Oil boiler too hot problem.

[Dougbikebloke]

Hello guys,
I have a Worcester heatslave 19/25. When the hot water is flowing enough to operate the flow switch and fire up the boiler, but flowing slowly enough to make the water get hotter till eventually the thermostat should cut off the boiler until the water has cooled enough to allow it to fire up again, it doesn't cut off whilst the water is flowing, until it reaches the limit stat shut off, which then needs to be reset. During normal use (when the tap is turned on to a higher flow rate) there is no problem, but if the bath is run slowly for 10 minutes, say, the water gets hotter and hotter until eventually the limit stat will shut it all off. The hot water thermostat on the front of the boiler works OK whilst no water is flowing in that it turns the boiler on if you adjust it up, then it stops when it's reached the set temp. I assume there must be another thermostat somewhere, that's overrides the flow switch?

 

[Exedon]

You have a sticking flow switch probably just needs cleaning.
It's the high limit stat cutting out burner.

 

[Dougbikebloke]

Thanks for the reply. The boiler fires up when the hot tap is turned on and shuts off when the tap is turned off. As long as water is flowing the boiler stays on until it hits the high limit stat and then needs resetting. The fact that the boiler shuts off when the tap is turned off surely means the flow switch is working correctly?

 

[oilhead]

The boiler is cutting out on the limit as the DHW heat exchanger is not taking enough heat from the primary source. How does the hot water flow compare with the cold water flow? You may have a scaled up DHW Ht Exch.

 

[Dougbikebloke]

Hi Oilhead,
The hot water flow is slower than the cold, but not by a lot. If I run the tap at full flow, I can run a bath without the boiler tripping on the limit stat. If I turn the tap on a slow flow, the water runs hot and keeps getting hotter until the limit stat trips. This slow flow would simulate a restricted heat exchanger, yes? But the boiler should still regulate by turning off the burner before hitting the limit stat, keeping the diverter valve open for hot water and pump going to still allow primary hot water through the heat exchanger to allow the cold to be heated, even on a slow flow, until the primary source cools enough to allow the burner to fire up again. That's what used to happen before this fault developed. Without the hot tap turned on , the hot water thermostat control fires the burner up if I turn it up and switches off the burner when I turn it down, so when the hot tap is running, should the burner turn off when I turn the thermostat down? If it should, it doesn't.

 

[oilhead]

The time lag of mechanical thermostats will cause your problem if you are not taking enough heat out of the primary water. You need to find a 'sweet spot' of hot flow that allows your boiler to lose heat out of your primary source but still give you hot enough water in your bath. When you run it now, do you have to temper it with cold?

 

[Dougbikebloke]

If I run the bath with a full open tap, the water may be a little tepid, so would need to top up the heat with the tap on a slower flow. The problem occurs when my Wife runs a bath, she likes it red hot, so she runs it slowly and goes off doing other things, when she comes back after 10 mins or so, the water is running slowly still, but stone cold because the limit stat has shut down the boiler.

 

[oilhead]

Experiment with running the tap a little faster. You could also invest in another limit stat, but I don't think that is your problem.