I have a boiler with hot water cylinder and expansion vessel in the loft. The cylinder is underneath the boiler, there is a pressure relief valve just in case both 2 port valves accidentally shut. The boiler fires up, runs for 1.5 mins then goes into standby? and green light flashes (potterton suprima) after another couple of minutes it fires up again and remains on for another 1.5 mins before shutting off again. It's done this for years, but surely this can't be right can it?
Hot water only runs for 1.5 mins
- Thread starter DIYedboy
- Start date
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Is everything working? Rads, DHW, etc? This relief valve I'm assuming is an auto bypass. Everything getting hot nice and quick?
Yes, everything works perfectly, I can feel the bypass circuit that the valve is in is hot, surely that loop should be cold unless the valve has opened (I've closed the valve down as tight as possible, but it's still getting hot).
Depends how big the loop is. If the bypass is knackered it will cause your boiler to cycle more often, because it will reach temperature more quickly. It will have been installed to meet a minimum flow requirement for the boiler under all conditions. There may well be some flow through it under normal operation, I haven't seen your system. You may also find the bypass is ok and you have another problem. Sounds like it's duff though.
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The cylinder is blue and was installed in 2010
The bypass loop is very small, probably 4 feet max (boiler n cylinder are only 12" apart.
I have a gate valve that restricts the flow returning from the cylinder, this is screwed down quite a lot so that the pipe leaving the cylinder is definately a bit cooler than the one entering the cylinder.
Would I be right in thinking that the thermistor on the boiler return pipe senses the return water temp is too high and puts the boiler in standby even though the cylinder stat is asking for hot water.
If that is the case the culprit must be the bypass valve which was also installed in 2010, are these things normally closed and only open if there's a pressure build up?
The bypass loop is very small, probably 4 feet max (boiler n cylinder are only 12" apart.
I have a gate valve that restricts the flow returning from the cylinder, this is screwed down quite a lot so that the pipe leaving the cylinder is definately a bit cooler than the one entering the cylinder.
Would I be right in thinking that the thermistor on the boiler return pipe senses the return water temp is too high and puts the boiler in standby even though the cylinder stat is asking for hot water.
If that is the case the culprit must be the bypass valve which was also installed in 2010, are these things normally closed and only open if there's a pressure build up?
The boiler modulates down to minimum and then turns off if the flow is still too hot.
It remains off for the anticycling delay time.
EXCEPT that the Suprima is an old design and does NOT modulate at all.
On hot water ONLY the temp differential across the cylinder coil should only be about 11 C with a non condensing boiler like yours!
So yours seems to be working pretty much as expected!
The auto bypass remains closed until most of the TRVs have closed ( if you have TRVs that is! ) and if the ABV is correctly set..
Tony
It remains off for the anticycling delay time.
EXCEPT that the Suprima is an old design and does NOT modulate at all.
On hot water ONLY the temp differential across the cylinder coil should only be about 11 C with a non condensing boiler like yours!
So yours seems to be working pretty much as expected!
The auto bypass remains closed until most of the TRVs have closed ( if you have TRVs that is! ) and if the ABV is correctly set..
Tony
I don't have any TRV's.
My hands aren't very accurate but there is definately a temperature drop between the in and the out on the cylinder
I checked today when the boiler was in hot water only mode and the water coming out of the tap was only just tepid, surely it shouldn't fire up for 1.5 mins then have a rest for 2 before firing up again (the pump continues to run whilst the boiler is having it's nap)
My hands aren't very accurate but there is definately a temperature drop between the in and the out on the cylinder
I checked today when the boiler was in hot water only mode and the water coming out of the tap was only just tepid, surely it shouldn't fire up for 1.5 mins then have a rest for 2 before firing up again (the pump continues to run whilst the boiler is having it's nap)
Open up your flow to the cylinder coil a tad, and turn up your boiler stat a bit. 
I had something comparable (but not quite the same) where the cylinder flow was necked down to 15mm instead of a gate, and the pump was a Grundfos Alpha, which modulates itself down. it helped to turn up the pump a bit to increase flow through the coil.
That's interesting lads, surely if I increase the flow, the water won't lose as much heat into the cylinder therefore it'll return to the boiler hotter and tripping the boiler into standby mode even quicker.
I assume that increasing the boiler temp will tell the thermistor to shut the boiler down at a higher return temp? thereby hopefully reducing/ removing the boiler's desire to go into 'standby' when it's still got a job to do
I assume that increasing the boiler temp will tell the thermistor to shut the boiler down at a higher return temp? thereby hopefully reducing/ removing the boiler's desire to go into 'standby' when it's still got a job to do
You said your hot water was tepid. We're offering suggestions. What happens when the boiler comes on from cold? If we had the chance to check your system and boiler, we would be able to diagnose properly, but we can only go on the information you provide. How hot is the flow from the boiler? Too hot to keep your hand on? I assume you have the stat set correctly on the cylinder?
Bolshy, when I said the tapwater was tepid that was to indicate that the cylinder contents was not up to temperature and therefore there wasn't a need for the boiler to have it's little nap.
I appreciate all the help and advice you boys are giving me, when I ask a question it is done to try and further my own limited understanding and not by way of argument (teacher/ pupil)
Yes the water coming out of the boiler is stonking hot and the cylinder stat works fine, the hot water circuit works fine and has done for years, but my question relates to the fact the boiler only wants to fire up for 1.5 mins before having a 2 min rest and then firing again. If this is what all old type boilers do then so be it.
I will put the boiler stat up and open the cylinder gate valve a bit more as you and JohnD have suggested
I appreciate all the help and advice you boys are giving me, when I ask a question it is done to try and further my own limited understanding and not by way of argument (teacher/ pupil)
Yes the water coming out of the boiler is stonking hot and the cylinder stat works fine, the hot water circuit works fine and has done for years, but my question relates to the fact the boiler only wants to fire up for 1.5 mins before having a 2 min rest and then firing again. If this is what all old type boilers do then so be it.
I will put the boiler stat up and open the cylinder gate valve a bit more as you and JohnD have suggested
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