Replacement boiler advice please.

[Peter Smithson]

My friends need a boiler to replace the conventional floor mounted one which works the central heating and the hot water system. The HW is gravity fed and the CH is pumped round a single pipe system of radiators. It's a fair sized 3 bedroom detached house.

They'd like a wall mounted one but one quote says they can't have a gravity fed HW system wall mounted. Any idea why he'd say that?

They also thought about getting a combi boiler as the idea of always having hot water available sounded good but on the minus side they've learnt that

1) combi boilers don't last very long - maybe 5 years or so.

2) In a hard water area, having mains water run through the heat exchanger doesn't do them much good either.

3) Some people say you can't have a combi with a single pipe system (for some reason).

Is that right? They're aware of the other things like only one hot tap at a time and reduced flow but it's these points that sound worse as they don't want to have to get a new boiler that often. The conventional one in there has been there for decades.

Don't know if it matters they have a conservatory so the flue would have to be angled so it goes out through the wall and up through the conservatorey roof as it does at present.

[Christian McArdle]

With gravity circulation and single pipe central heating, you would be strongly recommended to upgrade to fully pumped twin pipe system. It will be difficult to install any new boiler into your house whilst not breaking building regulations, let alone a modern type. Soon it will be absolutely impossible, as only modern condensing boilers will be allowed. These will not work with gravity flow.

Your fears of combi boilers are unfounded. The main problem with them is relatively low flow rates. This won't matter if you only have baths infrequently. As your storage system is seriously non-compliant anyway, it may be the cheapest way of getting up and running again.

Whilst I can't see that single pipe heating circuits are specifically banned at all, you are required to have separate heating controls, such as TRVs in each room. I don't know if you can get TRVs for this type of system, which have not been installed in the UK for many years (decades).

Christian.

[Andrew Gabriel]

Gravity fed HW system requires that the boiler directly heats up a tank of water. Boilers don't work that way any more -- they heat up a water pipe which has to be continuously pumped whilst being heated, or meltdown follows within a few seconds ;-)

There's no problem with a single pipe system. It's not what would be installed now, but the boiler itself will have no clue about it. (A condensing boiler with a single pipe system would however be quite a challenge I suspect.) You will likely need to add some thermostatic radiator valves to conform to building regs when boiler is changed.

[IMM]

Small tubed heat exchangers require a pumped circuit at all time.

No so.

Firstly some combi's have reduction controls. Secondly, on any system in a hard water area always have a phosphor scale reducer on cold water mains (the drinking taps can't be off it. Thirdly, most new combi's have plate heat exchangers which resist scale.

A combi is no different to any other boiler in this respect. Single pipe system are best ripped out, or amended to 2 pipe.

There are high flowarte combi's around, so don't go on old wives tales.

Not a problem with modern flexible flues.

[IMM]

The single pipe system will still work, but they inefficient and are best replaced. They are ca pig to get right after a boiler change. Having rad valves with them can also upset the performance too. Also the same with gravity hot water. Modern small capacity boilers require a minimum flow at a all times, so a pump is required at all times.

[Peter Smithson]

Thanks for all the replies.

Some of my misconceptions about combi boilers are from here

question - more than one of the people coming to give a quote have said that a wall mounted boiler isn't going to work with their single pipe CH and gravity HW system. They currently have a free standing one. Any ideas why some quotes are saying that?

Thanks

Peter

[Peter Smithson]

So it's really a matter of having to have a pumped HW system with modern systems rather than a certain kind of modern boiler. Odd that the chap said he could do gravity with a floor standing boiler but not a wall mounted one!!!

[IMM]

Ideal still do a cast iron floor mounted boiler, which when the new regs come in will be discontinued as it will be too inefficient. Changing from gravity to fully pumped is not a big thing an much more efficient too.

[Christian McArdle]

It's because you need a boiler designed at least 30 years ago to make it work. They didn't make wall mounted boilers back then.

Quite frankly, you should fix the system properly rather than spend money on

1970s technology that is about to be banned and will cost you dearly in fuel bills.

Christian.

[IMM]

They did. On the Continent they have been around for about 40 years. The first sold en-mass in the UK was the Glow Worm Spacesaver in 1969, but Continental makes were still available well before that, with forced flues and all that. Even then installers would not fit the cast iron Spacesaver (which could do gravity DHW), saying it would burn out. A fitted one for my cousin over 30 years ago and its still going today, although very inefficiently to today's boilers.

Very true. Bring it up to date.

[Dave Plowman]

There were/are plenty of gravity systems using an indirect heating coil. Modern boilers tend to have a much lower water content so need the pump to shift it faster. As it were. ;-)

[Peter Smithson]

I'm talking about gravity for the HW only - the CH is pumped. I know they've made boilers that do that 20 years ago as I've got one!

My friends are just getting quotes from people and getting different advice - not one of the quotes has suggested putting a pump in for the HW system which seems really strange to me.

I think I've figured it out - one of the people said they had to have a free standing boiler for gravity fed and the other people said they'd stick a wall mounted one in. But they didn't actually say they'd leave it as a gravity system - now my friends tell me! They assumed it would be left as gravity but it wasn't specified when the idea of putting a wall mounted boiler was suggested. So I suspect the wall mounted quotes would have involved a HW pump but they just didn't mention it verbaly.

Anyway, now they've just got some people saying it's OK to put a flue through two walls and another person saying its against regulation to put it through two walls! Might try and find these regs on the internet.

Thanks for the replies.

Peter

[Andrew Gabriel]

Oh, I know. The 'tank of water' I was referring to is the tank forming the boiler, often called the water jacket. That's what's gone.

[Andy Hall]

You're admitting to fitting a "cast iron clunker"?? For shame! You'll be telling us that you have a standing order payable to Conservative Central Office next....... ;-)

.andy

To email, substitute .nospam with .gl

[Ed Sirett]

I think it unlikely that any manufacturers will forbid a modern fanned flue going through two walls. They do specify maximum equivalent [1] lengths of flue which you will have to comply with. In fact given the price structure of the flue extending parts they'll likely be quite pleased with a flue through two walls.

[1] The length allowing for bends usually 1m per Elbow.

[John Stumbles]

---8 ... You will likely need to add some

AIUI (from the last time I ploughed through Part L looking for this) you don't have to have TRVs as long as the system is controlled by a room 'stat.

-- John Stumbles

-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

-+ No Rules, OK

[Andrew Gabriel]

That's correct, but most people have many more rooms than they do room stats.

[IMM]

Unless you bought a special import they were all cast-iron.

I would rather slit my throat first.

[Ed Sirett]

AIUI there are two ways to comply:

1) All TRV's + smart bypass interlocked to boiler. 2) Room Stat + TRV's on bedrooms, cooler and less often used rooms.