My Ideal Logic boiler goes into 'pump overrun' on occasion while the thermostat is calling for heat. It goes into overrun, then purge, when the temperature's reached as part of the normal cycle, but I've only just noticed this while it's actually heating.
Seems odd to me - if the boiler is trying to 'spread heat' why does it get so hot in the first place?
I would expect the pump to run continuously whilst the room thermostat is calling for heat.
I would expect the pump to be running (overrun) when the call for heat has been met and the boiler has just stopped firing. If the pump stopped immediately the static water in the heat exchanger would get too hot because it would be still heated due to boiler components in the heating path (heat exchanger etc.) still being hot. A bit like turning off an oven and the food still cooking until the oven cools down after a period of time.
The pump is controlled by main thermostat. The boiler firing is *also* controlled by water circulation temperature.
Some/many boilers control the pump, and have a built in overrun timer to run the pump longer than the main thermostat demands.
Yes
OK, thanks both, I think I get it! So it's not a problem then.
The Natural Philosopher formulated on Sunday :
Not in modern boilers. In modern boilers the pump is controlled by the boiler and the boilers determines when the pump needs to run. Basically as soon as flame is detected, through to burn turned off and remaining heat removed from the boilers heat exchanger. Very sensible too, why waste the heat? My boiler also exercises the pump and the fan, if they haven't had need to run for a while.
on 17/01/2021, The Natural Philosopher supposed :
Modern boilers do what is called modulate - the Kw output can be varied over a range of output, to suit the heat for HW or CH requirement.
RJH has brought this to us :
It might be, if the boiler manufacturer specifies that the pump has to be directly controlled by the boiler.
In my ancient boiler the pump is wired to and controlled by the boiler.
Assuming that the timers are set to on... if there a call for water heating or central heating the boiler will turn the pump on. The pump will remain on continuously until one of the thermostats indicates the temperature has been reached and then the overrun kicks in leaving the pump running until the boiler water outlet has cooled down. During the time that the pump is being effectively controlled by the thermostat(s) the burner will cycle on and off. More modern boilers may modulate the burner flame in order to reduce cycling.
On my boiler there is a temperature sensor attached to the boiler outlet pipe. This heats a bi-metal strip which in turn switches an over-run mirco-switch. The micro-switch supplies one source on mains to the pump when the boiler outlet pipe is still hot.
There is also a safety electrical over temperature cut off switch bolted to the boiler outlet pipe.
you say modern boilers. Mine, which I installed in 1989, has this. very necessary if you have zone valves.
I installed a modulating boiler in our last house in the early 1970s.
I don't quite understand what you are saying. How are you detecting that the pump is over-running? Which thermostat is calling for heat - the boiler stat or the room stat? Is the boiler controlling the pump?
By definition, pump over-run only occurs when the boiler is *not* firing, but has has some residual heat which needs to be dissipated to prevent it overheating. [The metal heat exchanger will be a lot hotter than the water in it, and will continue to heat the water in it if there is no flow - possibly to the point where the over-heat cutout trips].
On my boiler when the overrun became much too short in duration and cut off too early the water in the copper heat exchanger started boiling and the whole boiler started banging - loudly. The idea of the overrun is not to prevent wasting heat but to continue to distribute any heat via the flowing water to prevent localised short term overheating in the boiler.
On my boiler the over-run switch is electro mechanical and the "pip" on a micro-switch had worn. It was easily fixed with an adjustment screw being wound down quarter of a turn. The PITA was that a liquid screw lock had been used previously which required some heat from a large soldering iron to free the adjustment again.
Firstly by sound - I noticed it had stopped 'firing' while the room stat was calling for heat. Confirmed by the on-boiler display.
The room stat controls the boiler. The boiler controls the pump. I don't know the boiler's relationship with the pump, though.
Yes, that's what I'm asking. It would make more sense to me the throttle back the gas in the first place, rather than let it get hot to the point that it needs to cool down by pumping the hot water away.
Boiler pumps run continuously as long as there is a call for heat. The burner will cut out when the *circulation* temperature reaches the desired setting.
you're missing over run. Particularly when the circulation is controlled by zone valves, the heast in the boiler needs somewhere to go, so a valve stays open and the pump continues to run for a time controlled by the boiler.
Irrespective of how intelligent your thermostat it is basically an on/off switch. The boiler has no prior knowledge that the thermostat is going to tell it to turn off and without this prior information cannot throttle back in the way you suggest.
Whilst there is a call for heat the boiler is trying to almost instantly raise the temperature of a very fast flow of water in a VERY short run of pipe by 10C. A boiler may have some intelligence to increase the burner when the incoming water is at 20C, with an expected output of 60C which it cannot achieve instantly, and modulate the burner down when the incoming water is 50C, again with an expected output of 60C. To raise the water by 10C almost instantly the heat exchanger is at a much higher temperature even when the burner is turned down to low.
The boiler may be operating at the low burner mode for, say, half an hour before the room heats up enough for the room thermostat to cut in. The boiler, without warning, is told to shut down. The boiler throttles back the gas by the largest amount it can by turning the burners off. Even with the burner turned off the heat exchanger still is hot enough to raise a fast flowing steam of water by 10C+ and if the flow of water is stopped by very much more than 10C.
alan_m explained on 18/01/2021 :
The only thing the boiler can use, to know to throttle back, is an increasing flow (or return) from the boiler temperature - where the room stat is simple on/off. Throttling has to work almost blindly.
My Vaillant, with its fancy control system goes one step further. It knows the actual room temperature (or hot cylinder temperature) and knows the desired temperature. From the two values, it knows how much heat it needs to still produce, to avoid producing too much heat, so it can throttle itself precisely to hit the requirement.
That what my 1972 vintage boiler did. Wish I could remember the make.
charles brought next idea :
I have come across commercial systems which did similar, but I didn't know home heating systems had the technology until fairly recently.
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