Old Radiators - How Many BTUs Radiated?

In considering room heating-requirements, I can work out how many BTUs are needed, then find a modern, efficient radiator (or two) that would supply the required BTUs of heat, preferably at the lower heating-water flow-temperature of about 55degC (referred to as ‘Delta30’, apparently.) But, can I know for definite that this would be a significant improvement on the quite-old (1960s?) radiators that are already in situ? Ie, what’s the typical heat-output of an older double-radiator, say 1900mm x 600mm? (No, we don’t have the manuals…!)

“Delta” or “Δ” is the temperature difference between the room target temperature and the mid point of the temperatures of water entering and leaving the radiator. If 55°C is Δt30 then you want your room temperature to be 25°C. However if that is what the tables show then use it as an approximation.

Assuming that your old radiators are clear of corrosion and gunge then the surface area is what dictates the efficiency. Compare that with the nearest modern equivalent (without fins) to find your output. Installed correctly a 3 panel 1 metre radiator will have the same output as a 3 metre 1 panel radiator.

As your old radiators are presumably in imperial units and the new ones are metric a bit of leeway probably needs to be applied when finding an equivalent.

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This might help to identify the radiators


If changing radiators it is worth considering continental style aluminium multi-finned models. Some years ago I replaced all mine. Not all at the same time so a few do not quite match the rest, but I suspect visitors don’t notice.

They have a very low water content and vast surface area.

By way of explanation about amount of water in a radiator.

You want as little water in the radiator as will carry the energy (heat) you need. More than that and you have wasted energy heating it. Also by returning cold water to the boiler you will increase condensation and therefore boiler efficiency.

Thanks, both. The rads we have are the ‘first generation’ 1960s ones in the diagram, ie no convection fins. I’m thinking to upgrade/replace for two reasons, 1) to get more heat into the main living room, and 2) to move towards being ‘heat-pump ready’, for when the gas boiler needs replacing (eventually.) So, I’m going for a top-end estimate of required heat output (different advice and on-line calculators give slightly different answers!) (While not essential, would be of interest to know what the BTU rating of our existing ones are, for comparison.)

This will depend on the assumptions made by each one. Without very expensive measurements there is no way of knowing the U value of all walls, floors, ceilings, windows, doors and any air leakage for each room.

It is similar assumptions that make EPCs fundamentally flawed.

As I mentioned in another thread, you cannot have too big a radiator for a heat pump. Aesthetics and cost are the limiting factors.

Thanks, very useful.

… if I put a thermometer on the inlet- and outlet-valves of such a modern, aluminium, low-water-content radiator, what sort of temp-difference would it/should it show? (Assuming good installation etc.)

If the setup is perfect you should see boiler temperature in and room temperature out. The chances of that happening are nearly zero. Assuming a fairly normal boiler temperature of 60°C and 5°C pipe loss the inflow would be 55°C and hopefully the outflow is at least 10°C less, preferably 20°C less. You need a low flow rate pump, to ensure the water doesn’t pass too rapidly through the radiator. Alternatively shut the valve most of the way.
This is different from a heat pump which uses a relatively high flow rate with low temperature water.

If you shut down all your radiators that way and keep a height flow rate pump either the pump will get damaged or the bypass will be operating all the time, effectively ruining your efficiency savings.

Thanks, that makes sense. (Hadn’t realised that circulation pumps were designed/operating within different ranges of flow-rates.) I’m inclined to go ahead with a couple of large, Delta-30-capable aluminium rads in the living room (it’s a knock-thru), then see what effects can be noticed by tweaking; if I ‘go large’ enough, then at least we’re moving in the right direction of heat-pump-readiness.

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An initial search only finds steel rads (Flomasta/Screwfix, Milano Arruba/bestheating)… where did you get yours?

Mine are Faral, made in Italy. Bought over here.

This is a small one I had left over. Look in the background of continental films and you will often see similar.

I found this Screwfix selection:

Second hand ones available on eBay and there are identical looking imports from China. It looks as though Faral have left the UK market. Brexit?

Some of my older ones look like this. Also from Faral.

Thanks, very useful, I’ll keep looking. I’m convinced we need to design towards Delta30, ie big rads, to anticipate changeover - eventually - to ASHP.

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With a traditional heat pump you would probably need Δt15 to run the heat pump itself most efficiently but then you really need a several square metre radiator in each room, otherwise known as underfloor heating.

Most modern heat pumps can achieve 45°C, so Δt25, without too much loss of efficiency. Above that the COP seriously drops.

Everyone thinking of a heat pump in the future should aim for that bit better than Δt30 if possible.

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Another option for radiators is Jaga. They do a number of heat pump ready radiators and have a vast range of sizes and styles. A number of the styles also have a "dynamic boost (ie small fans which sit on top of the radiator fins and so massively aid convection and therefore you get effectively a oad more heat from the same size radiator). They are excellent and we have used them in both our last two houses. We run our combi boiler at 50 degrees in the autumn and spring and about 58 in the winter with the jaga radiators only on the ground floor and the conventional 1980s radiators (reclaimed from when we did our refurb in 2017) on the other floors.


Sorry - forgot the link. jaga.co.uk

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We need to replace a radiator in a bedroom, so I am looking for advice in case we switch to a heatpump in the future. Current rad is a single finned 1987 vintage. I had looked at aluminium for our bathroom to output more heat and to avoid corrosion.

Looks v interesting, Jaga have emailed me a catalog, I’ve fired off a couple of questions!, thanks.