Range rating the boiler

[David937]

Hello,

This is a very simple question and I just want to make sure I am doing the right thing.

I measures the radiators today and checked the outputs in the Stelrad Elise catalogue, I added all the outputs and it added up to approx 13kw.

At current the boilers output is set at 18kw but it can be lowered to 12kw.

Shall I lower it to 13kw?

Will it have any benefit?

 

[gas4you]

I would lower it to 14-15Kw and give your self a slight advantage.

You will not notice much difference at all. The running costs will be very slightly less and the boiler should stay in condensing mode longer.

But on a daily basis you won't notice anything, other than the feeling that you have set the boiler up correctly

 

[David937]

Thank you for the prompt reply.

I sometimes think that the radiators in my house are slighlty undersized, is it normal that a 80'C flow temperature is needed on very cold days to get the house to 19.5 degrees?

 

[gas4you]

Depends on the way your radiators were designed to run.

If they are on an old system, then that was probably designed with around 80C flow temp in mind. If you are talking about a lounge, then no this is obviously not acceptable, but if a hallway then they are often designed to be 20C, whilst bedrooms about 18C.

So whilst you may be correct, you can see there are a few extra factors to consider.

On a modern new system that I install, I always leave the boiler stat set to 60 on a combi, 65C on a heat only boiler with HW cyl.

 

[David937]

Do you mean that if it takes a 80'C flow temperature to get the lounge to 19.5 degrees then it not acceptable?

The hallway is usually 18 degrees.
The bedrooms are approximatley 20 degrees.
Living room and lounge are 19.5 degrees.
I keep my room at 17.0-17.5 degrees and get called a cold blooded reptile by my family. (My room is the attic and is zoned)

The average flow temperature last month was 67 degrees.

 

[gas4you]

Without seeing your property I cannot give a definate answer, but any system I design and install should always be able to achieve at least 22C in the lounge, with a flow temp or around 60-65C.

When checking your rads for heat output/size, be aware that todays rads are rated at less heat output for the same size a years ago, do to the mean water temp difference we now have to work with being lower, i.e. 50C.

 

[David937]

It is definatley capable of achieving 22 degrees with a 80 degree flow temp.
We have new rads but there are the same size as the older 85/75 ones and 7 are double.

 

[gas4you]

To me, whilst it is easy to say sitting here, whenever I replace old rads with new rads, unless there is a physical size problem, I always work out the rad size for the room at the new design parameters, not just replace like for like, which sounds as what has happened in your case.

All your rads should have been replaced with ones sized to work efficiently with your new 937, which really wants to be running at around 65C to maximise it's condensing capabilities, in any case no more than 75C.

Running at 80C flow, the design crtieria of 20C temp differential cannot be met and let the boiler condense, as the return will be at 60C, above dew point.

 

[David937]

Well what the guy done is he asked which rooms don't really warm up to a satisfactory level and calculated the required size of radiator for those rooms.

I checked d.41 and there is a 11 degree differential across the flow and return. At this moment the boiler is set to 57C because the outside temperature is 9C, if the outside temp drops to 0 or below the flow temp cannot be anything lower than 80C.

But isn't an average flow temperature of 67C reasonable?

BTW I range rated the boiler and now it is nearly silent esp on start up


Thank you for your help.