Central heating pump sizing

[John.g]

The whole thing is baffling, even though its highly unlikely that the pump would fluctuate on both CP and PP, it might be worth trying it on a "true" fixed speed or constant curve setting ie CC3 as shown above, if the boiler is still fluctuating then I think you can rule out the pump as the cause.

 

[John.g]

Trying CP3 28W 4.5m 0.8m3/Hr 62oC target DT 19oC Boiler started modulating fan speed 145<>177 consistent. Then later when I checked, it was 150 and steady, no modulation ??? Then within seconds of me looking at it, it cut out ??? Can’t see why though took 3 restarts and stayed running, started modulating again 145<>180

it seems, sometimes at least, that the boiler decreases it’s firing with each successive restart, which allows it to eventually stay running, is that how it works, or am I imagining it ???

Nothing really adds up from the above numbers, wonder if the thermistors are faulty or improperly attached to the flow/return pipes.
A deltaT of 19C&0.8m3/hr = boiler output of 17.67kw, clearly not correct. Fan speed of 145/177 = ~ 5.57kw output = deltaT of 6C, more believable but seems too low except restricted by d.00?.

 

[Reddwarf4ever]

Running very well today CC3 35W 5.0m 1.0m3/Hr boiler 63oC DT 19oC fan 150 running steady....been off a couple of times as room stat satisfied, so thats good, but been milder today.....only cut out once today for no apparent reason, but normalised fairly quickly, but wasn’t able to observe. Obviously CC3 uses a little more power than PP3, but maybe there are benefits.....(it did show lo flow just once after reinstalling the pump, but think it was due to being on hot water only and a lot of air in the system) tried to fit the insulation shroud, but typist for a 180 version and doesn’t fit mine, so it’s back to hessian wrap....dont Need the shroud now

Air noise has disappeared now completely, hiding in a radiator somewhere wonder if I should connect a valve to the vent, ( currently return and feed linked together ) to occasionally vent to the F&E tank ???

 

[John.g]

Something not adding up there, you have a deltaT of 19C ie flow of 63c & return of 44C? which at a flow rate of 1M3/hr equals to a boiler output of 22.1kw, no way.
The fan speed would indicate a boiler output of ~ 5.2kw so something has changed dramatically from your original readings. If you keep slowly reducing d.00 until the flow or/and return temps start falling then you will get a good feel for the real boiler/rad output, can then start trouble shooting.

Can you give a sketch of your combined vent & cold feed, mine has the vent carrying on up as per normal with the cold feed teed into it.

 

[Reddwarf4ever]

Sorry I meant DT 9oC......hope that makes more sense....will have. look at the F&E tank, a plumber did this for me and I insulated it all, so can’t see where / how it joins.....will jot down how it’s connected.

 

[John.g]

A deltaT of 9c = 10.47kw, more like it but the fan speed is a mystery as IMO it should be ~ 3024, as I said above, progressively reducing the output will tell alot, ie if deltaT still remains the same at a d.00 of 6kw as at 12kw then the boiler isn't outputting 10.47kw.

 

[Reddwarf4ever]

A deltaT of 9c = 10.47kw, more like it but the fan speed is a mystery as IMO it should be ~ 3024, as I said above, progressively reducing the output will tell alot, ie if deltaT still remains the same at a d.00 of 6kw as at 12kw then the boiler isn't outputting 10.47kw.

Could it be the f,ow isn’t 1.0m3/Hr ? As this is a calculated flow, not sure how accurate that is....

 

[John.g]

Will answer that shortly, what is the output clamped at just now, d.00?.

 

[John.g]

The calculated flow with a clean pump is quite reasonably accurate IMO, its another story if the pump is fouled as the efficiency falls off and as the pump uses power to calculate the flow rate then it can show a very inaccurate number. There was a case here where a UPS3 was suggesting no flow in all its different modes so suggested completely blocked boiler Hx etc but when BG opened the pump and cleaned it, everything returned to normal, the system was pretty dirty.
Its unlikely IMO that you have a problem like that because if the flow rate is for example 50% of the apparent then a 5.2kw boiler output would result in a 20/25C deltaT. However I would use all the tools at my disposal to establish the boiler output and one of these is by reducing d.00 until balance is achieved and you will then know the output. Once you have done this I would ask Vaillant why the fan speed is apparently so low, next time you look at it have a look at the target speed d.33 as well as its value, d.34.

 

[Reddwarf4ever]

D33 & D34 always identical did a test this morning, target 62oC DT 8oC Fan 167 ( 150-167 ) set D0 to 6KW ( 5KW resulted in output dropping to 60oC, but held steady while under observation )
so how does that sound with pump on CC3 ?