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Pigletjohn - boiler size help please!

(15 Posts)
amazonianwoman Sun 14-Oct-12 08:11:02

Our Baxi boiler is over 16 years old and broke down last winter (thermo coupling).

We're looking at replacing it with a condensing boiler (4 bed house, 2 showers which need pumps for good flow so discounting a combi - right?)

A plumber has quoted based on replacing our existing 23kw with a Worcester 24kw. But since the original was installed the house has been extended twice and there are 6 extra radiators.

Wouldn't we therefore need a bigger boiler??!!

nocake Sun 14-Oct-12 08:17:05

Until Pigletjohn arrives... there are two reasons why you might not need a larger boiler. Firstly the old boiler may already have been larger than the house required. Secondly, new boilers are more efficient so you can use a smaller one to get the same heat into the house (less heat is wasted by the boiler).

A good boiler fitter should have recommended the appropriate size for the house rather than just replacing like for like. I assume you're having more than one quote?

amazonianwoman Sun 14-Oct-12 08:58:35

Thanks. And very good points!

We had a quote earlier in the year, I need to trawl through emails to find it blush

annalouiseh Sun 14-Oct-12 10:39:31

we got one of these

got our boiler chap to fit it around £250 straight swap, you can have his num is you want
he done our whole house

PigletJohn Sun 14-Oct-12 13:05:16

24kW is ample for quite a big house. How big is yours?

Presumably you have 250mm of insulation in the loft, and you're had your cavity walls insulated?

Does it get warm enough with your old boiler in cold weather? Does the old boiler ever run flat out continuously yet is still unable to keep the house warm (after giving it an hour or so to heat up)?

Measure all your radiators and tell me how much you have altogether (e.g. radiators are 600mm high, and you have 5 @ 1 metre ans 5 @ 2 metre long, makes 15 metres length total. Count doubles as one-and-a-half actual length.

PigletJohn Sun 14-Oct-12 13:16:53


£250 to fit a boiler wouldn't even cover the cost of powerflushing your old system.

Bargepole required.

annalouiseh Sun 14-Oct-12 15:57:21

course not - he charges £150 for that.
i did say straight swap

PigletJohn Sun 14-Oct-12 16:28:05

I didn't mean to offend you unnecessarily.

Section 2.3 of the installation instructs for Ferroli boilers says that all the pipes must be careully flushed per BS7593 and Building Regulations document L

Boiler manufacturers reject warranty claims caused by dirt in the system, or not installed per their instructions sad

amazonianwoman Sun 14-Oct-12 17:29:33

Thanks Piglet, I knew you'd come up trumps grin

We have 14 rads in total, all doubles. I'll measure them tonight or tomorrow. It's a 1930s 4 bed detached, with 2 extensions which I presume are well insulated (both done before we moved in). As well as a good sized kitchen there are 3 other large receptions and a utility. It's not a cold house by any means, and can get too warm if the heating is on for too long. The extensions and the attached garage kind of envelope the house & keep it warm.

Thanks for the link Anna but we can't have a combi.

The plumber has quoted around £2700 but that includes moving the cylinder. And includes itemised powerflushing etc!

PigletJohn Sun 14-Oct-12 17:47:25

if your old boiler was 23kW, and the house could get too warm (haven't you got a thermostat?)

then a new 24kW boiler will also be capable of heating it. A modern boiler will modulate itself, i.e. will turn down its flame size if 24kW is not required. It might be able to modulate down to 8kW or so, and will be quieter when running at low power. It will do this from minute to minute, so on cold nights it might set itself high, then on warm evenings (especially as you have TRVs) as the house gets warmer, it will put out less and less heat, until the room thermostat or timer tell it to stop.

24kW is quite likely the smallest size in the range of boilers that your installer recommends, but it will have ample power to heat the house quickly of, for example, you have been away for a week in winter and the house has got relatively cold. Speed of heating will however be limited by the maximum output of your radiators.

amazonianwoman Sun 14-Oct-12 17:56:20

No we don't have a thermostat, just TRVs which I do try to set at optimum temperatures, except for one which was juddering when it reached temperature, but apparently the valve has been set on the wrong end o the radiator, so easily remedied.

We never leave the heating on all night, apart from during the very bad snow about 3 years ago. Is it easy (or necessary) to fit a thermostat? I'd also like a new control panel so I can set the HW and heating for different times, at present the most flexibility I have is for them both to come on at the same time twice a day.

digerd Sun 14-Oct-12 18:30:53

I had a new combi-boiler 24KW 4 years ago. My radiators were the 1960s finless which produced a powerful radiant heat giving high warmth at a lowish air temperature. The boiler was 40 years old and had never broken down apart from the pump, which I had to replace. After 10 years, I noticed the radiators were not giving off the heat they used to, and my neighbour said these old rads need flushing through every 10 years. However, I decided to change the rads for new powerful convectors, but after the flushing through and the new boiler was installed - the new rads were to be installed the next day- when the boiler was turned on, my rooms went from 12.5 to a very hot 19 degrees in 50 minutes. At 22 degrees, it was like 32 degrees and a summer heatwave. The old rads stayed just as hot for the first 30 mins and after 40 mins still very warm and took a whole hour to cool down with boiler turned off. But the next day, with the new convectors and a high tech thermostat, the air got stuffy but my body was not warm, and got colder as the water in rads got cooler very quickly due to modulation and new rads cooling quicker. My sister-in-law does not want a new boiler that modulates the flame down producing cooler water in the rads as she is always frozen in my home. My brother said as they have an old wall wheel stat, the boiler will NOT modulate down as it's the modern stat that enables the boiler to do that. He has 1984 rads and they emit a powerful radiant heat too as well as having fins, and keeps the heat long after the boiler goes off. Will his new condensing boiler modulate like mine with his old stat? And why do the modern convectors emit no significant heat and cool down so rapidly when boiler goes off?

digerd Sun 14-Oct-12 18:32:22

ps. no significant Radiant heat - correction

PigletJohn Sun 14-Oct-12 19:06:25


If you are having a new boiler fitted, get a room stat at the same time. It should go in the room you use most, but it must be in a room that has no other source of heat such as an open fire or a big cooker, or the stat will be tricked. The TRV in this room should be removed, or its head taken off and a cap used to set it fully open. You might prefer a programmable thermostat that alloes you to set differentr temperaures at different times of day and different days of the week. It will make your house more comfortable and avoid wasting energy by letting the heating run when the house is already up to temp.


Didn't follow all that. The boiler modulates by reducing the flame size when the water returning to the boiler, after going through the rads, has not cooled down much; i.e, the boiler can tell that not much heat is being used. The boiler runs more efficiently with small flame sizes, and more eficiently when it does not have to keep switching on and off. The boiler does the modulating, it is nothing to do with the room stat. It does not mean the water in the radiators is colder either.

If I understand you correctly, you are talking about low-water content radiators. Perhaps your old ones were also cast iron? The more mass the radiators and their water contain, the longer they take to warm up and cool down.

If your SiL complains of being too cold, I would suggest starting with an accurate room thermostat (modern ones are accurate to less than half a degree) and setting it to a temperature she considers warm enough. The stat will keep the room at that temp, unless e.g. she opens doors or windows or fiddles with the stat.

amazonianwoman Sun 14-Oct-12 20:27:17

Fabulous as always. Thanks for your advice.

Mumsnet ought to employ you as their resident DIY expert...

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