UFH to be connected to ASHP ?

[Fintray]

Don’t you want the delta T to go down so the heat pump isn’t working so hard ? Obviously you need a certain temperature differential to drive heat in to the house.

Moxi

The DT across the radiators is only c. 2C just now and they say the ideal is a DT of 5C so an increase of the differential but whether that is better I'm just not sure.

[ecogeorge]

I was under the impression that delivering a dT of 5C was required in order to minimise cycling of the heat pump - is this not the case ?

I probably does and I could possibly get away with a smaller heat pump in all but the coldest weather. My heat pump ran for over 300hrs without cycling once during last month and up to 11:00hrs today has been running 60hrs without cycling (0.1C avg. temp. and cop of 3.78), SCOP since March 9th 2024 4.12.
The flow through my heat pump is usually 33l/min which I intend to reduce to see the effect it has on DT and cycling.

Why reduce flow ? what are you hoping to achieve by doing so ??
George

[ecogeorge]

At previous ecogeorge towers heatpump (gshp) ran ufh at approx 34 c to achieve comfortable room temp (20-21c) any lower and house was cold in cold weather.
Did however have a 200 litre buffer tank in series cos wood burner and solar input.
George

[Fintray]

I was under the impression that delivering a dT of 5C was required in order to minimise cycling of the heat pump - is this not the case ?

I probably does and I could possibly get away with a smaller heat pump in all but the coldest weather. My heat pump ran for over 300hrs without cycling once during last month and up to 11:00hrs today has been running 60hrs without cycling (0.1C avg. temp. and cop of 3.78), SCOP since March 9th 2024 4.12.
The flow through my heat pump is usually 33l/min which I intend to reduce to see the effect it has on DT and cycling.

Why reduce flow ? what are you hoping to achieve by doing so ??
George

This.

[Marcus]

I probably does and I could possibly get away with a smaller heat pump in all but the coldest weather. My heat pump ran for over 300hrs without cycling once during last month and up to 11:00hrs today has been running 60hrs without cycling (0.1C avg. temp. and cop of 3.78), SCOP since March 9th 2024 4.12.
The flow through my heat pump is usually 33l/min which I intend to reduce to see the effect it has on DT and cycling.

Why reduce flow ? what are you hoping to achieve by doing so ??
George

This.

I'm only familiar with my own ashp (grant aerona 3 10.5kw), and I don't know if other makes regulate in the same way, but I can describe what this one does if it helps:-

The Grant runs with a fixed water flow rate (ideally 30l/min for the 10.5kw grant), and actively maintains a Dt of 5C when running 'flat out', which defines the heat output as 10.5kW (specific heat capacity of water 4200 joules/litre × 5°C = 21,000 joules/litre, × 30litres/min (0.5L/sec) = 10,500 joules/second).

If set to heat the ufh at a fixed flow temp of 25°C, it will ramp up to a Dt of 5°C, then run flat out until the flow temp reaches 25C plus 'hysteresis' - in practice, about 26.5C. It then throttles back to a Dt of about 4C, and stays there until flow temp gets to 26.5 again, then throttles back again to a Dt of 3C - and so on.

I'm not sure exactly when it starts cycling - it could be Dt < 2°C, or it could be a minimum compressor power - 400w perhaps.

I did experiment with turning the water pump down to 24 L/min to see if it worked more efficiently, as that's matched better to the ufh flow, but when running 'flat out' it then tended towards a Dt of 6°C

[Fintray]

Thanks for that info Marcus gives me something to think about.

[newtoallthis]

OP reporting back.
We have had discussions with three potential suppliers, all of them confirmed that we are entitled to the government subsidy.
We have selected one of them and the process has started.
The plan is to install a Vaillant 5KW machine, it will be connected to the existing UFH manifold (which will have all its controls disabled) and hence the
unit will operate all zones simultaneously.
We have a desire to install a very low temperature radiator in the hallway as it would be nice to have a source of heat we could use to warm coats, hats, gloves and scarves during cold weather - it would also add a bit of extra volume to the system to help when it has to defrost itself.
The heat pump will be connected to the existing thermal store where it will heat the stored water which in turn heats the DHW via very large coil.
I anticipate that the installation period will be early Summer as we are away a lot over the Spring so I won't really know how well it is performing until November or Descember next winter.

[sharpener]

OP reporting back.
...
The heat pump will be connected to the existing thermal store where it will heat the stored water which in turn heats the DHW via very large coil.

So you are planning to keep the whole TS at 45C (or more depending on yr view of Legionnaires' disease risk)? Presumably this will be done on a separate circuit via the usual DHW diverter valve, or the UFH will get supplied at the same temperature which will give you a lousy CoP. With 5kW to play with this may take quite a long time during which you will have no heating. Maybe this will not matter if the time constant of the heated slab is long enough. Fortunately you will not have to heat the whole 500l from scratch, just replace the heat you have drawn off during the previous 24 hours.

Or have I misunderstood your plan?

[newtoallthis]

OP reporting back.
...
The heat pump will be connected to the existing thermal store where it will heat the stored water which in turn heats the DHW via very large coil.

So you are planning to keep the whole TS at 45C (or more depending on yr view of Legionnaires' disease risk)? Presumably this will be done on a separate circuit via the usual DHW diverter valve, or the UFH will get supplied at the same temperature which will give you a lousy CoP. With 5kW to play with this may take quite a long time during which you will have no heating. Maybe this will not matter if the time constant of the heated slab is long enough. Fortunately you will not have to heat the whole 500l from scratch, just replace the heat you have drawn off during the previous 24 hours.

Or have I misunderstood your plan?

The plan is to heat the underfloor slab with a feed water temperature of something in the range of 30-35C and then to use the DHW diverter valve to heat the TS with a feed water temperature of around 60C.

With a 5 KW heat pump, the TS will typically take around an hour to heat a days worth of DHW.

The slab is so massive that it takes many,many hours to cool down and hence I have no concerns about the house cooling down to an appreciable extent. As an example, the UFH was last on at 06:00 yesterday, it has not been on for a little over 25 hours and yet the house is still pleasantly warm with the invidual room temperatures not obviously changing - and as I sit here looking out into the garden I can see that its covered in frost.

The slab takes a lot of energy to heat it up, but then it takes a long time to cool down - this is going to provide a challenge for optimisation but I'll worry about that when the time comes.

[sharpener]

Sounds good.

The slab takes a lot of energy to heat it up, but then it takes a long time to cool down - this is going to provide a challenge for optimisation but I'll worry about that when the time comes.

With such a long time constant, as long as/to the extent that the loss is only 1/3 of the capability of the HP then you would be able to run it entirely in the 8 hrs of Cosy cheap periods £££ . Just need to match long term integral of heat input with overall heat loss. I would have thought Vaillant's WC algorithm would do quite a good job of this - even without the fancier options such as active, extended or adaptive.