Gas supply question

[Dave Plowman (News]

Pal has a small terrace house built 1980s. Very well insulated (timber framed). Original boiler still works (basic electrics, non room sealed) All that ever seems to fail is the thermocouple.

However, he'd like to get rid of the hot water storage and header tanks and fit a combi. Quote for this involved fitting a new gas main to it. Running up the front of the house, through the roof void and down the outside rear wall and into the kitchen. No tests done - just what the chap said it would need.

But two neighbours have similar houses and combis and say they didn't have a new gas main fitted.

The house construction is a concrete slab with about 2" of poly? on top then a chipboard floor.

I did help fit a new kitchen some time ago which involved new flooring due to water damage, and all the pipes etc seemed to be within the poly layer, rather than inside the concrete. But dunno about the living room (and didn't really look at the gas) - and emptying that out to lift the floor a total nightmare as it would be in the new fitted kitchen.

The original gas supply coped with the boiler and the original stand alone gas cooker. Now all electric cooking.

So the question. Is it possible to test and measure the existing supply to see if adequate? Do all combis need the same gas supply? Pal simply doesn't want ugly pipes running over the outside walls of the house - or the inside ones, either. If that is the only option will stick with a storage system.

They are an elderly couple and would like a better shower than the current electric one - but otherwise not heavy hot water users.

[Scott]

Why not retain the storage tank and fit solar panels to assist in heating the water in the tank? This would be reduce gas consumption and be better for the environment.

Are combis not falling out of favour now?

[Andy Burns]

Some combis *do* need an uprated supply pipe, but I get the feeling most "plumbers" don't do the sums and just assume it's needed ... didn't work when Mum & Dad had a new boiler instlled by British Gas, the plumber said it needed doing, Dad (who used to work for British Gas and fitted the boiler that was getting replaced) did the calcs to demonstrate it would be perfectly good with the existing pipework ...

[Andrew]

If this is a backboiler with class 2 flue blocks in the party wall, then at some point a condensing replacement will be needed anyway and this will require similar upheaval to fitting a combi right now in the kitchen.

The electric shower should be plumbed into the incoming water mains anyway, so why not fit a more powerful shower (might need an upgrade to 10mm t&e back to the CU though).

[Colin Bignell]

Couldn't it be moled under the house?

Not all combi boilers are born equal. When I moved into this house, it had a combi boiler that would heat the water fine, but wasn't big enough to heat the house properly. When I looked at replacing it with one that was, that one needed a 22mm/3/4" gas feed, whereas the existing one only had a 1/2" steel feed from the meter.

[Andrew]

I think you'll find that all the boiler manufacturers specify 22m or even 28mm for the bigger combis (or further from the meter) to within the last metre, then 15mm. I believe the modern control systems in combi's are less tolerant of gas pressure drops and this would be an excuse to evade any warranty issues.

[Fredxx]

My latest boiler has a 22mm fitting on the inlet. And Gas Safe cowboys are told to use 22mm throughout as they can't work out the size.

Any half decent plumber should be able to calculate the loss at the demanded gas flow making assumptions about elbows etc. But the 1mbar of drop is quite onerous.

I quite like the idea of moling a pipe under the property! Not sure if it's now allowed for new pipe-work?

[Colin Bignell]

This was before the gas company replaced all the mains with plastic and fitted an outside meter box. At the time, the boiler was less than 1m vertically above the gas meter.

[Colin Bignell]

I was wondering whether it might be against some current regulation. When the gas company replaced all the mains along my road the new house feeds were all moled, but they only went under gardens or driveways.

[Animal]

A combi consumes a lot more gas power, boilers are now fussy re pressure drop, regs are too. In most cases a new fatter feed is needed.

[John Rumm]

You can certainly calculate the required pipe size given the maximum gas rate and the effective length of the run:

No - it depends on the input power. Combis tend to be at least 24kW - so if replacing a 15kW heating only boiler there might be an issue.

The problem I normally see is that fitters like rules of thumb and don't like doing proper analysis or sizing.

Keeping the boiler and upgrading the cylinder to unvented might solve the problem (near) mains pressure showers.

[Robert]

Many people cant get their heads round that if increasing the size of only part of the gas supply pipe it doesnt matter whether the larger size is at the beginning or end of the run.

[GB]

The OP says that there used to be a gas cooker running off the same supply pipe. Would that help?

[The Natural Philosopher]

By far the best solution is to replace existing header tank and hot water tank with a main pressure sealed tank. This can fit anywhere - loft for example - take either immersion or primary hot water circuit off the existing boiler, and will; be cheaper an less hassle to install than a combi. Which is in any case shit for hot water if more than one person wants some. Spend the extra cash on a water softener instead (if in a hard water area).

[David Wade]

Probably the only option if, like my old system, he has a 15mm feed to the boiler. The fitting instructions for my boiler say:-

"Under no circumstances should the size of the gas supply pipe be less than 22mm"

I do note for some smaller boilers 15mm is permitted provided the run is short, which seems unlikely in this case.

Dave

[Dave Plowman (News]

What I suspected. My view is I'd like a proper test. Not an expensive guess by a tradesman in a hurry.

The snag is to expose the existing pipe really out of the question. So there surely must be an easy way to measure the actual peak flow/pressure?

[Dave Plowman (News]

No it's not. The boiler is wall mounted in the kitchen, with a conventional flu through the back wall. House wasn't built with open fires or chimneys.

That doesn't address removing the storage cylinder.

[Dave Plowman (News]

That I dunno. Be a distance of about 10 metres under a concrete slab. And sounds expensive.

The existing boiler is one of the smallest I've ever seen - externally. The heating bills are very low, as the house is very well insulated - a function of the timber frame, I'd guess. And extra being added to the loft space. So the peak demand for gas would be set by the water heating.

[Dave Plowman (News]

Makes no difference in this case as the pipes simply ain't accessible.

[Dave Plowman (News]

My original post was already too long.

Perhaps I should have added that one reason they want to remove the hot storage tank is the possibility of a roof conversion. The stairs would go through the cupboard it's in. And having a storage tank in that new loft conversion likely to be vetoed immediately.