What size boiler needed

[sherylryan]

I think that's what's happening, i just can't work out why the pump is running so fast to circulate water around the closest circuit to the boiler. I'm stumped )

Below pics of pump operating different circuits, notice the ktichen manifold RPM compared to laundry and Radiators that have a longer circuit.

Also done an experiment with rads and kitchen (troubled manifold) both calling for heat, boiler flow never got over 43.6 degrees after 40 mins of running, i turned ufh manifold off and within 2.39 mins flow temp had risen 13.4 degrees to 57.

I am now thinking if the pump if pushing water that fast through flow to manifold is this indicating a blockage in the flow pipe?

Its may cause erratic operation of the UFH manifold TMV because you have hot water at high pressure on one side of the TMV and much lower pressure on the other side coming from the cold manifold return.

 

[John.g]

Can you give me your observed/estimated readings for the two UFH manifolds, ie, boiler/manifold flow temp, manifold(s) flow (mixed) temps & return temps & the total manifold flow rates.
Also a rough idea of the total rated rads output and the actual/estimated dT.

Does that pump display only the RPM in that box?, ie some can display the calculated flowrate if selected.

 

[sherylryan]

Can you give me your observed/estimated readings for the two UFH manifolds, ie, boiler/manifold flow temp, manifold(s) flow (mixed) temps & return temps & the total manifold flow rates.
Also a rough idea of the total rated rads output and the actual/estimated dT.

Does that pump display only the RPM in that box?, ie some can display the calculated flowrate if selected.

(BOILER FLOW TEMP SET TO 65)

Kitchen ufh manifold (troubled)
Boiler/manifold flow temp 48 c
Manifold mixed flow temp 45
Manifold return temp 35
Total flow 28.5 LPM

Laundry manifold
Boiler/manifold flow temp 70 c
Manifold mixed flow temp 45
Manifold return temp 35
Total flow 16.5 LPM

Total Radiator output 19kw
DT across Rads 12 degrees
Flow 70
Return 58

I will check the pump re the calculates flow

 

[John.g]

OK here are my numbers.
Why is the boiler temp so low at 48C (SP 65C) assuming a heat demand of "only" 19.88kw for the kitchen manifold??.
You can see why the Blr pump speed is far greater on the kitchen manifold vis the Laundry manifold due to the different flowrates required, 21.92LPM vis 4.71LPM respectively.

The very low pump speed on rads only at 891LPM for a flowrate of 19.23LPM is indeed puzzling, would have expected it somewhere up to 1700/1800 RPM based on the blr circ pump speed of 1870 RPM required for the kitchen manifold with its blr circ pump flowrate of 21.92LPM.



(BOILER FLOW TEMP SET TO 65)

Kitchen ufh manifold (troubled)
Boiler/manifold flow temp 48 c
Manifold mixed flow temp 45
Manifold return temp 35
Total flow 28.5 LPM
Blr circ pump flowrate, 21.92LPM (Manifold output, 19.88kw) Pump RPM 1870.

Laundry manifold
Boiler/manifold flow temp 70 c
Manifold mixed flow temp 45
Manifold return temp 35
Total flow 16.5 LPM
Blr circ pump flowrate, 4.71LPM (Manifold output, 11.50kw,) Pump RPM 1080.


Total Radiator output 19kw
DT across Rads 12 degrees
Flow 70
Blr circ pump flowrate, 19.23LPM (Rads output, 16.10kw, based on mean rad temperatures)
Pump RPM 891




Return 58

 

[sherylryan]

OK here are my numbers.
Why is the boiler temp so low at 48C (SP 65C) assuming a heat demand of "only" 19.88kw for the kitchen manifold??.
You can see why the Blr pump speed is far greater on the kitchen manifold vis the Laundry manifold due to the different flowrates required, 21.92LPM vis 4.71LPM respectively.

The very low pump speed on rads only at 891LPM for a flowrate of 19.23LPM is indeed puzzling, would have expected it somewhere up to 1700/1800 RPM based on the blr circ pump speed of 1870 RPM required for the kitchen manifold with its blr circ pump flowrate of 21.92LPM.



