Flow rate in central heating system

[Agile]

As for the delta t im pretty sure that for some reason anything greater than 15deg will signify a wait status in the boiler, S53. Its down to the sensors and their position and the bad software in the boiler(probably the most annoying design error with them)

As a differential of 20° would be normal operation then this is likely to be say 25° ? As the OP has said, a differential of 30° exists at startup and that triggers the S53.

On re reading the earlier posts there seems to be something else which both of us have missed. He said the problem occurs on ANY of the circuits. He has not been very informative about the balancing on the cylinder when asked. The cylinder coil could potentially be a very low resistance circuit.

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. Again measurement of the warmed up power input would give a guide as to the max power the boiler should be set to. These are aspects which need further investigation.

Tony

 

[edb49]

30°C is the trigger point, this I'm 99% sure about. I've sat in front of the boiler looking at d.40 and d.41 (flow & return temperatures) waiting for it to cut out. It will sit for minutes at a 29c difference, but as soon as it hits 30c differential I think it has 10 seconds to go back under this, otherwise it goes into S53.

On re reading the earlier posts there seems to be something else which both of us have missed. He said the problem occurs on ANY of the circuits. He has not been very informative about the balancing on the cylinder when asked. The cylinder coil could potentially be a very low resistance circuit.

The cylinder is a Megaflo CL250, by Heatrae Sadia. It looks like flow and return go in and out of it with no adjustment valve. There is a red stopcock that is fully-open and two thumb vents to let air out, but otherwise it's a direct in and out with no valves. Not sure if I am looking in the wrong place here though.

 

[edb49]

Right - on that rule of thumb:

1) Current flow rate = 1.15m^3/hour
At 1.15m^3/hr, we know:
Boiler pressure loss = 2.0m
Rest of system pressure loss = 2.9m

2) Desired flow rate = 1.6m^3/hour

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

This sounds like quite a big number, have I done my maths correctly?

 

[Norcon]

Have you tried lowering the boiler flow temp and narrow this differential down a bit and also help the efficiency?

 

[Agile]

The red valve on cyl is gate valve which can be used for balance.

I still would like to know the power input !

Tony

 

[Biggles..]

For those still interested and actually know what this is all about, agile included.

Your boiler stipulates the following 1600l/m minimum flow, max diff of 20 deg C and flow temp setting of 80c (its in the instructions you can argue that all you like) my manufacturers notes actually state 15deg before wait period.

Turning down the flow temp to 70 will reduce the return to 50, room temp 20 actually reduces boiler output by a third so know you've undersized the boiler.

i'll repeat it's not a balancing issue, if the cylinder was low resistance then if installed correctly with the cyl return as the last tee the return temp should be easily within the delta t of the boiler.

as for your calcs, no sorry they're wrong.

minimum flow 1.6-1.15 (your flow) squared.

0.45x0.45+0.2025

1.2025(increase)x2.9=3.5

3.5+4=7.5m head

Lee

 

[Biggles..]

Appolagies ed I think my calc is actually wrong and yours is right by the rule of thumb.

Too much red wine clouds the brain.

Lee

 

[mickyg]

can't be ar$ed to read the whole lot but a few questions..
1) has the ufh got its own pump set?
2) how many rads on the rad circuit?
3) how big hot water cylinder?

 

[edb49]

can't be ar$ed to read the whole lot but a few questions..
1) has the ufh got its own pump set?
2) how many rads on the rad circuit?
3) how big hot water cylinder?

1) Yes, the UFH has its own pump.
2) 14 rads
3) 250 litres

 

[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. Again measurement of the warmed up power input would give a guide as to the max power the boiler should be set to. These are aspects which need further investigation.

Not sure how you should measure this?

 

[mickyg]

1) Yes, the UFH has its own pump.
2) 14 rads
3) 250 litres

in which case a standard 15/60 on max should work absolutely fine with your system, if it doesn't something is not right with the system.

 

[Biggles..]

micky how can it when the desired flow rate is 1.6m3/hr for 20deg drop @38KW and the system head is 7metres.

15/60 cant produce those figures.

Your an ex group service guy you know that to avoid S53 you down rate,turn pump up or increase size.

oh by the way you have mail and does it work.

 

[mickyg]

micky how can it when the desired flow rate is 1.6m3/hr for 20deg drop @38KW and the system head is 7metres.

15/60 cant produce those figures.

Your an ex group service guy you know that to avoid S53 you down rate,turn pump up or increase size.

oh by the way you have mail and does it work.

I appreciate you've done the maths. I'm just saying it will work fine with no issues. I've seen many 38kw gw and vaillant steamers none of which had anything bigger than a 15/60. What pump do you get with a vaillant 637?
I think all this talk of pump size is detracting from what the real problem is as I suspect its not the pump.
Thanks for the mail it works

 

[Norcon]

The Op says the same happens with the HW circuit.
Surely its flow resistance is within the capability of the primary pump?
Is the balancing valve opened up enough?
And the coil uses what? About 24KW max. So the boiler should easily have a narrow differential temp on this.

Could the return to the boiler heat exchanger be partially blocked with sludge? The boiler tappings are 22mm and the primary return 28mm so thats hardly going to help.
Or are the thermistors giving the wrong readings?
What ever it is, it needs someone to have a look on site.

I assume the UF circuit resistance has not been added to the primary circuit resistance.

 

[Agile]

Not sure how you should measure this?

Look in the FAQs for measuring "gas rate" at the gas meter. Only do this when the system is stable and heating th ehouse.

As I said recently, there is something else wrong which none of us has identified yet. Otherwise the boiler would work fine on just the HW for example if it was a pump undersized issue.

Has the pump impeller been inspected to see if its blocked with dirt? 10 min job if the pump isolating valves work. Again see the FAQ.

Is there a blocked gause filter in the return?

So many possibilities which an engineer would probably find on site within 30 minutes!

Tony