Air Source Heat Pump Compatibility with Existing Underfloor Heating Pipes.

Hadders_123

Hi.  I posted this in 'house buying', but been advised to move it here.  
I'm looking at a (not completed yet) new build bungalow that has underfloor heating; but this is powered by gas.  We want to try to be as green as possible, so we would like to replace the gas boiler for an ASHP sometime after we move in.  The layout of the building looks suitable.  My question is what do I need to look for in terms of the specification of the under floor water pipes and the hot water cylinder.  My feeling is that the pipework will be compatible; but it would good to have this confirmed.  I'm expecting that the HW cylinder will also need to be swapped.  Incidentally, we also plan to put an 8kW solar system on the roof with a 5kWh battery.  Thanks in advance.

MattMattMattUK

The pipes themselves are normally the same (diameter, material etc), but the layout differs, in general with a boiler they are 150mm or 200mm apart, with a heat pump they are generally spaced 100mm apart, this is due to the lower flow temperature from the heat pump vs a boiler. The hot water cylinder likely need to be swapped yes, usually for a larger one and with a different flow/coil through it, although there are some compatible with both.

In terms of your usage in across both winter and summer, what would be the incremental cost of moving to a larger battery? The heat pump being air source is most efficient during the day when the temperature is warmer, in summer that is fine, you only need hot water and there would be more than enough excess generation to heat the the tank. In the winter you need both heating and hot water, ideally you want the heat pump to do most of the hard work during the day, especially heating the water, but your winter generation may be much lower (inclination of the sun impacting radiance, overcast weather etc.) however a larger battery combined with your solar and an E7 tariff may mean you never need to draw anything other than off peak power or use your own generation, it could mean that you are only ever drawing power from the grid in the 0-12p per kWh range.

Reed_Richards

MattMattMattUK said:

The pipes themselves are normally the same (diameter, material etc), but the layout differs, in general with a boiler they are 150mm or 200mm apart, with a heat pump they are generally spaced 100mm apart, this is due to the lower flow temperature from the heat pump vs a boiler. 

I always thought that a major constraint on underfloor heating is that you don't burn your feet.  I suppose if the pipes are buried deep enough it doesn't matter if they are hotter on a wider spacing but I would have thought that would require the pipe depth to be as much as the pipe spacing. 

If the gas boiler to be installed meets with modern requirements then I think it should be a "Low Temperature" gas boiler designed for a flow temperature of 50 C or less, which is closer to what a heat pump would provide.  But the heat pump will run a lot more efficiently the lower you can make the flow temperature so I guess that means you want the pipe spacing to be as small as practicable.      

BUFF

Reed_Richards said:

MattMattMattUK said:

The pipes themselves are normally the same (diameter, material etc), but the layout differs, in general with a boiler they are 150mm or 200mm apart, with a heat pump they are generally spaced 100mm apart, this is due to the lower flow temperature from the heat pump vs a boiler. 

I always thought that a major constraint on underfloor heating is that you don't burn your feet.  I suppose if the pipes are buried deep enough it doesn't matter if they are hotter on a wider spacing but I would have thought that would require the pipe depth to be as much as the pipe spacing. 

If the gas boiler to be installed meets with modern requirements then I think it should be a "Low Temperature" gas boiler designed for a flow temperature of 50 C or less, which is closer to what a heat pump would provide.  But the heat pump will run a lot more efficiently the lower you can make the flow temperature so I guess that means you want the pipe spacing to be as small as practicable.      

Iirc the current regulation is 55C flow or less. It would be nice to think that some builders do better than the minimum requirement (& I am sure that some do) but realistically I imagine that most new houses are built to the minimums ...

Hadders_123

Thanks for these replies.  So to make sure I'm understanding correctly - I should ask the builder what the flow temperature of the UFH is.  If it's 50C or less I'm probably OK to convert to an ASHP, any higher then the spacing of the pipes might mean it doesn't get as warm as with the gas boiler.  

I'm visiting the show home in a few weeks, so will be able to find the gas boiler model which I am assuming will give the same information.

Reed_Richards

The way UFH works, typically, is that the flow water from the boiler is mixed with the return water from the UFH loop so that the water entering the UFH loop is cooler than the flow water from the boiler.  This enables a mixture of UFH and radiators, which tend to require a higher water temperature.  The lower this target flow temperature is designed to be, the more economical it will be to run with a heat pump.  So although 50 C or less is okay for an ASHP, the lower the better, for your running costs.  That also applies for a gas boiler, but the improvement in running costs with a lower flow temperature is not so great.  

Mstty

One word of caution, has the house got any double height (vaulted areas) with under floor heating that you are likely to be stationary/sitting in.

A common complaint, even with designed new houses with ASHP, is the large open plan kitchen/dining and sitting area combined with double height large windowed areas. It proves difficult to get these areas up to temperature economically. Resulting in the need for a secondary source of heating such as a multi fuel burner.

Technically the same problem exists with gas as a kWh is a kWh and the space will needs what it needs but when you start turning the ASHP flow temperature up the efficiency of the system reduces and gas comes into its own due to price per kWh.

As an example we have a double height kitchen diner with 2 velux, 2 large windows and large french doors with window panels either side full height. This is at one end of the L-shaped downstairs layout. At the other end on the-shape the Lounge single height with bedroom etc above. With our economical ASHP settings we get 20-22oC on the Lounge on a day with no solar gain and 17.5-19oC in the kitchen/diner.
 

70sbudgie

We have UFH with a gas boiler. The temp for the UFH is 40°C, but I understand that it was designed for a lower temp. Now I better understand the systems, I view the conversation I had with the installer in a different light!

Reed_Richards

Mstty said:

One word of caution, has the house got any double height (vaulted areas) with under floor heating that you are likely to be stationary/sitting in.

I have always thought that double-height areas are there to look good and waste your money.  Hot air rises so you must end up with all the warmest air higher than anyone can access without the aid of a ladder.  I don't think architects pay any heed to running costs in their designs.  

Mstty

Reed_Richards said:

Mstty said:

One word of caution, has the house got any double height (vaulted areas) with under floor heating that you are likely to be stationary/sitting in.

I have always thought that double-height areas are there to look good and waste your money.  Hot air rises so you must end up with all the warmest air higher than anyone can access without the aid of a ladder.  I don't think architects pay any heed to running costs in their designs.  

We are habitual Rightmove lookers (we move every 5 years on average) the amount of new bungalow and house designs with massive open plan living areas with double height is an epidemic round here and as we are rural all with UFH fed by ASHP.

It's a design fetish that is going to cost people a lot of money in the long term in running costs. It's an architectural epidemic lol.