Radiator replacement - should we go for higher BTU than needed?

darkazuria

Looking for some advice on radiator BTU sizing.

We’re looking to replace a standard radiator with a vertical one in our upstairs spare bedroom/home office because it works better for the layout of the room.

Using online BTU calculators, the room seems to need around 2550–2650 BTU.

The plumber gave us a couple of options for the dimensions we're looking at.

- an aluminium vertical radiator rated around 2800 BTU (single panel style)

- or a steel vertical radiator around 3800 BTU (double panel)

The 2800 BTU would be enough, but would it be better to oversize and go with the 3800 BTU option?

Main thing I’m unsure about is whether 3800 BTU would be considered too oversized for a room like this, or whether going a bit bigger is generally recommended? 

We have a normal combi boiler.

Thanks

dnpark38

Will it matter if you have thermometer radiator valve to keep temperature same.

silvercar

Our installer advised us that vertical radiators should be oversized because heat rises and so they have a harder job heating the room at the level people are sitting (layman explanation). Oversizing means if nothing else, the room will heat quicker and the thermostatic control will switch that radiator off before others in the house go off.

Bigphil1474

If your plumber , who knows your system better than us, says either will do, then you know they are fine for your system. We have an oversized radiator in our living room which we manage by turning the TRV down so it balances out. AFAIK, having oversized radiators isn't a problem for your gas system, and may be future proofing you in some ways if you end up with a heat pump which tend to need bigger outputs. However, that's based on something I vaguely remember reading months ago.

FreeBear

https://forums.moneysavingexpert.com/discussion/comment/82006830#Comment_82006830

That might have been a comment I made..

When I replaced my radiators a couple of years back (along with the plumbing & boiler), I oversized nearly all the radiators by 30-50%. This enabled me to run at a much lower flow temperature and put me in a better position to install a heat pump when the time comes. As far as running at a lower temperature, it has proved to be worthwhile - The place heats up with 50-55°C flow at an acceptable rate with the added advantages of better efficiency, lower wear & tear on the boiler, and lower rates of corrosion. But running at a lower flow temperature only really works if the other radiators are also oversized.

Fitting just one higher output radiator does mean that one less needs to be replaced if/when a heat pump is installed. But that room will heat up quicker than the rest of the house. A TVR can be used to limit the heat rise, but is not an ideal solution. A better way is to limit the flow rate by turning the lockshield down a bit to reduce the mean water temperature.

Did try to find a simple article on the web that describes the relationship between mean water temperature and heat output that wasn't mired in maths. Still looking…. In the mean time, punch the numbers in to the Stelrad heat loss calculator and play with the MWT to see how it affects radiator sizing.

Albermarle

Aluminium radiators are more prone to damage/dents than steel ones. However the likelihood of it being damaged probably depends on its location as much as anything.

WIAWSNB

Just to add to what was said above, in case it helps; you are deciding between two rads, one of an adequate 2800 BTU output, and one of a much higher 3800 BTU.

Rads have two valves going to them - one is a 'lockshield' and the other is the 'control', usually a TRV. The 'lockshield' is essentially set to enable that rad to arrive at the required temp at the same time as all the other rads in the house - it's called 'balancing'. Balancing is carried out with all the control valves fully open - it's all about balancing the flow from the boiler to each rad, so that they all get the required flow at roughly the same rate. Rads closer to the boiler will need their l/s shut down a bit more, rads further away opened a bit more, that sort of thing. And you also use the l/s to determine how much supply each rad gets so that it provides the desired output. So, if you decide to go for the 3.8k rad, its l/s will be shut down a bit so it receives the flow for a 2.8k version. It'll then effectively be a 2.8k rad, and won't overwhelm anything.

You could fit the same size 3.8k rads in every room if you wanted to, from the sitting room to the smallest bathroom. But by tweaking the l/s for each one, you could have them behave like the actual rad sizes required, from 300W for the loo to 1.5kW for the sitting room.

In short - fit the rad that makes more sense to you, based on cost and cosmetics, as long as it isn't too small.

And - yes - as pointed out, the other main factor is future-proofing. The larger rad can provide the same heat output with a cooler water flow temp - you'd just need to open the l/s a tad to give it more flow. So, if you changed all or most of your rads to 'oversized' ones, then you could get the same heat out as before, with the boiler running cooler - the l/s's would just need opening up a bit more to compensate. You'd have more flow, but cooler water. Cooler water = more boiler efficiency. And it would make it more suitable for ASHPs, which do tend to run 'cool'.

If, instead, you have a 2.8k rad which is just currently 'adequate', then reducing the flow temp will make it underperform, and there's now't you can do about that.