Underfloor & radiator circuits - worth it?

[Bozza]

My self build is going to have a Mitsubishi Ecodan 11.2 KW ASHP with 250l cylinder.  Ground floor will be Underfloor heating with rads upstairs.

 

An optional upgrade - at £531 plus fitting - it to have both floors on different circuits, to allow the underfloor heating to run at a lower temperature, so should reduce running costs.

The house - 214 sqm, SAP rating is estimated to be 89 well insulated 3G glazing and south facing.  In NE Scotland.  Planning on it being a house for life.

 

for those with relevant knowledge of such matters, is the upgrade worth it?

 

ta
 

 

[scottishjohn]

have you considered suing rads that are designed to run with low temp ASHP

,basically they are much larger or are fan assisted

 

https://les.mitsubishielectric.co.uk/products/heating/domestic/radiator/i-life2-slim

[Bozza]

Great suggestion / product thanks but looking in detail the wattage output is too low I need to achieve 2.7 kw output in one of the bedrooms and only have room for one large rad.

[Hobbiniho]

Either your insulation level is average or your bedroom is massive to get a single radiator to output 2.7kw with low temperature ASHP flow temp it will have to be massive (what size rad are they quoting for that output?), the benefit of air rads is the fan that moves the air around the room

[Cpd]

7 hours ago, scottishjohn said:

have you considered suing rads that are designed to run with low temp ASHP

,basically they are much larger or are fan assisted

I really like the idea of these but am scared to look into what they cost ! 

[Temp]

9 hours ago, Bozza said:

 

An optional upgrade - at £531 plus fitting - it to have both floors on different circuits, to allow the underfloor heating to run at a lower temperature, so should reduce running costs.

 

Sounds a bit expensive just to split it into two circuits. Does it need a fancy computer to control the ASHP temperature or something?

[Bozza]

2 hours ago, Hobbiniho said:

Either your insulation level is average or your bedroom is massive to get a single radiator to output 2.7kw with low temperature ASHP flow temp it will have to be massive (what size rad are they quoting for that output?), the benefit of air rads is the fan that moves the air around the room

The bedroom is 18 sq metres but has three full height windows each 2.1 x 0.9m - all 3G.  The calcs they gave me was heat loss 678w, 20% uplift from cold state being 813w. 3:1 uplift for lower heat pump temperature, so 2522w output needed.  So two 700x900 rads or one 700x1200.

 

Insulation is good. 140 frametherm 35 in kit plus 25 over face.

[Bozza]

26 minutes ago, Temp said:

 

Sounds a bit expensive just to split it into two circuits. Does it need a fancy computer to control the ASHP temperature or something?

I don’t know tbh I presume so sort of thermostat with electronics

[Hobbiniho]

2 hours ago, Bozza said:

The calcs they gave me was heat loss 678w, 20% uplift from cold state being 813w. 3:1 uplift for lower heat pump temperature, so 2522w output needed. 

 

If the heat loss for the room is 678w its 678w whether you heat it with a coal fire or a nuclear reactor, im not sure where you are getting the 3:1 calculation from

 

Where the heat pump has to be factored in is with the lower flow temperature most radiators have the quoted output with a deltaT of 50 degrees so a flow temp of 70 degrees for a room temp of 20 with an ASHP you want a flow temp of 30-35 for most efficient running so you need a radiator (emitter) that has an output of 678w with a delta T of 10 degrees

[Temp]

Think that's exactly right. You need a rad that's capable of 2.5kW at 60C because it will only emit 813W at typical ASHP flow temperatures.

[Hobbiniho]

Yeh just figured it out, i thought @Bozza was meaning he needed 2.7kw output at low flow temp ? means that the fan rads posted will be perfectly adequate although not sure i would want one in a bedroom ?