Hi, hoping someone can give me the benefit of their wisdom and experience! We're in the process of getting quotes to replace our boiler in the hope we can bring our bills down - it's a hot water tank/circulation system, at least 20 years old, costing a fortune to run and every winter we dread it breaking down, so time to bite the bullet and replace it.

We've had four quotes now, but I'm more confused than when I started. Scottish Gas recommended a Glowworm combi. The next guy (independent plumber) said he could fit us a combi but would have to re-site it, as the only brand he installs (Worcester Bosch) wouldn't fit the space our old boiler is in, so a like-for-like replacement would cause less disruption. The third guy (larger local firm) said he would advise against a combi unless we were planning on replacing all the pipes at the same time, as pressurising the system might show up leaks in the pipes and if we took that risk, it would be a nightmare to find and fix them. However, I since discovered that they too only fit Worcester Bosch and wonder if he just didn't want to admit that the combi they stock wouldn't fit. Finally, we had another local guy in who recommended a combi, said the existing pipes would be fine, and he's only had problems with leaks in two or three over his long years in the trade.

So now I've ended up with two quotes for a combi and two for a tank system, which feels like comparing apples and oranges! My gut feeling is the combi will be cheaper to run (we've a three bed house but only one - electric - shower) but thoughts of ripping up floors to find an elusive leak do worry me - and I've seen quite a few posts on here about the difficulty of tracking down leaks. So - is it really a genuine risk? Anyone care to place odds on it?

Thanks very much for any expert advice offered!

Ok, an experts opinion, allbeit extremely biased.

First has anyone tested the flow rate and pressure of the mains, may not need doing if the water hits the sink and splashes over.  

The only reason a new boiler cannot go somewhere handy to the old one is regs, if there's likely to be a problem with flueing, you don't want to be blowing a cloud at the neighbours for example.

Scottish regs are different to ours, but to quote ours you will need to upgrade the system, to fully pumped, and fit TRV's, room stat etc.

There is a real chance of problems with leaks, if the pipes are buried in concrete for example probably as high as 80%, and that includes the radiators, run a mirror along the bottom and backs of the rads for signs of rust.

Finally the reason for changing it, reliability, on that point alone I would mark a combi as 2/10, against a heat only boiler 8/10.

You haven't said what make model the old boiler is, but there's a saying which goes, "IF IT AIN'T BROKE DON'T FIX IT" depends what condition it's in, but you will never recover the cost of replacing a working boiler, upgrading the system and insulation would.

Would I fit a Combi, in my own house, Not a cat in hells chance, unless it was a top of the range, Broag, or Atmos.

The cost to run a heating system depends mainly on how well insulated your house is and what temperature you heat it to. An old boiler might be 75-80% efficient and the new one 85-90%. That's will have an effect on your bills but don't expect them to halve. Insulation might be a better investment, depending on what you have already.

Some people dissagree but I feel Combi boilers are really only suitable for houses with one bathroom and average families. They don't really have the flow rate for multiple showers or old fashioned rain heads and body jets. However if you are used to an electric shower a combi is likely to be ok.

My preference is for a stored mains pressure hot water system of which a mega flow is one example. For any mains pressure system (combi or megaflow) it's essential you have someone check that the main pipe to the house can deliver both the static pressure AND the flow rate required.

'we can only have HW and heating on or off'

I'm surprised you can't have HW only, doesn't sound quite right to me. You may well benefit more from upgrading the system with a heating control pack and some minor plumbing than changing the boiler. Having more control over the system will make life a lot more comfortable and easier for you too.

Some TRV's, a well lagged cylinder and adequate loft insulation will also help maximise on energy saving.