Change from open vented to system boiler?

[phil1235]

Evening All,

I'm looking to replace (or rather to have replaced) a 10-year old open vented condensing boiler and wonder whether there is any advantage in going to a system boiler. (4-bed, 4 recep detached, 1 bath, 3 showers but not more than 2 used simultaneously). The existing header tank is no problem, lots of roof space. Old boiler is about 23kw, but a SEDBUK calculation comes out at only 16; add say another 2 for fast recovery cylinder? Was looking at a Broag Remeha Avanta 24v, but now maybe 18v or 18s. The rads and pipework are about 22 yrs old.
Welcome your views, please.

 

[Servotech]

A system boiler will give you a good flow rate along with hot water to the taps much quicker.
Hot water back up via immersion heater (in case of boiler breakdown) and a presurised system to boot.
So quite a few advantages there!

I do take it that you're comparing a system boiler as opposed to going down the combi' root.

Servotech,
(A "technical" grade plumber at your service)

 

[phil1235]

Thanks for that. No, definitely not looking at combi.

Is the SEDBUK estimate reliable, or should I stick with 80k BTU as now? (Have double glazed since old boiler fitted, which will have reduced result).

 

[bookahandyman]

Firstly the Broag is a very good boiler & for the small diff in price go for the 24kw, i only install this make unless the customer insists on something different , usually because of the price. There is a supplier in Southampton that is giving a full 5yr parts and labour warranty on the Broags. others just give 5yrs parts and 2yrs lab.
Secondly the system boiler is basically the same as you have now but pressurised. Meaning you wont need the tank in the roof or the pump in your airing cupboard, if thats where it is.

Side effects:- going to a pressurised system from a gravity fed unpressurised one is that any hidden or old compression joints ( even soldered ones) could leak. ive known some to hold out for 14months then suddenly just go pop and leak.

Other things may need to be changed depending on the design of your system. which i dont know what youve got, and could go on needlessly.

 

[muggles]

I think people are missing the point here a bit...

What's up with the boiler you have now? What make/model is it?

 

[mehran]

They can't of been that many condensing open vented boilers 10 years ago to even fit...unless the OP is mistaken

 

[phil1235]

Baxi Barcelona. Had several PCB problems, one new burner, 2 new electrode sets, then on Sunday before Xmas condensate trap gunged up (outlet wasn't frozen). Each annual servicing brings out more and more aluminium crut, so I guess the heat exchanger won't last for ever. It probably won't last us out here, but a new one now will, so might as well get the advantage of better reliability sooner rather than later. (Why buy a new boiler a few months before you move?)

Ref Bookahandyman's helpful comments, yes, I was a bit uneasy about old and inaccessible pipework being pressurised. Wasn't thinking about capital cost of 24 vs 18, but SEDBUK site claims that an oversized boiler is inefficient; if so, 10 years' extra gas could cost a lot more. But then won't modulation solve that?

 

[mehran]

a good modulation range helps. what you are looking for is a boiler that can modulate down as low as possible but still have its max output equal or greater then your max heatloss.


I would recommended the intergas 24/18, 6-18kw heating output, can be used both as an open vent or seal system boiler.

 

[bookahandyman]

based on what rooms u say ur house has i guessed 10 or 11 rads and a cylinder all at average 2kw gives 22 to 24 kw. just as a rough guide. if your rads are small you could prob get away with 18kw. only you know the sizes at the minute.
i always belive its easier to get up a hill in a range rover than a mini, esp if you go up the same hill everyday. likewise with a boiler, just a few spare kw wont hurt.

 

[mehran]

don't agree bookahandyman.

a boiler will spend almost all of its life in it's low-mid output range. as long as it maximum output is equal or greater to your heatloss you should size the boiler for its modulation range(as low as you can get it)


for example a boiler that can go for 6-18 is better then one that goes from 10-24kw if the max heatloss is <18kw

the only thing you get from over sizing is it makes the boiler cycle on/off more then it needs to.

 

[mehran]

btw here is another heatloss cal:

http://www.idhee.org.uk/calculator.html

 

[Gasman77]

err bookahandyman, your looking at this wrong, at 2k per rad, the op would need 12 x 1200mm x 600mm K2 rads to justify a 24 kw, what are the chances of that?

Don't give advice on something you know nowt about!

 

[ajrobb]

I am not a plumber but I would consider a 4-pipe system boiler with weather compensation. This easily supports different boiler flow temperatures for CH and DHW. You won't get the best economy if the boiler can't drop the flow temperature below 60°C for CH, which is too cold for legal DHW.

 

[ajrobb]

SEDBUK site claims that an oversized boiler is inefficient

That's odd, I'm sure I've seen some boilers quote considerably higher efficiencies at 15% and 30% modulation than at 100%.

 

[Nige F]

i always belive its easier to get up a hill in a range rover than a mini, esp if you go up the same hill everyday. likewise with a boiler, just a few spare kw wont hurt.

then petrol consumption is not your priority - why drive 4 wheels when 2 are adequate