(BOILER FLOW TEMP SET TO 65)

Kitchen ufh manifold (troubled)
Boiler/manifold flow temp 48 c
Manifold mixed flow temp 45
Manifold return temp 35
Total flow 28.5 LPM
Blr circ pump flowrate, 21.92LPM (Manifold output, 19.88kw) Pump RPM 1870.

Laundry manifold
Boiler/manifold flow temp 70 c
Manifold mixed flow temp 45
Manifold return temp 35
Total flow 16.5 LPM
Blr circ pump flowrate, 4.71LPM (Manifold output, 11.50kw,) Pump RPM 1080.


Total Radiator output 19kw
DT across Rads 12 degrees
Flow 70
Blr circ pump flowrate, 19.23LPM (Rads output, 16.10kw, based on mean rad temperatures)
Pump RPM 891




Return 58

The boiler will not get any higher than 48 degrees with heat only on kitchen manifold, I have again just now watched it SP is 65, flow no higher than 48 and boiler has been running for 48 mins continuously. I am totally stumped as to why the boiler does not perform when heating kitchen manifold.

 

[John.g]

The only logical explanation is that the demand is > than the boiler output?. I think you said somewhere that the boiler is actually derated to 23kw so not far away from the nearly 20kw kitchen demand, you might consider reduving the TMV setpoint to 38/40C, this should result in a reduction in the heat emitted by the loops and allow the boiler to get up to its SP temperature.

I will look up the boiler output nozzle rating and see what output it gives at 14.0bar.

 

[sherylryan]

The only logical explanation is that the demand is > than the boiler output?. I think you said somewhere that the boiler is actually derated to 23kw so not far away from the nearly 20kw kitchen demand, you might consider reduving the TMV setpoint to 38/40C, this should result in a reduction in the heat emitted by the loops and allow the boiler to get up to its SP temperature.

I will look up the boiler output nozzle rating and see what output it gives at 14.0bar.

It’s derated to 26kw.

The only logical explanation is that the demand is > than the boiler output?. I think you said somewhere that the boiler is actually derated to 23kw so not far away from the nearly 20kw kitchen demand, you might consider reduving the TMV setpoint to 38/40C, this should result in a reduction in the heat emitted by the loops and allow the boiler to get up to its SP temperature.

I will look up the boiler output nozzle rating and see what output it gives at 14.0bar.

It’s derated to 26kw, using a danfoss .55 80s, air setting 3.5.

I understand the different LPM kitchen vs Laundry but the kitchen flow is 5m from boiler and the laundry flow is 31m from the boiler, I thought the distance would also play a role in the laundry manifold flows.

Pump does not give me any flow rates.

 

[John.g]

Pipe (friction) losses are proportional to the square of the flow, so the losses at a flowrate of 4.71LPM are tiny with the piping sizes you have ~ only 5 to 6% of the losses at 20LPM. Your rads have no problem with a circulation demand of 19.23LPM.

 

[sherylryan]

Pipe (friction) losses are proportional to the square of the flow, so the losses at a flowrate of 4.71LPM are tiny with the piping sizes you have ~ only 5 to 6% of the losses at 20LPM. Your rads have no problem with a circulation demand of 19.23LPM.

I thought that was the case. Does the kitchen manifold need a bypass on it?

Just done laundry manifold, boiler temp SP 60c, flow out of boiler 71.6 degrees, return to boiler 43.1. Boiler stopped firing for 2 mins flow was still 62 degrees when boiler refires, return 44

 

[John.g]

Why does the kitchen manifold require a bypass??

What was the laundry manifold (mixed) temp? and was the manifold flowrate still 28.5LPM?.
If the boiler/manifold return was 43/44C then the manifold temperature had to be 6C to 8C higher so TMV not going much at a SP of 40C??

NOTE: Its the kitchen manifold SP temp that I suggested reducing to 38C/40C?? to see can the boiler increase to its cut out temp. Forget the laundry, it seems to be OK.

 

[sherylryan]

Why does the kitchen manifold require a bypass??

What was the laundry manifold (mixed) temp? and was the manifold flowrate still 28.5LPM?.
If the boiler/manifold return was 43/44C then the manifold temperature had to be 6C to 8C higher so TMV not going much at a SP of 40C??

