Give me hot water, boiler!

May 25, 2023 4:35 AM

We have what should be a pretty fearsome combi boiler, that puts out a pitiful amount of hot water. I'm fairly confident I'll need to phone a plumber in the end, but is there anything I can check myself first before doing so?

Our boiler is a fairly long in the tooth Worcester 35cdi ii. On paper it should comfortably suffice our needs, but it produces a pretty meek trickle of hot water. Our main bathroom is the other side of the house to the boiler, so by the time hot water reaches there, it's barely enough to run the shower.

Our boiler is apparently capable of generating 12-14 litres of hot water per minute. If I run the cold tap in the kitchen (right next to the boiler), it outputs about 14 litres of water per minute. If I run the hot tap, it outputs 4 litres per minute (pretty much exactly, if that's any kind of clue). This doesn't seem to change based on changing the target water temperature on the boiler.

We moved into this house not so long ago, so I don't have a plumber yet that I can really trust to fix this at a reasonable price. Before I call someone out anyway, is there anything I can check to see if I can fix the issue myself? Assume I'm an average idiot with no particular experience with plumbing.

posted by dudekiller to Home & Garden (8 answers total)

If you don't have it, here's the instruction manual. I would start by checking the pressure guage and if it is really low (there should be a red area showing too low or too high, and a green area where it's just right and the pointer should be towards the lower end of the green area, as it increases as the boiler runs) then following the instructions to top up the system (see Fig 1a). Also make sure you bleed the radiators to release any air that's trapped in the system. Anything other than that, I think you'll probably need a Gas Safe engineer.
posted by essexjan at 5:23 AM on May 25, 2023 [1 favorite]

The diverter value or the plate heat exchanger are the usual faults relating to water temperature I think. The system pressure is for the radiators.
posted by pipeski at 5:43 AM on May 25, 2023 [1 favorite]

That manual link didnt work for me -- here's a link going to the Bosch Site.

Is your water hard?
posted by gregr at 6:17 AM on May 25, 2023 [1 favorite]

Response by poster: We're in a soft water area.
posted by dudekiller at 7:08 AM on May 25, 2023

Sorry, here's the link to the manual.
posted by essexjan at 7:20 AM on May 25, 2023 [1 favorite]

I think the only thing you can really do yourself, is check that the pressure in the boiler itself is right as essexjan suggested and establish whether the boiler is actually firing continuously while the hot water tap is on.

It appears as if the most likely answers are, that the boiler itself doesn't have enough pressure (if there's a valve you can fix this at least temporarily and probably for a long while) or if the cold water into the boiler is being restricted in one way or another. The second of those will probably still require a call out as gas boilers are not DIY appliances.
posted by plonkee at 7:25 AM on May 25, 2023 [1 favorite]

I mean, is the house old enough that someone fitted a combi boiler into a system that's fed from a hot water header tank rather than directly from the mains? That'd certainly cause this.
posted by ambrosen at 9:02 AM on May 25, 2023 [1 favorite]

Response by poster: Thanks folks! It is an old house that was converted from a tank system some time in the past, but as far as I know there's no header tank hidden away. It's already well pressurised, so I guess it's off to a boiler engineer for me.
posted by dudekiller at 1:31 AM on May 26, 2023