At a house temperature of 10C there won't be much thermal energy in the circulating water to make defrosting quick and easy. I wonder if the heat pump is both trying to generate some heat for the defrosting and clear the ice at the same time and consequently using a lot of energy to achieve this with difficulty. A higher minimum temperature might fix this problem. Does the Grant documentation recommend a minimum set-back temperature?

Agree with the above.

Grant has historically provided kits for installers enabling them to treat the install more or less like a boiler, with obvious potential consequences.  The underlying chofu heat pump, and in particular it's controller, is apparently pretty good but the grant overlay can, in the hand of a poor installer, cripple it.  Are you running it more or less 24*7 on weather compensation without external controls, or have you got external thermostats, trvs and like paraphernalia.  If you have this may well be contributing to your high costs.  Also grant have in the past included a low load header, essentially a small buffer tank, in their installs, another source of probable inefficiency and, I suspect, possible problems with defrost.

@jamespa One problem has been that we moved into this house after the installation was done and received minimal documentation. Only today through this forum have I found the detailed installation manual and am working my way through it. I need to work out if weather compensation is even enabled to be truthful. We have one CP4 programmable thermostat for setting the room temperature. I am reasonably sure we don't have a low loss header as I am presuming that is installed indoors and I cannot see any such unit.

@johnr As I mentioned in my reply to JamesPa below, I have only found the detailed install documentation as we were left minimal information about the installation when we moved in. I cannot find any recommendation for a minimum set-back temperature but will be contacting the installer tomorrow and will ask. What would you recommend? Would increasing it to 14C be advisable. Or higher?

@davidm06  I think 14C is better than 10C as it keeps a bit more heat in the house. I recall, without checking the documentation, that Vaillant defaults to 15C.

Have you searched Grant's website for their manuals?

@davidmo6 When we had the gas boiler we used to set the house to 15C and it took a day to warm up on the last day of our holiday. Now with the heat pump even though Vaillant’s default is 15C we set it to 16. In fact it took 2 days to warm to the 20C we wanted. Our house ha a lot of thermal mass whereas yours may not.

Do ensure you do a legionnaires cycle before you get back that is default on the Vaillant absence mode but you’re obviously doing it manually. 

39kWh a day is low in these cold snaps, our two highest were 48 and 52 on our 7kW pump. IF your pump is correctly sized then you will need more than that.

Thank you so much for the advice. We will change the base temperature. I set it to 10C as we were away for 3 weeks.

On another note as you mention the legionella cycle , when we had our Altherma, heating the water up was all automatic. I had asked our installer when in the week this happened on our install but they didn't reply. I realise now that this is an extra piece of kit with the Grant which we don't have and so it has probably never happened as I doubt the previous owners ever did this manually. Strikes me as rather bad practice especially if like us, we were not involved with the installation and we're never told anything about what to do!! 

Posted by: @davidm06

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Thank you so much for the advice. We will change the base temperature. I set it to 10C as we were away for 3 weeks.

On another note as you mention the legionella cycle , when we had our Altherma, heating the water up was all automatic. I had asked our installer when in the week this happened on our install but they didn't reply. I realise now that this is an extra piece of kit with the Grant which we don't have and so it has probably never happened as I doubt the previous owners ever did this manually. Strikes me as rather bad practice especially if like us, we were not involved with the installation and we're never told anything about what to do!! 

The Vaillant Arotherm R290 does the legionella cycle natively, ie using the heat pump itself. 

Most (possibly all) R32 heat pumps dont, they use the immersion heater instead.  Im pretty sure that the Grant fits into this category.  I have no idea how it goes about triggering the immersion, but it probably needs some sort of relay (which could be wireless) connected to a contactor on the heat pump control.

The need or not for legionella protection is debatable (or at least debated) unless you are vulnerable, but Im pretty sure that MCS requires that it be provided and you are thus entitled to it.  

Posted by: @davidm06

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We have a Grant Aerona3 R32 17kW installed in 2022. I have some concerns as our costs are very high and out of line with our experience with a Daikin Altherma installation that we had in Wales for 13 years. It seems to related to a defrost mode as when the outside temperatures fall to below zero, the pump starts working like mad continually.

Hello @davidm06, Grant 10kW owner here. We experience the same, the Grant Aerona R32 is a defrost-happy unit, one which will defrost every 40-50 minutes automatically, with no user-configurable parameters to adjust this. At that frequency of defrost, our unit runs pretty much flat out drawing 2.8kW almost continuously, and struggling to reach target leaving water temperature. The power consumption massively increases when it is regularly defrosting.

Our heating system is not short of volume, but frequent and repeated defrost cycles impacts the unit's ability to provide sufficient heating to our home in lower temperatures of around freeing point. We have enabled and pre-set the 3kW immersion heater in our low loss header to energise during defrost, and also during initial system start-up. This consumes more power of course, but does mitigate the tardy performance of the unit when it is continually defrosting or from a cold start. Realistically speaking, our 10kW ASHP becomes a 7.5kW ASHP when defrosting regularly, which is inadequate to heat our house when the outdoor temperature is below freezing. The kW capacity of Grant Aerona R32 ASHPs reduces in sub-zero ambient temperatures, exacerbated by loss of heat caused by defrost cycle disruptions to the unit operation.

As others have mentioned, the energy used for defrosting the evaporator coil comes from the hydronic water circuit, if this circuit is kept at a very low temperature, or too many radiators and zones are closed, reducing the circuit volume, there's insufficient heat energy can be drawn from it to achieve a quick defrost. 

That's a very clear explanation and not exactly a ringing endorsement for a Grant Aerona!! Thank you.

Hi, ours is a 13kw system.

Are you talking about heating the home or DHW when you say 10c ?

Or is that outside temps