Boiler Rating

[robertito]

I have an aging gas boiler and a fully-pumped heat-only system. It works well, in the sense that with the boiler thermostat at maximum the radiators can just disperse all the heat, so there's no short cycling.
I'm under pressure to get a modern modulating boiler, but my concern is this : should I get a higher rated boiler, on the basis that the output will be greatly reduced when it gets fully up to temperature? Living in a typical 1930's 4-bed semi-detached lagged and double-glazed house in southern england, what size boiler would you suggest ?

 

[gas4you]

If you are warm enough in your property now, then just get a boiler to match your radiator heat outputs added together plus 3Kw for the HE cylinder.

Something like the Vaillant Ecotec Plus can easily be range rated down to match your system exactly.

If you go for too big a boiler you will just be wasting money and energy and possibly cause yourself problems if way too big.

 

[D_Hailsham]

I'm under pressure to get a modern modulating boiler,

What reason is being given for you to spend a lot of money on a new boiler?

Let me guess! It's more efficient; it will save money on gas; it will save the planet; etc etc.

In theory - yes it will do some of those things - maybe not the last! But that is not the only thing to take into consideration. What about the money spent on the boiler and installation? That has to come from somewhere, savings possibly - then the interest lost has to be taken into account. And if you have to borrow the money, there is the cost of financing the loan at anything up to 30%pa. And will you have finished paying for the boiler before it needs replacing (modern boilers seem to have a life of about 5 years)?

The savings made on your gas bill are unlikely to cover the interest lost or paid.

From a purely economic point of view, the only time to replace a boiler is when it is unrepairable or too expensive to repair.

You also need to consider what other options you have for reducing your fuel bill - double glazing, cavity insulation, loft insulation. These are one-off costs and save money permanently. Depending on your circumstances, you might be able to get a grant towards these improvements . See EST Grants and Offers

 

[robertito]

Thanks, D_Hailsham, I've decided to take your advice and keep my existing boiler until it goes seriously faulty. At the moment it works well. But I'd still like to understand modulating boilers: would one of the same nominal rating work as effectively, or would it throttle back to too low an output once the water got fairly hot ? For example if my house needs about 20kW, and that's what the existing boiler supplies, should I eventually go for 18kW, 24kW or something a bit bigger ?

 

[D_Hailsham]

But I'd still like to understand modulating boilers: would one of the same nominal rating work as effectively, or would it throttle back to too low an output once the water got fairly hot ?

If your house needs a 20kW boiler, that's what you get; though slightly larger will do no harm. But don't assume that your existing boiler is the correct size; it was quite normal 20 years ago to install oversized boilers.

If you use the Sedbuk Whole House Boiler Calculator, you can find out what size boiler you should have. It does however assume that you are installing a regular boiler, not a combi. The size of a combi is always determined by the hot water requirement as this will be much greater than the heating requirement.

The difference between a modulating and non modulating (on-off) boiler is that, when the water leaving the boiler reaches the temperature set by the boiler thermostat, the on-off boiler will just turn off until the temperature has dropped, say 10°C; it then turns on again. If it did not do this, the boiler would continue heating the water, possibly until it boiled.

In a modulating boiler, once the required temperature has been reached, the boiler output is reduced to maintain the water at the set temperature. How far it reduces will be determined by the amount of heat being given out by the radiators, which is determined by the heat loss, which changes with the weather.

Heating systems are usually designed for a worst-case with an outside temperature of -1°C and a room temperature of 20°C. So it has to raise the inside temperature by 21°C. If the outside temperature is 11°C, the heating only has to raise the temperature by 10°C, so less heat is required from the radiators, and so from the boiler. The modulating boiler will run at full power to get the house up to temperature and then throttle right back to provide just enough heat to maintain the inside temperature.

Further refinement can be obtained by installing weather compensation (boiler dependent), which measures the external temperature directly and adjusts the boiler output accordingly.