Combi boiler only doing half the job

[Adrian Clark]

Hi all,

I have an old Barlo combi boiler (must be at least 12 years old) that has gradually developed a problem over the past couple of months.

The central heating works fine... all radiators quickly heat up without the boiler seeming to be struggling at all. However, the supply to the hot taps just won't go above "warm". I'm not sure how reliable the temperature gauge is, but it never goes above 58 or 60 degrees. This is the case even when the radiators are piping hot.

It definitely never used to do this, although it's possible the problem may have been developing over several months (hard to tell in the summer, but I occasionally found that baths weren't as hot as I'd expected).

Can anyone suggest what might be failing here? And whether it's worth a repair bill, or whether we should bring our planned autumn boiler upgrade forward a few months?

adrian

[A.Lee]

Diverter valve is probably at fault. There is a valve in the boiler, which closes when the hot water is on, and allows all the heat to get to the domestic hot water. This valve will be sticking open slightly, thus heating up the radiator water too, and not getting enough heat to the domestic hot water. Ours has gone once, and it was a real PITA replacing it, as the bits are supplied as a new seal kit, and it is an awkward job, though total cost was only around £40. Alan.

[Adrian Clark]

Thanks Alan - sounds like that must be the problem. Do you mean you fitted the new valve yourself? I suspect that may be a bit beyond my hamfisted abilities, but at least I have some proper information when I ring for quotes!

adrian

[A.Lee]

Yes, I did it myself. I try and do all of my own house jobs myself. If I dont feel confident in my abilities, then I'll get someone in. The diverter valve is basically just a plumbing problem, so I had no hesitation in doing it, if however there was a fault with the gas side/supply on my boiler, then I wouldnt DIY. Alan.

[Mathew Newton]

You can determine if it is/isn't by letting the radiators/system cool and calling for hot water. If the diverter is not diverting, even partially, then the CH flow pipe (and radiators given time) will start to heat up.

Mathew

[John Stumbles]

A boiler of this age may use a central-heating-type 3-port valve, like this

than a 'normal' combi-boiler diverter valve. In these (or at least the elderly Myson I had a good look at a while back) it seemed that the system was arranged so that with the valve in the non-energised position the primary water would go round the water-water plate heat exchanger (PHE) to heat DHW, and when CH was called for the valve would energise to send primary water round the rads circuit. In other words, the opposite of a more modern combi arrangement. In this case I'd expect a the problem described by the OP to be either silting up of the primary side or scaling up of the secondary of the PHE.

Does the boiler fire up and stop again whilst delivering hot water? Or does it run continuously but only deliver lukewarm water? And have you actually measured the temperature and flow rate and compared with the specs? (Or the specs of any other similarly-rated non-condensing combi?) Could it be that somehow the boiler is allowing a greater-than-normal flow rate of DHW and thus it's not getting as hot.

[Ed Sirett]

Given the time scale of the symptoms the likelihood is that it is a diverter valve problem. That does not rule out say a problem with the gas valve modulator or a problem with limescale build up in the 2nd heat exchanger.

[Adrian Clark]

Thanks for the continued advice, chaps. I've had a local heating/ plumbing person round to have a look at it, and he agrees that it's the diverter valve, but reckons that it's generally on its last legs anyway.

So... I have a quotation for a brand new Vaillant (plus extras, including thermostatic valves on all rads) to consider! Wil be contacting a couple of other engineers to see if they (a) think there's a cheaper short-term solution; or (b) can do the full replacement job for less.

adrian