Atag boiler short cycling
- Thread starter Jon c
- Start date
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My car can do 120mph, it doesn't mean I should drive it like that. It massively increases fuel consumption
Yes it can, but it'll fire up at around 70% of its maximum output before modulating down once the flame has stabilised. If the initial firing & flame stabilisation phase is producing more heat than the system can absorb, the boiler will shut down & cycle.
If the system is clean and the radiators are all heating up properly then I would be looking at 2 things.
1. Why is the boiler is cycling - there could be a circulation issue somewhere and the boiler is reaching temp too quickly, quite possible given it's a small bore system and there may be restrictions in the system - I would open up all the valve and take off all the trv's, just to test if it still cycles the same. As suggested a thorough balancing exercise my help.
2. The house still isn't getting warm enough - I would then look to the actual size of radiators and are they large enough to actually warm your house properly when it gets this cold.
1. Why is the boiler is cycling - there could be a circulation issue somewhere and the boiler is reaching temp too quickly, quite possible given it's a small bore system and there may be restrictions in the system - I would open up all the valve and take off all the trv's, just to test if it still cycles the same. As suggested a thorough balancing exercise my help.
2. The house still isn't getting warm enough - I would then look to the actual size of radiators and are they large enough to actually warm your house properly when it gets this cold.
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So I've had a play around this morning.
With all trvs and lockshields fully open it seems to run for longer,but still quite a big difference between flow and return temps when the boiler cuts out. I turned the temp down to 70⁰c, and balanced the system by opening all the upstairs lockshields by only a quarter turn and leaving the trvs how we had them set to try to divert the heat downstairs. Downstairs lockshields were all at least a full turn open. This makes the problem much worse, it will only run for a minute at a time. Also the flow temperature on the boiler climbs very rapidly, from around 45 to 80 within a minute and the return temp does not increase at all! The same was happening before I turned the upstairs lockshields to only a quarter turn open and the temp was set to 80. When it's set at 70 the flow temp goes to about 80 before cutting out. I can see how this might suggest it's oversized or the flow is very poor due to microbore. I've had another look at the pump, it is definitely set to max speed and the 2nd and 1st speed settings are actually working, just much quieter which made me think it was turning off earlier.
With all trvs and lockshields fully open it seems to run for longer,but still quite a big difference between flow and return temps when the boiler cuts out. I turned the temp down to 70⁰c, and balanced the system by opening all the upstairs lockshields by only a quarter turn and leaving the trvs how we had them set to try to divert the heat downstairs. Downstairs lockshields were all at least a full turn open. This makes the problem much worse, it will only run for a minute at a time. Also the flow temperature on the boiler climbs very rapidly, from around 45 to 80 within a minute and the return temp does not increase at all! The same was happening before I turned the upstairs lockshields to only a quarter turn open and the temp was set to 80. When it's set at 70 the flow temp goes to about 80 before cutting out. I can see how this might suggest it's oversized or the flow is very poor due to microbore. I've had another look at the pump, it is definitely set to max speed and the 2nd and 1st speed settings are actually working, just much quieter which made me think it was turning off earlier.
stop messing with the pump, set it at max and leave it set at that
I have opened the upstairs trvs up to about one full turn and kept the temp set at 70, this makes no difference and the boiler still only runs for a minute or so. With the lockshields open about 2 turns, it's still no better. With the trvs all open as well it works much better, will run for 5+ minutes. When the hot water is on it also helps it run longer. Also when the boiler was set at 80 yesterday that seemed to help it run longer. So it seems to me that the original design of the system using long runs of microbore to get to the downstairs rads is not very good and this is causing poor circulation. Unless the upstairs rads allow flow (when the trvs allow flow) the circulation is not enough. Combined with the fact that boiler may be oversized it is cutting out due to the heated water not being taken away quick enough. Maybe the intermittent nature of the problem is due to the trvs sometimes being open and sometimes being closed. Does that sound about right?? The problem with leaving the upstairs trvs open is that it gets far too hot upstairs yet downstairs does still not get warm enough when it's cold outside. Is there anything that can be done without major works like increasing the size of pipes?
That's thanks to my house being a new build
. I don't know why they decided to shove it all in such a small area, they've basically put the hot water cylinder on top of a shelf and the pipework shown is underneath this. I'll check the torque as you describe.
It looks like it's more and more pointing towards a circulation issue. The small bore pipe can furr up too, that and the valves are another pinch point.
Basically the boiler is getting too hot and cuts off, so the flow through the boiler isn't sufficient to remove the heat quickly enough, that looks like a circulation issue - the trouble with that will be finding out what's causing it - restricted pipework, restricted HEX, lazy pump, dodgy TRV's etc
Small bore can be a nightmare when it gets older especially where runs are significantly different lengths, I have it but as it's all buried in the walls it's a huge job to replace. That's why I'm now looking at UFH for downstairs.
Basically the boiler is getting too hot and cuts off, so the flow through the boiler isn't sufficient to remove the heat quickly enough, that looks like a circulation issue - the trouble with that will be finding out what's causing it - restricted pipework, restricted HEX, lazy pump, dodgy TRV's etc
Small bore can be a nightmare when it gets older especially where runs are significantly different lengths, I have it but as it's all buried in the walls it's a huge job to replace. That's why I'm now looking at UFH for downstairs.
Makes sense...could powerflushing help? Would love to have underfloor heating downstairs but obviously it's not practical as the floors are already laid and we're not looking to replace them. As you say the microbore lengths do vary massively. I think the longest length is to the largest rad in the kitchen, not exactly good design! Could a halfway solution be to add some fairly large extra rads downstairs on 15mm pipe of course?
sorry but your install is a totall disgrace, if anyone working for me thought that was acceptable they would be shown the door
I mean no disrespect (as I know how helpful you are on here) but that's an example of the worst kind of responses when help is being sought yet is seemingly commonplace on Internet forums. It's of little use to the OP, at best might just make them feel rubbish (and ripped off if they'd paid for it) and is akin to the old 'If I were you, I wouldn’t start from here' joke about being asked for directions!
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Not gonna disagree with that, it was done about 12 years ago when the house was built (although I haven't lived here that long) but obviously nothing can really be done about it without major work.
No doesnt need major work, but you should start by rotating the pump 45 degrees so that the large screw is facing you, the pump is not designed to be verical like that , horizontal will be fine, the rest is what it is , not much point in changing it now
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