Central heating pump sizing

[John.g]

Will do the calcs in a minute, is the flowrate still indicating 1.0M3/hr?, this is important.

 

[Reddwarf4ever]

Will do the calcs in a minute, is the flowrate still indicating 1.0M3/hr?, this is important.

DT 6oC fan steady 150 assume boiler limited to 5KW Target 62oC, actual 62-63oC
Pump 35W 5.0m 1.0m3/Hr ( maybe CC2 would help reduce the return temperature ? )

be good to make up some calculations to put in an excel sheet....with various unknowns......have to look through the posts and set this up

UPDATE
boiler outlet fell to 54oC with D0 5KW, increased to 7KW fan now 170 outlet climbing slowly now at 59oC.....think it will settle around 6KW with the fan around 160-170...DT currently changing during heat up

 

[John.g]

Ok then, if I assume that the heating demand is 5.5kw from the fan speed, a deltaT of 8C means the flowrate has to be 0.59m3/hr, Also, it means that the total rads output at max (50 deg) output is 7.9kw, which implies that you only have maybe 4/5 rads in service just now?.
If I assume the heating demand based on a flowrate of 1 m3/hr and the same deltaT of 8c then the output is 9.3kw and implies that the max output of the rads in service (at 50C) is 13.3kw.

A different approach is to monitor your gas meter.
Your gas flowmeter probably measures the flowrate in m3/hr, the older type (here anyway) measure in 1/10ths of a m3 which means a very quick/accurate spot check can be made in a short time, if the meter only turns over every m3 then you would have to monitor it over a hour, a m3 of gas around here is ~ 10.8 kwh so easy to calculate the real boiler output from this assuming a boiler efficiency of ~ 88% (or whatever).
Have a look at your meter and see its scale.

"UPDATE
boiler outlet fell to 54oC with D0 5KW, increased to 7KW fan now 170 outlet climbing slowly now at 59oC.***.think it will settle around 6KW with the fan around 160-170...DT currently changing during heat up"

I think the fan speed is a pretty good indication of your boiler output, from the above, fan speed 1650 rpm so output = 18*1650/5200, 5.7kw.

So, how many rads are currently turned on or in service?.

 

[Reddwarf4ever]

All 12rads, 3 without TRVs fully open, all hot, house warming up well.

D0 now at 7KW DT 8oC outlet stable 62oC fan constant 1500rpm

“18*1650/5200, 5.7kw.” Are the 18 and 5200 constants ?

 

[John.g]

Yes, I'm assuming that when the boiler is running at 18kw that the fan speed is 5200 RPM.
The gas meter will/would provide a good check especially if its turning in 1/10ths of a m3.

 

[Reddwarf4ever]

Been looking back at previous posts. Not sure if I have a definitive solution currently on CC3 today was particularly mild....watched the boiler as it went down to the lowest firing ‘ Fan 150’ outlet temperature was 63oC ( Target 62oC ) ...but slowly climbed and the boiler could not reduce firing any further so cut out when it reached 65oC. Couldn’t see what i could do about, other than raise the target temperature. Before I bought the new pump, I was mistakingly running the outlet at 72-78oC amd the only problem I had, I believe was the pump, but at that temperature I was certainly not in condensing mode. If I currently run the outlet higher than 64oC the inlet will be above 55oC.....I assume a higher flow will allow the rads to radiate more heat, as they are constantly kept closer to the boiler outlet temperature, but the return temperature will rise also. Maybe I should have bought an ultra low firing boiler, which goes down to 3.5KW, although these seem to be much more expensive and can’t justify the cost to change it now, even though it’s 11 years old.

during colder days or in the evening, it manages ok, but as we get closer to the warmer weather, the boiler will struggle more. I wonder if a lower flow would suit my boiler better, won’t release so much heat from the rads, but the return temperature should be a lower and help it fire and very low KW......

not sure now which way to go

 

[John.g]

I would suggest not reducing the flow rate below ~ 0.6/0.7 m3/hr.

Is the room stat switching in/out during these low boiler demands?.

Also, a boiler should be reasonably able to cycle without any real problems, I would consider a 5.2 kw minimum output as quite acceptable.

You might try return temp control again some time even though it didn't control to the expected (return) target temp, it would be interesting to see then what parameter shuts the boiler down on minimum output, ie the target return temp+3C or the actual return temp+3C or whatever.
From post #203
Changed D17 to 1, set the temperature by the dial to 55oC ( should be the inlet temperature) took some readings while it was firing, but abandoned for now, as not sure quite what was happening....D41=37oC D40=47oC D05=36oC wasn’t firing very hard

Edit: "d.05 Determined heating flow set target temperature" maybe this is why your setting doesn't influence it?, it implies that the heating curve does but if so, without outside temperature sensing one would think it should follow the target temperature, vaillant will know, maybe.

 

[Reddwarf4ever]

Will have a try again......so difficult emailing vailant, they take forever to reply & tend not to answer all the questions, just the first one....but will try and ask....thanks

 

[John.g]

Before c/o, see if d.05 is the same as your set flow target temperature, also after c/o, see if altering your (return) target temperature also alters d.05.

 

[Reddwarf4ever]

Will have a look at what you suggest.
But the simple science is that I need a boiler that will go below 5KW, or find a way to dissipate more heat into the house, can change two downstairs rads to double fins, possible 30% extra heat from each, maybe enough. Currently target 64oC inlet 55oC, fan 150-155 modulating. Have tried CC2 to see if it helps, not sure if it has or not, trying to determine best pump setting to give a condensing return temperature and allowing the boiler to have some control over the outlet temperature. When the weather was colder, at the start of this post, it was a lot easier, however as the weather warms, it will get more difficult.

Trying to work out if a higher or lower pump flow is better in my circumstances. Also is it safe to assume that if I see steam from the boiler vent it’s in condensing mode ? Or would I see a little steam anyway ?

Thanks