Hot water - erratic boiler behaviour

[giw12]

Hi

My Potterton Osprey is playing up a bit....

It's an open vent system with separate hot water tank (i.e. not a combi) and the central heating side works fine.

Water though is a bit unpredictable, so today I did me an experiment.

1. Emptied the hot tank till it ran cold
2. Let it fill up
3. Switched water on via the programmer

The boiler fired up immediately so all good so far. However, over a 30 minute period, it fires up, runs for 2-3 minutes then goes off for 4-5 minutes before repeating the cycle. I try the water after each on-off cycle and it's getting hotter but nowhere near the 60 degrees setting on the thermostat.

After the 30 mins the water was in fact just under 40 degrees.

Given the boiler's rating (it's designed to cope with fairly sizeable commercial installations) then it should be heating the water fairly quickly I'd have thought.

Shouldn't it just click on and then run until the water in the tank reaches the required temp?

Thoughts much appreciated.

Gary

 

[Hugh Jaleak]

Boiler will run until one of the stats on the system switches it off, whichever stat is satisfied first. Seems although the external stat is still calling for heat, the boiler stat is reaching its set temp first. This means poor circulation, as you are relying on the water circulating from the boiler to transfer the heat to where required, if the heated water is not being shifted from the boiler very quickly then boiler will keep switching on and off on the boiler stat as the heat slowly dissipates.

IIRC the Osprey is for fully pumped systems, if CH is working ok then I'd hazard a guess the motorised valve on HW side isn't opening properly thus restricting flow through the cylinder.

 

[Agile]

It seems more likely to me that the boiler is overpowered for the absorbtion of the cylinder!

Would need a thermal survey to determine whats going on.

Tony