Supporting a smaller ASHP with a backup heater in deepest winter aka "Hybrid Heat Pump"

[Bashers]

I'm looking at DIY fitting a heat pump at home.

Rather than getting an oversized 12 or 14kwh ASHP to cover that one week a year when it drops to below zero, i wish to get a smaller unit that is properly sized to support the house 95% of the year and use the exisitng boiler as the stand in when things get really icy.

A lot of the models from Mitsi, LG and others support this 3rd party boiler / Hybrid function and the logic sound good to me.

However it begs the queston, what happens to the ASHP water loop when its really cold, the gas boiler is now running and the ASHP is offline for days on end.

I know they run a defrost when the condensor coils freeze, and will run a cycle to keep the water circuit above 5 or 10 degrees if required.

However looking at the LG THerma V units, the design prociple is to use a 3 way diverter to switch between the ASHP and the 3rd party boiler - so now its unable to run a defrost cycle, or receive water heated by the gas boiler as its out of circuit??

[ProDave]

My LG simply turns on the water pump periodically to keep the water outside above an undocumented lower limit.  In fact there is really nothing documented about this (very poor) and I have not found any settings to adjust anything about this function.

 

So it WILL expect water to flow when it wants it, from somewhere.

 

It won't run a defrost cycle as the compressor and heat pump is not running.

 

Rather than re invent the wheel, why not let e.g the LG heat pump turn on it's own internal 3kW electric heater if the heat pump is unable to maintain the set flow temperature demanded?

 

It won't be the heat pump unable to function because it is too cold, the problem you might face is it can't get the flow temperature high enough if heating demand exceeds the heat it can provide.

[JohnMo]

Some heat pumps will let you run in full hybrid mode - i.e. both heat pump and boiler on together, other will not not so its one on or the other.

 

Defrost cycle is not frost or freeze protection - very different, defrost cycle would never run with heat pump off line..  Freeze protection comes from freeze protection valves (opening the pressurised system to atmosphere to dump water) or antifreeze, the same as your your car (but a safe version of glycol).

 

[Bashers]

Quote

Rather than re invent the wheel, why not let e.g the LG heat pump turn on it's own internal 3kW electric heater if the heat pump is unable to maintain the set flow temperature demanded?

Whilst thats an easy fix, thats most defintely my most unwanted results really.

Im trying to save money and running a 3kw resistance heater for a week during subzero temps could cost me close to £30 per day at my current 40p peak rate

I think i need to find one thats probably bivalent hybrid mode rather than one that switches itself off when its cold (LG Therma V) and switches in the 3rd party boiler

[ProDave]

1 hour ago, Bashers said:

I think i need to find one thats probably bivalent hybrid mode rather than one that switches itself off when its cold (LG Therma V) and switches in the 3rd party boiler

the Therma V does not switch itself off when it gets cold, it may switch on the resistance heater as well.  That is very configurable. I have mine disabled so it is never used, but my HP is well within the total heating power requirement.

[SteamyTea]

23 hours ago, Bashers said:

Rather than getting an oversized 12 or 14kwh ASHP to cover that one week a year when it drops to below zero, i wish to get a smaller unit that is properly sized to support the house 95% of the year and use the exisitng boiler as the stand in when things get really icy

If you drop the room set point temperature a degree or two, that will still allow the ASHP to perform as normal for a few extra days a year.

 

16 hours ago, Bashers said:

trying to save money and running a 3kw resistance heater for a week during subzero temps could cost me close to £30 per day at my current 40p peak rate

My understanding, which may be wrong, is the the resistance heater does the last temperature uplift i.e. 55⁰C to 65⁰C or whatever.

It does not replace the work the ASHP is doing.

30 quid is 75 kWh, but I suspect your total daily load is 75 kWh.

If so, the ASHP should supply around 90‰ of that, leaving just £3/day for resistance heating.

[Bashers]

3 hours ago, SteamyTea said:

If you drop the room set point temperature a degree or two, that will still allow the ASHP to perform as normal for a few extra days a year.

 

My understanding, which may be wrong, is the the resistance heater does the last temperature uplift i.e. 55⁰C to 65⁰C or whatever.

It does not replace the work the ASHP is doing.

30 quid is 75 kWh, but I suspect your total daily load is 75 kWh.

If so, the ASHP should supply around 90‰ of that, leaving just £3/day for resistance heating.

Thats an acceptable compromise actually. thanks for the advice

[SteamyTea]

3 minutes ago, Bashers said:

Thats an acceptable compromise actually. thanks for the advice

You will need to see if that actually happens, and I am not sure how you would do that for your particular ASHP.

Others may know.

[ProDave]

I am pretty sure the LG puts up a little symbol on it's control panel when the resistance heater comes on.

[Iceverge]

Given the COP of an ASHP is proportional to the temp difference of the coolant and the ambient air is there a case for always doing a little bit of the heating via the ASHP in a hybrid setup. 

 

EG if electric is 3 times the price of gas per kWh then allow the ASHP to operate only when it's beating a COP of 3. (Ignoring inefficiencies etc) 

 

 

 

For instance if your desired flow temp is 40 Deg, your return temp is 20 deg and on a cold day a COP of 3 is reached at 30deg.  Then let your ASHP do 20-30 Deg and the gas boiler do 30-40 Deg for minimum cost. 

 

 

 

 

 

 

[Bashers]

Do you think the heater element is binary, on/off or is it modulated by an inverter as would seem sensible

[dpmiller]

on/off.