Flow rate in central heating system

[D_Hailsham]

Another odd feature is that he says he has tried to reduce the max output but the boiler does not seem to accept the instruction.

If you read his post carefully he says that he reduced the output to 7kW, which is almost at the bottom end of the modulation range. He has not said if he resolved this problem. I suggested he set it to 35kW.

At startup the whole system is cold and the TRVs all open thus giving the lowest flow resistance.

To set an ABV to be passing at that setting would mean the full boiler output would never be obtained.

I agree all TRVs should be open and the flow resistance is the minimum. But, if the rads have not been balanced properly they will start closing too early, before the ABV comes into operation. This will result in a reduction in the flow rate and hence an increase in the system pressure.

I would like to know why the OP thinks the system is properly balanced.

Did anyone notice this?

There is no stat for the rads! There are independent stats for all the rooms with UFH, but the rads just rely on the timer in the heating controls.

So what turns the boiler off when the rads get up to temperature? All the TRVs will have closed, but the boiler will continue to run until something stops it - normally the boiler thermostat, but could an S53 also occur?

Might be a good idea if the OP fitted a room stat (and got rid of one TRV).

Right - on that rule of thumb:

Boiler pressure loss = 4.0m (from manual)
Rest of system pressure loss = 2.9m * (1.6/1.15)^2 = 5.6m

Total pressure loss = 9.6m

The maths is OK but a system head of 5.6m, excluding the boiler, is very high. It implies that the pipe have not been properly sized for the heat flow. Assuming you have 38kW at 80/60 at the start, and a flow rate of 1.63m³/hr, your main boiler connections should be 28mm. This will give a pipe velocity of just under 1m/s which is acceptable. But the rest of the system needs to be properly sized as well.

A proper calculation of the index circuit would help.

 

[edb49]

Hmm. I think I may have been heading up the wrong path here.

I've compared the UFH pump to do the main circulator pump. The main pump has quite a bit more noise.

The UFH pump just has an electric motor whir. The main circulator has this noise but also you can hear what flowing through it, a bit like when your turn a tap on with an empty hose connected to it, you hear water flowing through the hose until it starts spurting out the end.

Second thing I've identified is the system is losing pressure. About 3 weeks ago the system was at 1.4 bar, and yesterday I checked it was at 1.0 bar. I've repressurised to 1.5 bar, but I guess drop in pressure must mean:
1) Leak
2) Air being generated somewhere and vented
3) Pressure being released somewhere?

The pump has isolator valves so I will take it off this afternoon and have a look at it.

 

[edb49]

Once all the rads are up to temperature, and assuming there is no demand from the UFH and HW, then I assume the boiler will modulate down to its lowest output until the flow exceeds the set point, at which point it will shut down.

I don't think this will trigger an S53 though, as the delta between flow and return would be about as small as it can be, assuming the boiler is inputting very little heat and the water has the shortest circuit back to the boiler's return.

 

[edb49]

Another odd feature is that he says he has tried to reduce the max output but the boiler does not seem to accept the instruction.

If you read his post carefully he says that he reduced the output to 7kW, which is almost at the bottom end of the modulation range. He has not said if he resolved this problem. I suggested he set it to 35kW.

I have reduced the output to 25kw, being about 2/3rds the peak output. With that reduction I'm getting a 26c delta between flow and return. In theory if I reduce it to 12kw I should see a 13c delta? Or does it not work like that?

I would like to know why the OP thinks the system is properly balanced.

I'm assuming it is properly balanced, as it has been working perfectly with a different boiler for 10+ years.

 

[Biggles..]

You won't get S53 when cutout at boiler stat.

Boiler optimum is 15deg delta t cutout for S53 is 20deg, that's from a Vaillant engineer(dm)

I'll leave it to the rest,high resistance in your system-not a one pipe in places is it.

 

[Deleted member 105431]

as above from lcgs I recommend you call out an engineer familiar with these boilers but first try these settings

d0=30
d1=10
d71=75

the password for level 2 access is 35. I would say the primaries must be at least 28mm upto the controls (motorised valves) and you will need the pump lcgs quoted. The pump must be on it's highest setting.
The undefloor heating should be timed for at least 16hrs a day in the winter too.

 

[Norcon]

I've compared the UFH pump to do the main circulator pump.

Are you saying the 25-55 is NOT the primary pump. (ie - boiler pump)?
Oh dear.

The main pump has quite a bit more noise.

What size is this one then? And they should run pretty much silent.
You say system is losing pressure.
Full of AIR now may be?

There might be another possibility.
The primary pump is probably positioned on the flow and coupled with the high resistance heat exchanger and lower heating circuit resistance resulting in the pump being starved of liquid leading to cavitation.

You need a minimum inlet pressure of 0.5m head at 82c to avoid cavitation and ensure quiet running of the pump. (Grundfos spec)

I'm assuming it is properly balanced, as it has been working perfectly with a different boiler for 10+ years.

It won't be properly balanced for a boiler with a 4m head through the heat exchanger at the design flowrate needed as the previous boiler may have been much lower.
See balancing in FAQ's.

 

[Agile]

Six pages on and I still dont know if this boiler has an internal pump !

Its not correct to run the system with TRVs on every radiator and just rely on the boiler to cycle. There should be a room stat to actually turn it off.

If there are TRVs on every rad then the system should have an ( auto ) bypass.

There is something seriously wrong with the flow rate. The pump needs to be isolated and then just the head removed to check that the impeller is not clogged with debris.

After refitting the pump it should be bled from the central screw, ideally with each valve open in turn. It should be pretty quiet in operation and any "water" noises would imply trapped air.

I would also recommend the "bucket test" ( see FAQ ) while the head is off to check that the pump valves are not clogged.

Eventually we might find the problem but its taking a very long time so far. More important than balancing the rads is balancing the flow between the hot water and heating but this does not appear to be the cause of the problem because the excess differential is stated to occur as well if its just CH or HW or with both!

Tony

 

[edb49]

The 25-55 180 is the main pump, the UFH has a separate 15-40 130 pump.

 

[Biggles..]

i like the way agile gives advice without knowing this boiler doesnt have an internal pump.

 

[Agile]

Did I miss where the OP stated the exact boiler model?

 

[Biggles..]

Did I miss where the OP stated the exact boiler model?

yes obviously.wink

 

[edb49]

What should the next course of action be?

I think the current pump isn't sufficient anyway; my reasoning is that the boiler on its own produces a resistance of 4.05m at the 1.633m^3/hr desired flow rate.

At this flow, the most resistance the pump can cope with on its highest setting is 4.7m. That gives 4.05m for the boiler, and 0.65m for the rest of the system. I am guessing that 0.65m is very unlikely to be "the answer" for the rest of the system - so the pump is undersized and needs to be uprated.

So should course of action be:

1) New pump
2) Powerflush
3) Re-balance system

If there is any compotent professional in Maidenhead please let me know about them! As I've mentioned, already had three separate guys out and don't think any of them made much progress.

 

[Biggles..]

Did I miss where the OP stated the exact boiler model?

yes obviously.wink

 

[Biggles..]

as above from lcgs I recommend you call out an engineer familiar with these boilers but first try these settings

d0=30
d1=10
d71=75

the password for level 2 access is 35. I would say the primaries must be at least 28mm upto the controls (motorised valves) and you will need the pump lcgs quoted. The pump must be on it's highest setting.
The undefloor heating should be timed for at least 16hrs a day in the winter too.