couple of days which isn't a problem now but would have been in the winter

[JamesPa]

4 minutes ago, Dillsue said:

Balancing and WC setup can be done now. I wouldn't wait til winter to set things up in case you hit problems.....I couldn't balance the whole house and have had to shut the heating down to add a pump and control for it. Potentially could have been without heat for a couple of days which isn't a problem now but would have been in the winter

Interesting, presumably to boost a high resistance leg.  Was it just plumbed in series or did you employ any separation?

[Dillsue]

The original part of the house had 8mm copper drops from ceiling to floor on 5 rads and I couldn't get any flow to 3 of them without shutting down the lockshield on the rest of the house. A small pump on the 22mm pipe feeding them has sorted that. Still got another rad with a long 10mm run to get flowing but Im confident Ill get that with balancing. Everything is plumbed direct so no separation

[SteamyTea]

TL;DR

 

We looked at phantom power draw a while back with regard to crankcase heaters, has this been ruled out?

[Michael_S]

Our series two has a built in immersion in the heat pump unit in a very small horizontal 'cylinder' which we have both disabled via dip switches and physically disconnected.  There is a downside that this is part of the de-icing mechanism and we did have some issues when there was a 3 day hoar frost earlier in the year.  WE also run a secondary circulating pump due to undersized internal plumbing but have to be careful it does not generate too much flow through the HP during start up as the heat pump sees this as the flow sensor being stuck open and errors out.