Choice of HT ASHP for retrofit

[nowty]

You seem to be trying to replicate what you have with your oil boiler, high temp and short heating windows. With a HP, that may lead to disappointment with both heating performance and your leccy bills.

With HPs you need to change the way you heat otherwise the efficiency and lifetime of the HP will suffer. Oversize your radiators and run at lower temps over longer periods of time.

If its really cold in winter I'm running almost 24/7. At the moment I'm running 5am till midnight and I also need storage heaters to supplement the HPs.

I understand your thinking about having the thermal store but it will create a permanent step change in inefficiency to your system.


From Kensa who I regard highly in their knowledge of heatpumps, although their brand is far too expensive for me.
https://www.kensaheatpumps.com/installe ... fer-tanks/

What about thermal stores ?
The unwary could easily think that fitting a thermal store was a good substitute for both a buffer tank and a hot water cylinder, but when sourced from a heat pump this is not a good idea. Although the thermal store will usually function OK as a buffer, they don’t make good partners for heat pumps as all the space heating load then has to be served at DHW production temperatures – this will typically lead to very poor efficiency and result in high electricity bills.

A buffer tank can also be used for daily energy storage, although currently in the UK due to the lack of viable variable rate electricity tariffs these types of installations are rare. To serve this purpose the buffer will often have to be several thousand litres – finding a suitable location for this in a property without a basement can be a challenge. It also needs to be very well insulated to avoid the heat losses negating any running cost saving. The heat pump runs when the electricity is cheap, heating up the buffer. Later on, when electricity is expensive but heat is required, the energy stored in the buffer can be delivered to the house with only a low wattage circulator pump required to run. Systems like this don’t actually save any energy, indeed they may actually increase energy consumption slightly due to the system losses, but they can, given the right tariff and good design, reduce the running costs significantly.

[sharpener]

I've been looking at a Grant. It seems that you don't actually need the expensive controller you can build your own controls inluding cheap-rate electric, DHW, and the rest.

Yes, things like night setback could be v useful, p42 of installer guide see also user guide.

Have you found any performance curves anywhere @Stinsy? I want to be sure what output I would get at the max flow temp or close to it.

[Stinsy]

I've been looking at a Grant. It seems that you don't actually need the expensive controller you can build your own controls inluding cheap-rate electric, DHW, and the rest.

Yes, things like night setback could be v useful, p42 of installer guide see also user guide.

Have you found any performance curves anywhere @Stinsy? I want to be sure what output I would get at the max flow temp or close to it.

If you're looking for info bare in mind it isn't really a "Grant" HP. They're just the importer, it is really a Chofu HP
(made in Japan) and the Chofu installer guide aimed at the NZ market is more comprehensive.

[sharpener]

With HPs you need to change the way you heat otherwise the efficiency and lifetime of the HP will suffer. Oversize your radiators and run at lower temps over longer periods of time.

I understand your thinking about having the thermal store but it will create a permanent step change in inefficiency to your system.

From Kensa who I regard highly in their knowledge of heatpumps, although their brand is far too expensive for me.
https://www.kensaheatpumps.com/installe ... fer-tanks/

What about thermal stores ?

A buffer tank can also be used for daily energy storage, although currently in the UK due to the lack of viable variable rate electricity tariffs these types of installations are rare What does this mean?????. It also needs to be very well insulated to avoid the heat losses negating any running cost saving. The heat pump runs when the electricity is cheap, heating up the buffer. Later on, when electricity is expensive but heat is required, the energy stored in the buffer can be delivered to the house with only a low wattage circulator pump required to run. Systems like this don’t actually save any energy, indeed they may actually increase energy consumption slightly due to the system losses, but they can, given the right tariff and good design, reduce the running costs significantly.

Thanks @nowty, useful stuff. Actually Kensa seem to support the use of buffer tanks if designed appropriately.

Oil boiler is turned down to 55C as this is sufficient. I already have over-sized rads in the big living room which give quick warmup but there is also the original u/f circuit which can still provide a lot of heat even when the thermal store temp has decayed to ~40C in late evening. Two 300l tanks will store ~18kWh at 60C more cheaply than a Sunamp and I have enough space. And inside the thermal envelope so losses will be partly captured.

Even allowing for the drop-off in COP at high temps the modelling shows I will save by running the HP off the free PV during the day and the cheap elec at night. The HP will in addition reduce the power needed for hw by a factor equal to the COP and and I have just doubled the PV panel area so making more availabe for space heating. If there is a surplus I can use it to heat up the fabric of the building with the u/f.

As a worst case data point the price/kWh I pay for oil is approx 1/3 to 1/4 of peak rate electricity so after the COP of the HP I will not be significantly worse off even if I have to use it at peak rate e.g. to heat the bedrooms in the late evening.

[jonc_uk]

Two 300l tanks will store ~18kWh at 60C

Looks like are assuming 600 litres with a 25 degree differential.. 17.5kWh.

If you need a flow temp of 55C and can heat with your HP to 60C, you have stored 3.5kWh.

I have a 750L TS buffer fed by my Kensa HP which will heat the top of the tank to 55C and I have a floor flow temp of 35C. I still don't consider this an effective thermal store for the heating (unless the Eddi's are diverting power to it). I do pass the hot water feed through it though to pre-heat the water before it gets to the high-temp TS.

[Swwils]

Get the flow demand down to 35C.

Go from there.

Otherwise the oilgas will be cheaper over the lifecycle of the system

[sharpener]

I have a 750L TS buffer fed by my Kensa HP which will heat the top of the tank to 55C and I have a floor flow temp of 35C. I still don't consider this an effective thermal store for the heating (unless the Eddi's are diverting power to it). I do pass the hot water feed through it though to pre-heat the water before it gets to the high-temp TS.

I am struggling a bit to understand the design objective of your TS if not for heating. And why heat it via the Eddi's (I asume with direct heating) when the same energy could via the HP be multiplied by its COP?

[Stinsy]

Frankly I wouldn't fixate on running it at 45℃ other than in the cheap period for your DHW. Just run it at 35℃ and add an A2A if you find that insufficient.

I'm on my 2nd winter running my gas boiler at 35℃. Sure my small rads take a very long time to heat the house from cold, but even when it is -6℃ outside I can hold a steady 19℃ inside.

[jonc_uk]

I am struggling a bit to understand the design objective of your TS if not for heating. And why heat it via the Eddi's (I asume with direct heating) when the same energy could via the HP be multiplied by its COP?

That is my low-temp TS, which really acts as a buffer. I have 3x 750L TS - low / med / high. Each can be heated via HP or gas boiler / CHP and each has 3x 3kW heating elements near the bottom.

Hot water is heated to temp by the high TS, but passes through the other two to pre-heat.

Store heating priority is high --> low for gas and low --> high for HP. (in reality, HP never heats the high TS)
High TS is heated during cheapslot via a timer.

The Eddis priority is the high TS, but dumps to the low TS if the high one is up to max temp.

In operation, the low TS only receives heat via the Eddis in the height of summer, and not that much. It is more of an export limitation mechanism then.

Hopefully that makes more sense!

[sharpener]

I'm on my 2nd winter running my gas boiler at 35℃. Sure my small rads take a very long time to heat the house from cold, but even when it is -6℃ outside I can hold a steady 19℃ inside.

I'm a great believer in weather comp and it's extraordinary it's not better known/promoted as it's so cheap to implement. Pity you can no longer get the 3rd party add-on that Danfoss used to make, cut 25% off my gas bill in previous house. But still want to have a decent flow temp in reserve - it does freeze even in Devon - just not a system quite as complicated as @jonc_uk <g> !