# to oversize radiators for lower boiler temperature ?

#### ecobuilder

I just need some heating advice.

We mainly heat our house with a woodburner in the sitting room
(space heating only, no backboiler)

I've had a new gas combi fitted which will heat an underfloor heating
system in our kitchen/ dining room. The underfloor heating is working really well and is only needing 28 degrees output on the boiler.(at the moment! )

I'm in the process of specifying 4 double panel convector radiators (2 up and 2 down), and have chosen their k.w outputs based on the 50 degrees output specified in the catalogue. (a 1.6, a 1.4, a 1, and a 1 kw.)

My question is;
Is there any benefit in over sizing the radiators (by say a factor of double the 50 degree size ) so that they can run on a lower boiler heat output ?
(And therefore save on gas consumption); And maybe even run at the same low output as the underfloor heating circuit ? so that I can set the boiler temperature at the low output for both circuits (the radiators and the underfloor heating).

I do have a mixing/blending valve/manifold on the underfloor heating circuit so this can blend down the boiler temperature to the correct lower temperature, if they require different temperatures, but is this efficient ?
the boiler running higher (say 50 degrees) to feed the radiator circuit and then having to be cooled down to 28 degrees for the underfloor circuit.

Would it not be more efficient to have the boiler running at the same lower temperature for both circuits ? Hence my question about doubling the size of the radiators, and can a larger radiator run at 28 degrees anyway.

.
can a larger radiator run at 28 degrees anyway.

28c mean radiant temp yes , 28c flow/supply temp no.

28c MRT would require a flow temp of 58c (20c DT) , the revised radiator size for a 1.6 kw output @ 60 MRT would be 4kw.

Do you mean the difference between Room temperature (20C) and mean rad water temperature (48C)? If so, I've never heard it referred to as the MRT.

Why 60C "MRT"? Rad outputs are now quoted for a 50C MRT to comply with BS EN442. I know there are some suppliers (mainly retailers) who still use the older MRT as it makes the output appear higher.

If the required rad size (at 50C MRT) is 1.6kW, you will need a 3.6kW rad to supply the same amount of heat at a 28C MRT.

You might like to read this.

The underfloor heating is working really well and is only needing 28 degrees output on the boiler.(at the moment! )
Is that a flow temperature of 28C?

Is there any benefit in over sizing the radiators (by say a factor of double the 50 degree size ) so that they can run on a lower boiler heat output?
You could install larger "oversized" rads and the boiler would be able to run at a lower temperature; but you will never be able to run at the same flow temp as the UF.

The return cannot get any lower than the room temperature (think about it!), so a return temp of 21C is the lowest you can get. If the flow is 28C, the enlargement factor is 36; i,e you would need a 36kW rad to produce 1kW of heat.

Doubling the size of the rad would allow you to run at a flow of 60C and return of 40C. Tripling the size allows a 53C flow, 43C return.

Do you mean the difference between Room temperature (20C) and mean rad water temperature (48C)? If so, I've never heard it referred to as the MRT.

Yes , we learn something every day Dh. , and MWT meaning?

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