I realise this is probably going to need a gassafe person – I just want to know likely cost and whether its likely to be parts he carries before he starts sucking his gums, dismantles the boiler and leaves us with no heating for a week!

I have a Glowworm Swift Flow 100 combi boiler (probably 18 yrs old).

Hot water doesn’t get above luke warm. When hot water runs the boiler seems to “pulse”, by which I mean the flame fires up then drops back repreatedly and indefinitely.

Central Heating seems to work fine.

If you run hot water when the CH has been on for a while you get a minute or two of properly hot water then it goes back to what you would expect.

Soft water area (central scotland). Boiler does NOT get an annual service. The pressure (CH side only?) does need topped up from <1 bar to >2 bar every three or four months – although I’ve never noticed it make a difference.

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SO, possible diagnosis, costs and complexity?

Sounds like a faulty thermostat in the hot water tank, or maybe a valve depending on how it’s connected up.

Mine did something moderately similar. It was the flow sensor, the water was flowing but it didn’t always register. I popped the cap off of the sensor, cleaned out some gunk and scale, refitted the cap and it’s been good for a couple of years. We’re hard water round here, though.

I’d guess (as a non plumber) the second problem (CH) is your expansion vessel which is an easy thing the check and worst case scenario you can fit (diy probably?) an external expansion vessel. Or it could be leaking pipes / rads (but you should notice the damp patches).

Either way that problem shouldnt affect your HW though?

bencooper – Sounds like a faulty thermostat in the hot water tank, or maybe a valve depending on how it’s connected up.

I’m no expert but I didn’t think there was a hot water tank in a combi system?

Further info it if helps:

– when I run the hot water there is a green light on the front of the boiler comes on (this is what it should do). This doesn’t oscillate / pulse the way the burner seems to so I’m guessing the flow sensor is working (although I’d be delighted if I could just remove and clean).