Boiler running without pump dangerous ?

[mike42]

I have a feeling I'm asking for too much for this to be answered without a "why are you asking?" but here goes my 2 questions just in case anyone is kind enough to constructively reply.

1/ I have a Heat Only boiler on an S-Plan. If the external pump failed would the boiler just run for a bit and then switch off as the pipes would get hot very quickly ? Its an Intergas HRE OV and fairly sure it has stats on both flow/return and probably elsewhere.

2/ Now extend that to a real hypothetical. If there was no path from the boiler to the vent (yes I know thats dangerous) but hypothetically speaking, my guess would be that the water in the pipe would heat up and expand dangerously before the boiler had a chance to switch off. So I'm guess an internal or external joint or valve would dangerously blow off.

 

[Notch7]

Assuming the boiler is working ok, if the pump wasn’t working the temperature of the water in the boiler would heat up rapidly until the set temperature was reached and the boiler would stop firing.

I don’t where the sensor is, I think there’s one near the heat exchanger

I’m pretty sure the boiler stops firing very very quickly when the sensor reaches temperature, my guess would be seconds.



Do boilers have a way to tell if the pump is running? I’ve no idea.

 

[desktop987456]

If the external pump failed

There is only external pump in a heat only boiler? So there is only one pump which is external.
As far as I am aware (and pros can advise better), there is a flow switch in heat only boiler (I know viessmann heat only OV) has one. so no flow no go for this boiler that I am aware.

 

[ericmark]

I know my boiler, has two external pumps, two motorised valves, and when the motorised valve micro switch sticks on, the boiler runs for around 5 minutes before switching off due to the internal temperature of the boiler, once the DHW has reached max temperature, until the DHW gets hot it simply heats that water, I am running on a C Plan with an oil fired boiler.

I take it as a warning that the micro switch has stuck, when the boiler runs for a very short time.

I know the normal is for the pump to be on the outlet of the boiler, and should the motorised valve not be open, the by-pass valve will lift, so water still circulates but not through the radiators. However, my pumps are on the return, so no by-pass valve can lift as the pump sucks the water around the system.

 

[stem]

If you have a wet central heating system at some point in its life the pump will fail. It's a fact, they don't last forever. Boiler manufacturers know this and design their products accordingly. Many boilers as well as having control thermostats also have an overheat thermostat. As @Notch7 these will stop the boiler firing when triggered.

To control any expansion, if your heat only boiler is fed from a feed and expansion cistern, they include capacity for that, an open vent and an overflow to outside.

If your heat only boiler is of the 'system' type it will have an expansion vessel and a pressure relief valve that will be vented outside. They normally curve towards the wall to keep any rapidly discharged spurt of hot water away from people who might be passing.

 

[mike42]

Thanks all - good variety of comments. The flow switch is something I hadn't thought of so in theory you could cap off the flow and return at the boiler and it just wouldn't start. Not that I recommended trying that test at home.....

I maybe be in a situation where I'm relying on a motorised valve which _starts_ to open and simultaneously the boiler is triggered to start via opentherm or low voltage (24v) signalling. So unlike an S-Plan the valve would not be fully open before the boiler starts. Plus the valve could fail to open.