I remember the plumber telling me if the pumpset was changed a bypass would need to be fitted, I'm not sure why )

We have no bypass on our system at all.

Laundry manifold mix temp was flow 45 return 35, boiler return 44. I am picking a lot of water returning back to the boiler through the stupid pumpset on laundry manifold.

Another question I've been thinking about today, Our expansion vessel connects into the firebox (I think that's what its called) of the boiler, I am wonder when they removed the internal pump this vessel was removed from flow pipe. I'm not sure on this just asking out of curiosity. Where actually is an expansion vessel suppose to be connected to? Thanks john you are very helpful.

You can see in the 2nd image an updated model boiler than ours and the expansion vessel connects into the flow pipe.

 

[John.g]

Must go out for a hour or so, will revert later, can you run that test on the kitchen manifold.

The Laundry manifold TMV is performing perfectly as you can see by your findings, below, with mixed flowtemp of 45C (boiler at 70C) , there is not a lot of water returning to the boiler, only 4.,71LPM?.

Laundry manifold
Boiler/manifold flow temp 70 c
Manifold mixed flow temp 45
Manifold return temp 35
Total flow 16.5 LPM
Blr circ pump flowrate, 4.71LPM (Manifold output, 11.50kw,) Pump RPM 1080.

 

[sherylryan]

Must go out for a hour or so, will revert later, can you run that test on the kitchen manifold.

The Laundry manifold TMV is performing perfectly as you can see by your findings, below, with mixed flowtemp of 45C (boiler at 70C) , there is not a lot of water returning to the boiler, only 4.,71LPM?.

Laundry manifold
Boiler/manifold flow temp 70 c
Manifold mixed flow temp 45
Manifold return temp 35
Total flow 16.5 LPM
Blr circ pump flowrate, 4.71LPM (Manifold output, 11.50kw,) Pump RPM 1080.

if only our kitchen one would perform nicely, yes will run that test now.

 

[sherylryan]

if only our kitchen one would perform nicely, yes will run that test now.

Ok so here goes

Boiler setpoint = 65 c
Pump RPM 1350


Boiler flow and return - Boiler fired continuously for 61 mins,
F = 48.8 - R = 32 after 5 mins
F = 53.6 - R = 34.7 after 15 mins
F = 55.6 - R = 36.6 after 30 mins
F = 56.2 - R = 37.3 after 40 mins
F = 56.6 - R = 37.5 after 45 mins
F = 56.7 - R = 37.5 after 50 mins
F = 56.8 - R = 37.6 after 55 mins
F = 56.8 - R = 37.6 after 60 mins
FINAL FLOW = 56.8 C RETURN 37.6 C

Manifold TMV = 40 c
Flow and return
F = 45.0 - R = 34 after 6 mins
F = 47.2 - R = 35 after 16 mins
F = 48.6 - R = 37 after 31 mins
F = 48.6 - R = 37 after 46 mins
F = 48.6 - R = 37 after 51 mins
F = 48.6 - R = 37 after 56 mins
F = 48.6 - R = 37 after 61 mins
FINAL FLOW = 48.6 C RETURN 37

Boiler/Manifold Flow temp 56.8 - setpoint 65 c
Manifold Mixed temp 48.6
Manifold return temp 37
Total flow 27 LPM

Interesting that the radiators and Laundry manifold get 10 degrees above boiler set point but kitchen manifold never reaches boiler set point.

 

[John.g]

The above gives a manifold demand of 21.85kw so the boiler output must be there or there or there abouts the same since it can't reach its SP and cut out, I suppose the manifold temp of 48.6C isn't bad control since we don't know how accurate the TMV SP index is, you might reduce it to 30C sometime to see reduced manifold output and boiler cut out

For interest, you can see the bypass flow required to increase the boiler return temp from 37C to 45C.

The position of the EV connection is effectively into the pump suction in your boiler and it the new original seems to be into the pump discharge with the pump pumping into the return, probably OK in both cases.

Boiler return 37C (no bypass)

Boiler return 45C (with bypass)