Radiator sizing is the most important thing when it comes to heat pumps, especially on older retrofits. Not only will it ensure your heat pump actually works but is the single biggest contributor to how well it works.
Sizing radiators for boilers it's pretty simple, the output from radiators is stated in the catalog but assumes they will be sat at around 70°C. The slight complication with heat pumps is that they are more efficient at lower temperatures and can have limited flow temperatures.
In fact, boilers are also more efficient at lower temperatures but not as much as heat pumps. The maximum temperature you really want to run heat pump-powered radiators at is 50°C, although, the lower the better.
The process to figure this out is quite complex, first, you need to complete an in-depth room-by-room heat loss, then use what's called conversion factors to work out how big the radiator you're looking for is, but this is quite long-winded, and this is the consumer series, we're here to simplify it for you.
So, once again this is part of an article you can find below, where you can find all this broken down and the cheat sheets, but another reminder, this is NOT a replacement for actual real-world calculations. It is a guide to give you an idea what to expect and to give you a level of understanding so you know what to ask installers.
Installers will find this helpful too as it can give a rough reference for quoting BEFORE doing all your long-hand measurements and calculations and means you will be able to walk around with customers to quickly size and place radiators in the building without having to flick through catalogs.
As always we need to work out how much energy we need in any specific room when it's a cold -2°C day outside, to do this we can use our Heat Loss Cheat Sheet, again this is not a replacement for calculations but gets us in the ballpark, I'll link to that above and below.
Now the first thing you'll need to be aware of is the different types of radiators available. You have type 11, type 21, type 22, and type 33. If you look down from the top of your radiator you'll see which you have, The first number represents the number of panels, and the second number how many of these convector fins.
If you don't have these types of radiators with convector fins, you most likely want to upgrade your radiators anyway purely down to age, let alone the reduced output they have.
So once you know what types of radiator you have you simply measure how much m2 your radiator is and multiply it by this table.
This table shows the number of watts delivered per m2 of the radiator when the radiator is at 50c.
If the number of watts your radiator can produce at our new 50c flow temperature matches or exceeds your room's heat loss you'll probably have no problem with the rads you've got.
|50°C Radiators (DT30)|
So let's say we need 500W or 0.5kW in a room, measure the height of the radiator in meters and multiply by its width in meters. If its a 600mm x 1200mm type 11 radiator we multiply 0.6m by 1.2m which gives 0.72m2 and then multiply it by the 800 watts from our table. This gives 576W which is above the 500W required
If you find you've come up short, you can use this same cheat sheet to find out how big your new radiator needs to be.
Let's say you need 1200W in a room. Typically you would go for a type 22 radiator, take the 1200W requirement, and divide by the 1500W per m2 equals 0.8m2 or radiators to place in the room.
If we were looking at 600mm high radiators which is most common, we would divide the 0.8m2 required by the 0.6m high the radiator is and it shows that you'll be looking at a radiator 1.3m wide. You can then walk around sizing radiators to rooms very quickly and simply rather than having to keep flicking through catalogs.
|40°C Radiators (DT20)|
Some extra things to be aware of, this assumes you aren't going to cover up your radiators in some way and they have a decent amount of free airflow, for example, they don't have a shelf above them. Please don't do that.
It's also worth noting that you can get high-temperature heat pumps, but we always advise low temperature as you'll have much lower running costs and higher comfort.
If you've tried this method and have any feedback or improvements let us know and I’ll update the article.
If you're a homeowner looking for a heat pump install, check out our Heat Geek Map or our Heat Pump Assured installation service where we personally guarantee the installation. Or look on our map for highly trained engineers in your area.
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The calculation for radiator type 11 output is incorrect using the DT30 table. The calculation uses type22 W/m at 1500W and not type11 at 800W/m
Thanks, we will check and amend as necessary.
This has been helpful but I believe that you have a typo, you say to use type 11 as an example and then you use type 22 as the figure.
Thanks for the amendment.