Combi or an Unvented cylinder?

[Bruce GT3]

Hoping to get some advice here

I've just moved into a new property (a bungalow built in 1980) which has an outdated boiler and conventional hot water cylinder. The boiler is running off a bulk LPG cylinder and the pipework for the central heating runs through/under the concrete floor screed feeding several radiators.

As an upgrade I want to replace the boiler for something more efficient. However, the problem is that because it's a bungalow and there's very little head for the hot water cylinder the hot water flow rate is abysmal and can take anything up to 5 minutes before taps run hot.

I am tendering for quotes from several companies, both large and small local firms alike, but am getting conflicting advice. I'm pretty sure that a combi will suit my needs best in terms of cost savings and hot water demand. One firm has advised against a combi because of the underfloor CH piping and the risk of leaks - and recommends an unvented cylinder supplying hot water, with a low pressure feed for the central heating. Another has told me that although there is a risk of the pipes leaking if higher pressure water is fed from a combi it is a "1 in 100" chance of it happening. I don't know what to do next Has anybody here had experience, good or bad, with running a combi boiler into underfloor piping?

Additionally, can anyone recommend one of these "descaler/desludging/softener" type products that claim to keep your pipes clear and do all sorts of other "magical" things? Do they work? Are they a good idea or not?

TIA,
Bruce

 

[david williams]

common problem with fitting combi on old pipework and radiators,pressure to much leads to leaks.it is always a gamble will your installer gaurentee it will not happen??.also existing hot water pipework in 22mm ,should be changed to15mm.iwould go for low pressure heating and unvented cylinder

 

[Bernie2]

Interesting!

The question is I suppose "Do you connect a low pressure system up to a high pressure?"

Depends on the state of the pipework. But being underfloor you can't see it, so its hard to say.

The careful company don't want to subject pipes they can't see to high pressures. Basically I suppose, if they did leak, repairing them is a slow floor up job. And of course a chance the customer may say "You should have known that would happen, so I am not paying for any leak repairs!"

So the low pressure option is obviously the safest.

As to water flow, well probably a combi would suite but the central heating side is also high pressure.

So providing you are prepared to chance the leaks I would say a combi is probably the best for a bungalow.

I would though test the mains pressure and flow rate to determine whether a combi would work properly before I got one.
Also I would recommend power flushing the system and one of the Fernox inhibitors.

Try the Fernox or Sentinal websites for a bit more info on inhibitors.

On reflection it all depends on the state of the underfloor pipework. If the house was built in 1980 it means the underground pipework is probably soft copper, which is fairly thick walled, so unless the water is particularly aggressive, the water velocity has been within limits and the pipes have been protected, they should be okay.

You could of course get the pipes pressure tested before you connected them up. But that may tell you little other than they hold water at the time of testing. It may not tell you how long they are likely to last.

If it was me, I would take a chance and go for the combi.

 

[Bruce GT3]

Thanks for the replies so far chaps.

Company A that wouldn't install a combi was for that precise reason - couldn't guarantee that it wouldn't leak and a nightmare if it did. Company B that did recommend the combi wouldn't guarantee it wouldn't leak, but did advise that it was up to me to acceopt the risk

One other system I have found whilst surfing is these vented mains pressure hot water systems or "thermal storage". Anybody have any thoughts on them? Looks a bit like a Megaflo in reverse, where there is a cylinder of water and a mains cold feed that runs through a coil in the cylinder to heat up the incoming water. Not sure (technical idiot!) but would that give me mains pressure hot water AND low pressure hot water for the central heating? Best of both worlds possibly?

 

[stevetheplumber]

Thanks for the replies so far chaps.

Company A that wouldn't install a combi was for that precise reason - couldn't guarantee that it wouldn't leak and a nightmare if it did. Company B that did recommend the combi wouldn't guarantee it wouldn't leak, but did advise that it was up to me to acceopt the risk

One other system I have found whilst surfing is these vented mains pressure hot water systems or "thermal storage". Anybody have any thoughts on them? Looks a bit like a Megaflo in reverse, where there is a cylinder of water and a mains cold feed that runs through a coil in the cylinder to heat up the incoming water. Not sure (technical idiot!) but would that give me mains pressure hot water AND low pressure hot water for the central heating? Best of both worlds possibly?

i run a thermal store in my own house and find it very good unlimited hot water delivered at what ever preasure yor existing main supplies very quick response for rads in winter and no hi preasure cylinder to worry about no preasure relief valve no pipework for same i think they are better than unvented cylinders due to there inherent simpliciy

 

[Bernie2]

Yes I agree with Steve.

The thermal store or heat bank was a simple response to the unvented cylinder.

You don't need to be specially ticketed to install unvented cylinders for nothing.

They can make a big BOOM!

So its probably best not installing one if you don't have too.

If I ever change my own system, I may consider a heat store. We are quite happy with a simple open vent boiler and 2-2 port valves on the c/h and dhw.

Its quick to heat up and for me the best about it is, it just gets on with the job without complaint year in and year out, unlike the usual combi's.

Soon as they put pcbs in boilers its been problem after problem. Didn't anybody tell them electronics like it cool. And why so complicated?

Couldn't they have just kept the sections separate and not lumped them all together on the one pcb?

But then I live in a house where a combi would be best.

But all those breakdowns!!!

 

[Bruce GT3]

Thanks again chaps.

I really like the sound of the thermal heat store/heat bank.

Are there any manufacturers you would recommend over another? I see Gledhill do several cylinders designed for use in this type of system. Anyone else I should look at?

 

[Plumbstar Tom]

Just remember that the thermal store is a big hot water tank at very high temp so high that its water has to be mixed with cold on its outlet.

I'm pretty sure that it was a young baby was trajically that was killed not so long ago by a thermal store coming through the ceiling. So just be careful fitting it

Council aware of boiler fault that killed baby - Telegraph

 

[flat pack]

you need a unvented ticket to install this ,
stick to a combi or get a unvented plumber in

 

[smoggs03]

i would go for the combi all its going to do is show you have a leak (if you do)where an open vent will merrily top its self up causing damp etc. pipes in concrete are going to leak sooner or later anyway. if you choose the right company they won't just leave you in the lurch i know the one i work for advise at point of sale it's your responsibility but in reality the WORST case is it's fixed at cost of pipe (we can't afford bad press @ our prices) if your on our service contract your laughing enjoy free repair!!