Hi ChrisR, I admit I don't test hi limit stats as I don't have a reasonable way of checking these in the field, other than pulling the control stat and waiting for the boiler to reach (in some models) 110 deg C. I work only on oil boilers, and the combi's for oil have a thermal store of around 70 litres which I think is not the same for gas combis. Is this right? However I check the PRVs as it's an easy check, then I can check the diaphram/bladder in the expansion vessel and its pressure. I note that the servicing instructions for most boilers deal only with the combustion side, but when I'm at a customers I look at everything from the oil tank to the water side of the boiler. Manufacturers instructions are the basic service charge, anything else is extra with customers agreement.
With the volume of water in the boiler (for oil models) the internal expansion vessel is often just enough to cope with the boiler, and the installer has not calculated the correct size required for the system and so diaphram failures are frequent. (I charge as a seperate item for these tests after discussing it with the customer.) As oil boilers have a large store tank I think I don't see leaking PRVs after testing as dirt particles have probably settled in the tank, where I think you have a different problem with gas boilers with their lower water content. Is this the case?
Dirt in the system is a bit of a nusience, if I was to install a new system i would put in an inline strainer (and isolating valves) to catch as much as possible and then clean the strainer annually (but not for free). The cost of extra parts is less than £10
The manuacturers information I have eludes to a British Standard for these PRV and expansion vessel tests, so I will try to find out which standard it is and let you know.
With the volume of water in the boiler (for oil models) the internal expansion vessel is often just enough to cope with the boiler, and the installer has not calculated the correct size required for the system and so diaphram failures are frequent. (I charge as a seperate item for these tests after discussing it with the customer.) As oil boilers have a large store tank I think I don't see leaking PRVs after testing as dirt particles have probably settled in the tank, where I think you have a different problem with gas boilers with their lower water content. Is this the case?
Dirt in the system is a bit of a nusience, if I was to install a new system i would put in an inline strainer (and isolating valves) to catch as much as possible and then clean the strainer annually (but not for free). The cost of extra parts is less than £10
The manuacturers information I have eludes to a British Standard for these PRV and expansion vessel tests, so I will try to find out which standard it is and let you know.
