Radiator surface temperature

[Davidson]

Hi, Can anyone confirm to me whether the surface temperature of a domestic radiator is determined only by the flow temperature of the water going through it ( i.e. c. 75 deg). I am currently having a battle with a heating engineer who is arguing that it is down to a lack of in-ceiling insulation which is the reason why the radiators are not piping hot and not that he has installed a boiler which is not large enough to heat our 15 radiators.

I completely understand that the inability of a radiator to bring a room up to a reasonable temperature could be due to lack of insulation (or rad being too small) but I have never heard of lack of insulation being the reason why a radiator is not piping hot.

 

[ShaunCorbs]

What's the size of the boiler also what temps are you getting ?

 

[bacon_sandwich]

Radiator output in KW is proportional to temp of water at the radiator. Say it leaves the boiler at 75 degrees, hits the first radiator at 70, next at 68 etc etc down to say 60 for the last on the line. Obviously if you are losing temp in the pipework , thats bad, but thats what pipe insulation is for.. lol

 

[Scott_D]

What size pipework goes into the radiator?
Has it been teed into the supply for another

 

[Last Plumber]

Pay for an independent report on your Heating system.

 

[Davidson]

Thanks for the replies so far. To provide a bit more information the boiler is a 4 to 18 kw biomass boiler which is feeding 15 radiators with a combined output of around 16kW. All the radiators and pipework have been recently installed by another heating engineer and all pipework lagged. The problem we are encountering is that despite setting the flow temperature at 75 deg C the boiler has been struggling to get passed a flow temperature of 60 deg C and as a result all the radiators in the house are luke warm to the touch. When half the radiators are shut down the radiators do become a lot hotter. Initially it was thought there was a problem with the boiler so it has been swapped over but still the same problem. They have just hooked the second boiler up to increase the capacity and all 15 radiators are now piping hot - no wonder when the boiler output is now 36 kW feeding 16kW of radiators. Our conclusion is that they proposed a boiler of insufficient capacity in the first place yet why would the radiators all not be piping hot if their combined output (16kW) is less than that of the boiler (18kW) ? This I am told, is down to the fact that we have areas of flooring between the downstairs and attic which are not properly insulated ?

I have argued that the radiators are not getting piping hot because the boiler does not have the capacity to heat the water in the sealed system to 75 deg C. It is being suggested to me however that it is the lack of insulation which is preventing the radiators getting to temperature.

What we cannot get our head around is why the surface tempaerature of a radiator (to the touch) is in anyway connected to insulation - surely that is room temperature which is determined by room insulation, not the surface temperature of a radiator. I had always considered that if a boiler's flow temperature is set at 75 deg C then the first radiator will heat up to to near that temperature (with a little lost in the pipework) and then this will decrease slightly as you go up the radiator run. Any thoughts ?

 

[ShaunCorbs]

How's your hot water heated ?

 

[Davidson]

How's your hot water heated ?

Hot water is heated by biomass boiler - we have a 210 litre unvented tank fed off the mains. We have been programming the water in the tank to be heated at a separate time from the central heating.

 

[Last Plumber]

Do you know the pipe size of the Heating circuit?

If it is a two pipe system then the rads should all be similar temps if balanced correctly. The water in a two pipe does not travel from one rad to the next, it travels through each rad and back to the Boiler.

 

[bacon_sandwich]

Trouble is here is that although you may have just enough capacity at 18kw, there could be a long warm up time. If you had left the boiler on continously it may have eventually warmed the place up. However 18KW for a 15 rad place is a bit optimistic. If your house was a passivhaus i may have some different thoughts, were there any calcs done for heat loss for various outside temps as the clue may be there. Anyway if you now have 36KW i guess you can turn it on and have a toasty house in an hour or so.

 

[johnduffell]

Agreed the boiler doesn't have the capacity to get the rads up to temperature, either that or the pump is not strong enough to get the boiler onto full whack.

However as an aside, for best efficiency assuming it's a condensing type boiler 75c is way too hot. You really want slightly bigger rads and run them at a much lower temperature, the minimum to get the room warm in a reasonable time.