Combi boilers and pump overrun

[Agile]

This is one weird thread.

I like the concept of the Intergas but with such a large block of ali that doesn't need a pump overrun to dissipate heat after hot water demand how can hot water demand be met instantaneously, the heat up time is slower I'd have thought. Has anyone ever compared it to a plate hx?

Which combis dump heat into the CH circuit after hot water is satisfied? I think the old Swiftflow might have but do others? Ours do not, the integral bypass is there for that.

There are two particular problems with the IG but they do have a facility to mitigate it.

As you suggest the big ali block does have a significant heat up time from cold , perhaps at least a minute.

But they have an intelligent preheat which keeps it preheated at the same times as it was previously giving DHW the day before. Using that, as I always suggest customers do, the DHW delivery is then instantaneous!

All older boilers with permanent pilot lights and fans do run the fans at low speed. This sucks in air and required an at least annual cleaning of the fan blades.

Older boilers with a water activated diverter valve will dissipate heat to the CH during pump over run. Those that are temperature activated will only need to over run for 10-20 seconds but the poor hysteresis in the thermal stats often used will make that more like two minutes in most cases, even when the CH is cold.

Tony

 

[vulcancontinental]

This is one weird thread.

I like the concept of the Intergas but with such a large block of ali that doesn't need a pump overrun to dissipate heat after hot water demand how can hot water demand be met instantaneously, the heat up time is slower I'd have thought. Has anyone ever compared it to a plate hx?

Which combis dump heat into the CH circuit after hot water is satisfied? I think the old Swiftflow might have but do others? Ours do not, the integral bypass is there for that.

There are two particular problems with the IG but they do have a facility to mitigate it.

As you suggest the big ali block does have a significant heat up time from cold , perhaps at least a minute.

But they have an intelligent preheat which keeps it preheated at the same times as it was previously giving DHW the day before. Using that, as I always suggest customers do, the DHW delivery is then instantaneous!

All older boilers with permanent pilot lights and fans do run the fans at low speed. This sucks in air and required an at least annual cleaning of the fan blades.

Older boilers with a water activated diverter valve will dissipate heat to the CH during pump over run. Those that are temperature activated will only need to over run for 10-20 seconds but the poor hysteresis in the thermal stats often used will make that more like two minutes in most cases, even when the CH is cold.

Tony

Ah, touch and go as we call it. I guess that's why it is introduced on the boiler, because of the heatup time. I don't suppose it costs much, anyway better than a 24/7 preheat.

 

[Alfie25]

Ok I think I get it now. So what happens when the CH is turned off with regards pump overrun? As the circulating water will already be very hot, won't it take ages to cool down? For temperature controlled pump overruns won't this take a long while to achieve?

 

[Alfie25]

Sorry to come back to this one but still trying to understand. For boilers that have a pump over run based on a temp drop rather than time, how long could the pump be running before it is satisfied and turns off? I'm talking about after the CH has been on, so the system water will be very hot.

Baxi are pretty clear and says theirs runs for 3 mins (time based, not temp), but Viessmann just says theirs works on temperature, so would we be talking hours before the system water cools to the preset level?

 

[Dan Robinson]

Dunno. Lost my piece of string

 

[Alfie25]

Fair enough Dan I do appreciate it's a bit of an open ended question but seeing as you're reading this, what's your experience with temperature based pump overruns? Do they tend to work like a thermostat and the system water has to fall to a set internal temp?

Could it effecitvely take over an hour to drop? Assuming the system water set high (e.g. 80 degrees C), what level would it have to drop to in order to satisfy the pump over run?

 

[Agile]

Alphie, you seem to not be noticing is that a CH system has a design drop of 10-20 C and so dumping the heat into the rads will quickly reduce the temperature.

Boilers vary but typically would want to reduce the internal temperature to say 65 C before turning off the pump over run.

Tony

 

[bazdaman]

Glow worms seem to drop to about 65 ish Tony ive never really heard a boiler pump stay on for longer than 5 mins.

 

[Alfie25]

Alphie, you seem to not be noticing is that a CH system has a design drop of 10-20 C and so dumping the heat into the rads will quickly reduce the temperature.

Boilers vary but typically would want to reduce the internal temperature to say 65 C before turning off the pump over run.

Tony

True, I may be over-estimating the time it would take for the temp to drop. I suppose it all depends on what parameter is set in the Viessmann for when the over run stops. Maybe 65 like you say, which shoudnt take too long to achieve from a peak of 80. I'd have thought for cheapness it would just be on a timer rather than temp controlled but obviously not. Heard mixed things about the 100 series, but then the joy of the internet is you hear the best and worst of everything!

 

[Alfie25]

For the sake of completeness and if anyone reads this in the future I thought I'd post back my conversation with Viessmann technical.

They say the pump overrun on the 100W is temp based, and after the demand for CH is turned off, boiler internal temp needs to drop by 34 degrees for the pump overrun to stop. In reality they advised this could be 20 minutes or it could be over an hour which is not unknown. My house is pretty well insulated so I'd expect it to be at the longer end of the spectrum.

Bizarelly, the 200W is time controlled, as per most other combis, with a default pump overrun time of 12 minutes but can be set anywhere between 0-20 minutes. Seems strange to me that the feature can effectively be turned off by setting to 0 minutes, and that the 200W (which is more feature-packed than the 100W) seems less 'intelligent' in this respect.

The long and short of it is a boiler with a pump overrun of potentially an hour would annoy the life out of me so I'm going to look for a different boiler, hence the new thread.