[air]

50C is ideal if you have a typical Irish underfloor heating system installed by contractors that don't understand what they're doing with regard to pipe layout and spacing.
This is the kind of incompetence that has given UFH and heat pumps a bad name in this country.

A good rule of thumb is that every degree of flow temp costs 1% in system efficiency.
A flow temp of 35C is more typical in other countries where installs are properly designed by people who know what they are doing.

[G-Man]

Originally Posted by air

A good rule of thumb is that every degree of flow temp costs 1% in system efficiency.
A flow temp of 35C is more typical in other countries where installs are properly designed by people who know what they are doing.

Does that mean for every 1C increase in heating water temperature, you loose 1 percent of efficiency.... So you would design your heat budget with an eye to keep water temp as low as possible and perhaps then having a cheaper or more optimally sized system.

[unkel]

Originally Posted by air

A flow temp of 35C is more typical in other countries where installs are properly designed by people who know what they are doing.

Surely your water for baths / showers would need to be at least 50C and regularly over 60C, or am I missing something here? I know nothing about heat pump systems - apologies if question is stupid.

[air]

Exactly, by reducing your pipe spacing you increase the total pipe surface area exposed within the slab so you can achieve the same heat output with a lower flow temperature.

The power consumption of the heat pump is directly proportional to the lift in temperature it has to achieve.
If it's 5C outside and you're lifting it to 50C flow temperature that's a 45C lift. However if you are only lifting to 35C, that's 30C of a lift (33% reduction), so you should see far better system efficiency, probably 20% or more.
The heat pump will also work at a lower condensing pressure so should last longer all else being equal.
For this reason it's generally better not to use a heat pump for DHW if it can be easily avoided.

You can look up proper pipe layouts online also, these dissipate the most heat at the room perimeter where losses are greatest and keep the flow and return pipes near one another to help achieve an even floor temperature.
This will ensure better comfort at lower air temperature by helping eliminate any air movement due to temperature differentials, again further improving efficiency.

[Gamma1]

I have a B3 bungalow with PV, electric car condensing oil boiler, wood stoves and pellet burner. Have looked extensively at air to water HPs and not for me for these reasons
Have a mix of different pipes - gunbarrel, Cu, plastic.
Efficiency falls with reducing outside temp and increased water temp
No real savings.
The only type of system I am considering is air to air HP based on one outdoor unit and multiple indoor units (3 for me is fine). Efficiency here will be higher overall as there is no intermediate water.
There will be minor savings - mostly against wood but I will probably do it for the sake of convenience.

[KCross]

Originally Posted by unkel

Surely your water for baths / showers would need to be at least 50C and regularly over 60C, or am I missing something here? I know nothing about heat pump systems - apologies if question is stupid.

Separate "zones" for that.

The UFH should optimally be running at ~30°C (what @air called flow temp) and will be the largest energy consumer by a long way.

The DHW will, of course, be at ~50°C and will run a boost every few weeks for legionella which takes it up to ~60+°C.

[mp3guy]

Originally Posted by KCross

Separate "zones" for that.

The UFH should optimally be running at ~30°C (what @air called flow temp) and will be the largest energy consumer by a long way.

The DHW will, of course, be at ~50°C and will run a boost every few weeks for legionella which takes it up to ~60+°C.

How does the HP split that temperature? Does it just mix with 50c water, or can you only heat the UFH at different times to the DHW?

[KCross]

Originally Posted by mp3guy

How does the HP split that temperature? Does it just mix with 50c water, or can you only heat the UFH at different times to the DHW?

The HP manages it itself. I'm sure there are different implementations but I suspect a series of valves to heat one or the other. Heating DHW would be a very small proprtion of the overall time it spends doing space heating.