Selecting a Central Heating Boiler

3 Dec 2006
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United Kingdom
I have a Glow Worm Compact 75 combi non-condensing boiler feeding the 11 radiators in my 4 bed bungalow. Its about 7 years old. Personally, I think it sucks as the flow rate of hot water is pitiful and I have to chuck many litres down the drain before it heats up with bath. Topping up the bath is impossible without running the hot water into the hand baisin until it heats up again. Secondly, I think the radiators are too small to heat up the house and I'd like more or bigger ones. I've had the boiler checked and there isn't anything wrong with it.

I have a cold water tank but no hot water one.

Consequently, I'd like to replace it and have a couple of questions:

1) You can't use a combi boiler with a power shower can you ?

2) Would it be reasonable to assume that to get a really good non-powered shower I should then select a boiler which gives me the highest flow rate at 35 deg even if that would be capable of heating a bigger house ?

3) Any thoughts on the Baxi Combi 133 HE (16 ltrs per hour @ 35 deg) ? Anything better ?

4) Do you get Combi boilers which heat up the water for a few seconds before sending it down the pipe ? Or have a small storage tank ? ( I want to be able to top up the bath with hot water )

5) I assume that I might as well got for a Condensing SEDBUK Class A one ?

Sundry other facts:
I have a cold water tank, but not a hot water one. Boiler must be wall mounted and < ~500mm wide. Price is less of an issue. Don't want to fit a hot water tank.

Any help gratefully received !
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The Glow Worm Compact 75 was never any great shakes in the Hot Water flow dept, and even in quite small properties we found the Compact 100e was a far better bet all round. I think your boiler was too small from the start. Both are ancient history now, though I work on and service lots of them.

The best you can do is run the hot water really quite slowly, and make sure the thermistor inside the back of the control panel is fully turned up so you are getting max burner pressure in Hot Water mode..get a Corgi to do it as he should have a manometer on the gas valve at the same time.(and i don't mean the knob on the front)

No you can't use a power shower with a combi. if your water pressure is good you hardly need a power shower.

Putting a higher capacity combi boiler in than strictly necessary may offend the purists, but it does mean you avoid the problem you currently have, and most boilers now have fully modulating gas valves which will only draw the gas they need relative to the demands you are making at any particular time. So in a word YES.

I don't know much about Baxis, so I'l leave that for others. I do hope you meant 16 Litres/minute, and not litres/hour as you wrote. Its the compact 75 that does 16Litre/hour (it should do about 9 to 10 l/min if i remember rightly) :LOL:

Most combis have a 'pre-heat' option. All this means is that hot water is continuously circulated round the hot water heat exchanger reducing (in theory) the time to getting hot water at the tap. Some combis include a small capacity 'heat store' of heated water..a mini cylinder if you like.

I'd only go for a heat store if there is a long pipe run to your point of use. The condensing combis we install now are much 'meatier' in their hot water production, and we don't get many complaints. I think you will find the performance of a good combi spectacularly better than your current boiler.

Yes, go for a SEDBUK "A"...if price is not an issue.make sure the system is powerflushed too before you wreck the new boiler.

Nothing to add to that, except DO check your water supply first. If you can't get say 20 litres/min of cold (kitchen sink + outside tap), then your new 40kW combi will be working part-time.
Thanks for your replies.

Don't have an outside tap unfortunately.
The kitchen tap is of very small diameter. It only put out 6 ltrs/min.

However, I figured if I ran water out of the bath (which has bigger taps) and measured the water level in the water tank, then if the level in the tank stablised as the ball cock opened then I would know that the mains supply into the house was at least equal to the the amount that the bath could dispense. This proved to be the case, and I then measured the the amount flowing out of the bath tap. Over several readings (and splashings), this averaged a little over 23 litres/min. Does that sound ball park right ?
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Discount the kitchen tap flow rate - the stop cock feeding it wasn't fully open ! I can't open it fully without it splashing up the sides of the sink and getting me into trouble with my fiance ....

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