Just some thoughts on personal experience after 4-5 months with a Valiant Aerotherm 7kwh.
On cost, I’ve tried Cosy and Agile. Cosy is proving more economical in colder months. Below cop 2.8 and average outside temp 5deg or below (14.5 Deg temp difference). That’s typical for Cambridge area during winter months where I live.
Assuming 95% gas efficiency and adding back gas standing charge saving Cosy on my system is always cheaper than gas.
At recent -2 to -4 degree days my system has struggled to maintain target 19-20deg indoor temp. This due to 2.1 cop and likely system slightly undersized. On coldest days gas supplied 180 kWh but ASHP gives 150. Solution being looked at are heat exchanger ventilation fan in utility room instead of intermittent extraction and possibly turning down/off emitters in unused rooms - this will mean they act as cold emitters but better to put limited heat where it is needed and then reheat these rooms back to normal once cold weather passes . We’ll see if this works.
If average cop is sum of daily cop divided by number of days this gives different result from total heat over period divided by electric input. Latter is truer figure as it gives true weighting to winter months when most energy is used/produced.
My system cycles at 1.3 hours per cycle.
Using Havenwise to optimise system and seems to be working for me but early days
Interesting that your 7kW unit hasn’t been able to keep your house at your set point temp during the cold snap. Can you provide more details about your house build?
150kWh heat produced over 24hrs equals to just over 6kW constant output, your 7kW unit should be capable of 7kW constant output in -3C conditions and constant defrosts. What is your heat curve and indoor temp set point set to? Which room temp mode are you running (Active, Inactive, Expanded)?
I’m very intrigued about Havenwise, you’re the first example of anyone using it with a Vaillant ASHP that I’ve seen. Could you detail more about how it works for you?
1902 Edwardian semi with 5 bedrooms. External insulation on two thirds, vacuum glazing, underfloor insulation. Key comparison is 180 kWh max on gas versus 150 kWh heat pump - see attached snapshot on cold day. Other graphs show tail off in unable to meet target around -3deg.
Solution is to reduce heat loss by targeting incremental improvements - front of house walls would be best step but hassle of internal is step to far at moment. Hence maybe not heating unused bedrooms for just coldest days.
Havenwise means I can forget flow temps etc and also optimises load shift for best economics plus helps grid meet peak demand.
Interesting. Do you know the approx total floor area of your house?
Your electrical consumption seems very high; my house is a 4-bed 145sqm Victorian semi with uninsulated solid brick walls on 2 out of 3 elevations, shitty ancient double glazing on the front elevation, and my 7kW unit consumed 46kWh total on the 12th Jan to keep the whole house at a constant 21degC, at a combined CoP of 3.5 including 250L DHW to 50C.
Is it possible the Havenwise optimiser is calling for un-achievable flow temperatures, causing the ASHP to enter a defrost death spiral?
@martin53 I’ve moved this here to keep the electrification of heat thread on topic as that’s a thread specifically for discussing the results of the electrification of heat heat pump trial.
Hi Ringi, could you give any examples of what you mean by this description?
I agree that using direct electric heating - an immersion for DHW or a fan heater or convection heater for the space - is probably the fastest and cheapest way of bringing the temperature up to spec. But may be it’s worth seeing if the heat pump can be persuaded to behave better?
The effectiveness of the heat pump is determined by the maximum input: 3kwh in my case. The rated output 7kwh (or in practice 20% more the installer says!) is just a calculation based on assumed cop. On coldest days the cop is 2.1 (and similar for many that I have seen at this outside temp). So 3x2.1x24=150kwh per day at best.
Yes, a mini block or immersion on coldest days would help close the gap from 18.5 Deg average to desired 19.5-20.
I’m going to try prevention first so better insulation and heat extraction ventilation in utility room to turn it from cold to warm room which also means shared walls do not suck heat from adjacent rooms.
Not heating unused bedrooms on coldest day may also help as aim is to find that extra 20% heat which will close the gap to 180kwh per day.
More disruptive step/expensive is insulating internal wall on front of house with aerocell or similar but that’s down the road - my wife is not ready for that
On the bright side, the ASHP is supposed to not be oversized and so reduced cycling so it’s definitely achieved that!
Initial price comparison (for my system) are Havenwise cheaper than my attempts at load shifting. Cosy cheaper than Agile on most winter days. Cosy cheaper than OVO heat pump plus as get time of use on all electricity not just heat pump.
So maybe Havenwise with Cosy in winter and Agile in summer but that’s down the line.
It seems like your system is operating at a very low CoP during design temperature periods. Do you have a buffer? There could be distortion happening if secondary pump flow rates are not balanced.
As I mentioned previously, your power consumption is very high for the typology of house you’ve described.
What is your weather compensation curve set to? Do you have an idea of how the Havenwise system is actually attempting to control your ASHP, in terms of flow temperatures and how it responds to your Cosy ToU tariff?
It could be that Havenwise is ramping up the power during the cheap periods, inducing frequent defrost cycles, after which the heat pump attempts to ramp up again, causing ever more frequent defrost cycles from which it can gradually no longer recover. This is a defrost death spiral, and leads to hugely reduced CoP, which you are seeing.
Yes, you may be right. I was thinking that just short term to meet heavy demand it might bridge gap until outside temperature rose again. So the heat going directly into the rooms that I wanted warm would more than make up for the leaking of heat to the lower temperature rooms which would only be minority of the house but we’ll see.
My house is 216sm Edwardian with 16500 kWh heat and hot water demand per year, so not great but not excessive.
Cop versus temperature difference shows steady linear decline so no defrost downward cycle that I can see.
