Radiators - Over size?

[Dr Murdoch]

I'm replacing a few rads and I thought I would over size. Its an old house (1840's).

My thinking is this (and please correct me if i'm wrong!), if I have a high powered rad with TRVs it will cut off when the room reaches the set temp, therefore not wasting excess energy? And furthermore, I could reduce the water temp from maybe 70 down to 60ish? Those saving money?

Am I way off the mark?

[PhilboSE]

Use an online conculator to get a BTU number for the room. Try a couple, as they can vary by 20% (it’s not an exact science).

Then buy rads with at least DOUBLE that number. Firstly, your presumption is correct, secondly it’s a bit of future proofing against lower temp / flow systems that may be needed in the future.

[98elise]

Dr Murdoch said:

I'm replacing a few rads and I thought I would over size. Its an old house (1840's).

My thinking is this (and please correct me if i'm wrong!), if I have a high powered rad with TRVs it will cut off when the room reaches the set temp, therefore not wasting excess energy? And furthermore, I could reduce the water temp from maybe 70 down to 60ish? Those saving money?

Am I way off the mark?

Yes using a TRV you will use about the same energy to heat the room, and probably quicker.

You're also right you have the option to run at a lower temperature. This reduces the output from the radiators.

I always oversize mine by at least 30%.

[Trustmeimadoctor]

go as big as you can fit then go for a triple panel one in the same size

bigger is always better

[Dr Murdoch]

Thanks both.

The online calculator indicates that I will need a circa 3600btu rad, so I've ordered a 7200btu.

[paulwirral]

Your much better off going smaller but more rads to spread the heat around the areas your replacing them in if the layout allows it , instead of one huge rad put two or three smaller ones in .

[OutInTheShed]

paulwirral said:

Your much better off going smaller but more rads to spread the heat around the areas your replacing them in if the layout allows it , instead of one huge rad put two or three smaller ones in .

I agree.
Plus moving rads to the coldest walls can help in some cases.

Personally, I think it makes sense to work in Watts.
If people must use redneck-throwback units, the correct one is BTU per hour.
If you've used say 15kWh of gas in an evening, to keep 3 rooms warm, that gives you clues about realistic wattages.


My lounge has a total of 4kW of rads in it, according the rating, which from memory would be with the rad at 70degC and the room at 15?
Typically, if the great outdoors is about 5 degC, the rads only need to be about 30 for a room temp of 20, once everything has warmed up and settled down. You do need more power to warm things up in the first place, and you need to allow for the coldest plausible weather.

Also, all the calculations go out the window when people keep opening doors and so on.

Adding more insulation so your existing rad can be turned down is better still.

[Trustmeimadoctor]

paulwirral said:

Your much better off going smaller but more rads to spread the heat around the areas your replacing them in if the layout allows it , instead of one huge rad put two or three smaller ones in .

Yes but rad placement is usually tricky if adding multiple and often requires more floors up or ceilings down and walls chased

The key is surface area the more you have the more output you get and the bigger area you spread the output over the comfortable it is, hence ufh is great huge emitter outputting it's energy evenly over a rooms area

[NerveAgent]

If you go for big rad, low flow temp constant heating. Hot and cold spots aren’t really a thing.

[Trustmeimadoctor]

They are less of an issue but it's still not as good as ufh

[Black_S3]

Stellrad website has a good chart showing the kw output of their rads at 50 and 70…. It’s definitely a thing now that you should be thinking about low temperature heating when installing anything new, that said there’s still other considerations like pipework and expansion vessels before you blindly go massively oversized…

[paulwirral]

Pipe size is a major consideration, I installed two huge double panel rads for a mate , I told him they’d never work on mini bore but he knew better .
I balanced them up as best as I could but they only ever get warm .
Meanwhile back in my own house my vast number of smaller rads spread all over te rooms work great !

[Trustmeimadoctor]

Flow on microbore on low temp system with big rads isn't really an issue

70/50c type 22 1800x600 2382w 102lph

50/30 type 22 1800x600 901w 39lph

35/25 374w 32lph

[TheK1981]

Before spending any cash check if you are eligible for any grants/funding, I got cavity wall and loft insulation for my mid-90s house, mine was through British Gas, was about 8 years ago but there’s always various schemes running,

After that I went big on radiators, got much bigger than I had before, cost more but warms up in 20 mins rather than 2 hours and costs less as the boiler is on much less time.

[Cheib]

paulwirral said:

Pipe size is a major consideration, I installed two huge double panel rads for a mate , I told him they’d never work on mini bore but he knew better .
I balanced them up as best as I could but they only ever get warm .
Meanwhile back in my own house my vast number of smaller rads spread all over te rooms work great !

This is very true, it can also be how the pipes have been installed as some connectors can really impede flow rates.

We’re just doing a refurb of our house which had a dreadful heating system. It’s become clear that the pipe work was a major problem. We literally have started again.

Once you start reducing the system temperature flow rate does become more important and as others have said two rads is better than one in a lot of cases especially with bigger rooms.