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Guidance with installing a new heating system

 
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Air Source Heat Pumps (ASHPs)
Last Post by Hamilton 6 months ago
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bobflux
 bobflux
(@bobflux)
Reputable Member Member
Joined: 7 months ago
Posts: 186
20/04/2026 10:26 pm  

Posted by: @hamilton
↑
cement and sand mix, 7cm (5 for pipes + 2 on top), tiles all over

So 5m3 of mortar. With density 1.6-2 and thermal capacity 800-1000 J.kg/°C if the slab heats by 1°C that means 1.7-2.7 kWh is stored.

So you can store a bit of heat in the slab during the cheap kWh hours, but probably not enough to keep the house warm for the rest of the day.

Slab reaction time should be a few hours, so if you have large south facing windows letting in plenty of sun you may find it useful to use a thermostat to stop the heat pump when the sun heats the house enough. Otherwise it'll keep heating the slab and a few hours later when that heat gets released it gets too hot.

Now I see you have exterior insulation: if the walls are heavy masonry (stone/brick/concrete not hollow blocks) and not insulated on the inside, this adds a huge amount of thermal mass, quite effective at smoothing temperature variations. In this case you can probably exploit the cheap kWh hours more to store heat in the masonry, also store free heat from the sun.

Posted by: @hamilton
↑
PE-RT Type 1

This doesn't say whether it has EVOH oxygen barrier or not.

Posted by: @hamilton
↑
9 circuits, at 10cm centre to centre, planned layout image attached to this message.

 

10 cm, you're going for ultra low temperature then! Good for optimizing heat pump COP, but there are a few gotchas.

For example, if you run 25°C flow and ambient temperature is 20°C then you can't have 5°C deltaT between flow and return, more like 3°C, which means for the same power you need higher flow.

If the pipes are long, at low flow temperature, and low flow rate, you can get into a situation where most of the heat has left the pipe way before reaching the end, which gives uneven heating. A way to fix that: in rooms with 2 loops, just run the 2 loops together, ie route two pipes side by side, but reverse the flow direction of one pipe, a bit like a countercurrent heat exchanger. This gives more uniform floor temperature.

100m ID 13mm PEX pipe gives Kv ~ 0.43 m3/h (that's the flow for a pressure drop of 1 bar). With manifold/flowmeter losses you get about 3 l/min in this circuit for a circulator-friendly 2.8mH2O (0.28 bar) pressure. With 9 circuits that gives 27 l/min (1.6 m3/h) which is absolutely fine for your future heat pump, it gives 9.5kW at deltaT 5 which means it'll have enough flow to run at a much lower deltaT on a 5kW heat pump.

Note I'm assuming it's balanced for the same flow in all circuits, which means the shorter ones need to have the balancing valve throttled on the manifold. Otherwise all the flow goes to the shorter circuits and the longer ones don't heat.

I'd turn the 2x 105m circuits into 3x 70m circuits, making them closer in length to the others. That should make balancing much easier and less fiddly.

With good insulation, I think one pipe every 10cm is a bit overkill: I have one 20mm PEX pipe every 15cm and flow temperature is something like 25°C lol.

 

 


This post was modified 6 months ago by bobflux

   
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 Hamilton
(@hamilton)
Active Member Member
Joined: 6 months ago
Posts: 6
Topic starter 21/04/2026 7:31 am  

@bobflux Great, thanks for the advice. The pipe has an Oxygen barrier. I'll pass your information to the installer, when the time comes...



   
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