Is your central heating doing it's job?

When I moved into my new house in February 1997, it wasn't long before I noticed that even with the gas central heating on full, it never got very warm in the house. It took some four hours to reach 70o in the living room. I found it hard to understand why - there were plenty of radiators. Asking British Gas for help brought only a demand for £30 call-out fee.

In fact it's all very simple. The radiators were simply too small! Fitted by the builders for cheapness rather than effectiveness, they only put out half the heat needed for my little house. The boiler was only running at a fraction of it's capacity.

This page is intended to give you a quick way to check if your house has the right central heating system fitted. It's been culled from free handout sheets given out by DIY centres like Wickes and B&Q.

Heat Ratings

The output of a radiator is measured in British Thermal Units (BTU.s) per hour. (So is air-conditioning, come to that!) 3000 BTU is roughly equivalent to a 1 kilowatt bar heater, just to give you some idea.

Room Temperatures

How warm should a room be? Well, personally I like it quite a bit warmer than most people, but the following values are given in one of the leaflets:

Living/Dining 70-72o F / 21-22o C   Bedrooms 65o F / 18o C
Kitchen 68o F / 20o C   Bathroom 70o F / 21o C
Hallways / WC's 65o F / 18o C      

How much heat is required?

Here's a chart to allow you to estimate the BTU's needed for your room:

Room Size (in metres) To reach 65o F / 18o C (BTU's/hr) To reach 70o F / 20o C (BTU's / hr)
2.4 x 2.4 3500 4100
3 x 2.4 4100 4800
3.6 x 3 5500 6400
4.3 x 3.6 6150 7200
4.8 x 3.6 7700 8900
5.4 x 3 7450 8600
5.4 x 3.6 8450 9800
5.4 x 4.2 9450 11000
6.1 x 3.6 9075 10550
6.1 x 4.2 10200 11860
6.1 x 5.4 12300 14200

All of which is fine, but there are a number of other factors come into it. This is all based on a house with 230mm solid brick walls, 2.4m high ceilings, 100mm loft insulation, each room having one external wall and average size windows.

So, work out the number of BTU's you need to heat your room properly, and then adjust it. The following factors mean you need less BTU's to reach the same temperature:

For solid floor Subtract 10%
For uninsulated cavity walls Subtract 10%
For foam-filled cavity walls Subtract 20%
For upstairs bedrooms Subtract 25%
For double glazing Subtract 5%

However, you may need more if some of the following apply:

For two outside walls Add 15%
For three outside walls Add 40%
For northern aspects Add 10%
For no loft insulation Add 15%
For high ceiling - 3 metres Add 20%

which may give you a rather larger total, if a few of those start to kick in, as they may well in a detached house.

Radiator outputs

The BTU rating for a radiator is normally specified in the catalogue. It basically depends on how big it is, and you can double the value if it is a double radiator. Actually a double is not quite the same as two singles of the same size, but for rough and ready purposes, you can ignore this. The different makes don't really vary either. But get a catalogue from a plumber and work it out.

Here are some example ratings:

Length (mm) 420mm high, Single (BTUs/hr) 620mm high, Single (BTUs/hr) 620mm high, Double (BTUs/hr)
500   1890 3581
600 1640 2276 4312
800   3047 5773
900 2474 3433 6504
1000   3819  
1100     7966
1200   4590  
1300 3586   9427
1400   5362  
1600   6133  

As you can see, the ratings relate linearly to size. There's probably a formula you could use, if you knew if. If I had CGI access on this server, I'd write a little form that you could use interactively - put in your radiator sizes, and it would return the BTU rating. But the above should keep you going.


So what you need to do, is work out the required BTU rating for the room, and then see what you actually have. The result may be a disappointment! Remember that builders want to put in the smallest they can. This is actually true of central heating firms too, curiously. Having done this calculation for all the rooms in the house, I knew I needed to double the BTU's, and the best way to do this was to fit a couple of double monsters downstairs, each of 9500 BTU. I had several tradesmen attempt to convince me I was crazy! They were so used to trying to install the minimum for building firms, they couldn't adjust to the idea that the customer would really rather fit a bit more, and turn down the thermostat if needed!

It's probably a good idea to take a bit of paper, and work out the values needed and installed for each room in the house, not forgetting rooms with no radiator!

What about the Boiler?

The boiler also has a BTU rating, which makes it pretty easy to see if it will cope with the load or not. Some boilers (I have a Myson Economist, which seems quite common around here) have a gas-pressure screw, which is adjusted on installation to set the boiler BTU to the value needed. Don't alter this yourself - this is a job for a trained Corgi-registered Gas engineer to adjust. But I found that in fact I had oodles of spare pressure on the boiler - it was capable of anything up to 40,000 BTU's and I was only using 18,000 BTU's. I ended up installing 30,000 BTU's in the house, and even so could fit more.

Upgrading radiators

This is actually DIY plumbing, although I had it done by a tradesman, who in consequence charged me lots more money than it required. But that's another story.


I hope all this has been useful. Go around to the DIY's and plumbing and heating centres listed in your yellow pages - the free leaflets will tell you much that is very informative. Happy heating!

Last updated 3rd October 1998.

This page has been online since 11th December 1999.

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