Central heating pump sizing

[Reddwarf4ever]

Thanks, will try the lower outlet temperature first, but if not, where would this auto bypass be fitted exactly ? Thanks

 

[Sparkgap]

Thanks, will try the lower outlet temperature first, but if not, where would this auto bypass be fitted exactly ? Thanks

Basically between the boiler flow and return. I usually put them at least a couple metres away if possible but before any zone valves or controls. An AVDO or similar valve will start opening as the TRVs close and maintain the boiler flow rate. Also since there will not be more water forced through the open rads any noise will be reduced.

 

[Reddwarf4ever]

Basically between the boiler flow and return. I usually put them at least a couple metres away if possible but before any zone valves or controls. An AVDO or similar valve will start opening as the TRVs close and maintain the boiler flow rate. Also since there will not be more water forced through the open rads any noise will be reduced.

If this valve is between the boiler flow and return, is it effectively a boiler bypass ? Can’t see how that increases the flow through the boiler ?

 

[Last Plumber]

If this valve is between the boiler flow and return, is it effectively a boiler bypass ? Can’t see how that increases the flow through the boiler ?

It creates a short circuit which opens more when the circuit closes down to help the boiler maintain a minimum flow rate. I personally do not think this will make any difference to the issue you have, although I agree it needs to be there.

 

[Reddwarf4ever]

It creates a short circuit which opens more when the circuit closes down to help the boiler maintain a minimum flow rate. I personally do not think this will make any difference to the issue you have, although I agree it needs to be there.

Thanks

 

[Last Plumber]

Thanks

I suspect that adding the by-pass to your system would (if set weak enough) prevent the delay/waiting code from being activated because you would likely increase the return temperature to something the boiler wants to see. However, you would still have a circulation problem and I personally think the anti-cycling timer would come into play more.
These are just my thoughts on what I have read regarding your heating system.

 

[John.g]

What about taking the TRV rads out of the picture altogether by noting the TRV settings on each and then just shutting them and just run the boiler on the other 4 rads, the heating demand from them may still be > 5 kw to allow the boiler to run continuously if the boiler flow temp is 70/75C, this will also give a reasonable feel for the flow rate and deltaT?.

 

[Reddwarf4ever]

What about taking the TRV rads out of the picture altogether by noting the TRV settings on each and then just shutting them and just run the boiler on the other 4 rads, the heating demand from them may still be > 5 kw to allow the boiler to run continuously if the boiler flow temp is 70/75C, this will also give a reasonable feel for the flow rate and deltaT?.

Hi
if I shut off some TRV rads, won’t that further restrict the flow through the boiler ?

 

[Last Plumber]

Hi
if I shut off some TRV rads, won’t that further restrict the flow through the boiler ?

Here is a few lines from post 32: -

Are all your lock shields open?
Is the pipework off the top of the boiler vented properly? (no possibility of air lock in flow or return).
Are the pump valves fully open and clear?
Once the obvious has been ruled out, look for partial blockages.

Is there any reason that doesn't make sense? Am I missing something or have I missed something you said earlier?

 

[John.g]

4 rads with wide open valves should have no problem in circulating adequate water through the boiler, if the deltaT is again too high then IMO there is restriction somewhere or possibly a pump problem if set to speed 2 or speed 3.

 

[Reddwarf4ever]

4 rads with wide open valves should have no problem in circulating adequate water through the boiler, if the deltaT is again too high then IMO there is restriction somewhere or possibly a pump problem if set to speed 2 or speed 3.

happy to change the pump.....but if I am doing that anyway, maybe a higher throughout would be best......want comfort more than a low wattage pump.....any thoughts ?

 

[SJB060685]

If you double your flow rate through fixed pipework you will double the velocity of the water. This can not only cause noise problems throughout the system, it can also cause valves etc to fail earlier through erosion. More importantly though if you double the flow rates this increases the frictional resistance against the flow by around 4 times, which means you would need an increase in pump power to overcome the system resistance.
Just a thought but have you considered placing a flow monitoring valve on the flow or return by the boiler? This would at least give you an indication of the flow through the boiler.

 

[John.g]

Its a vented hot water cylinder and if also a vented primary system then with (and if) that circ pump is on speed 3 there should be some signs of vent pump over in the F&E tank if there is a blockage in the system. I think running on those 4 rads only would tell a lot especially if their rated outputs or dimensions are known.

 

[Reddwarf4ever]

Its a vented hot water cylinder and if also a vented primary system then with (and if) that circ pump is on speed 3 there should be some signs of vent pump over in the F&E tank if there is a blockage in the system. I think running on those 4 rads only would tell a lot especially if their rated outputs or dimensions are known.

All 4 radiators 100cmx50cm, the CH vent to the FE tank was joined to the outlet of the FE tank several years ago, as I kept getting excessive air into the system, the plumber told me this was the usual thing to do with that problem......never had the problem before the CH system was expanded during the extension.....that’s the trouble with modifying stuff, especially plumbing....

I tried running the boiler at 65oC ( set at 7 KW ) and all went really sweet for a while, DT 24oC across the boiler.....but then for an unknown reason it tripped, then just kept overshooting the target temperature and tripping......wish it controlled better, just doesn’t seem to cut back enough when close to the target temperature ......set at say 70oC often goes to 71oC, which is acceptable, but then at 72oC it trips......wish I could adjust this to make it more ‘ lenient‘

 

[John.g]

All 4 radiators 100cmx50cm, the CH vent to the FE tank was joined to the outlet of the FE tank several years ago, as I kept getting excessive air into the system, the plumber told me this was the usual thing to do with that problem...never had the problem before the CH system was expanded during the extension..that’s the trouble with modifying stuff, especially plumbing.***

I tried running the boiler at 65oC ( set at 7 KW ) and all went really sweet for a while, DT 24oC across the boiler..but then for an unknown reason it tripped, then just kept overshooting the target temperature and tripping...wish it controlled better, just doesn’t seem to cut back enough when close to the target temperature ...set at say 70oC often goes to 71oC, which is acceptable, but then at 72oC it trips...wish I could adjust this to make it more ‘ lenient‘

You have a combined vent and cold feed, mine was installed like this 48 years ago and works very well to prevent air ingress etc so even if you have restrictions it won't pump over. The 4 rads, if doubles, should emit ~ 5.5kw total at 75C.
The normal reason for a boiler cutting out is because the heating load is less than the boiler minimum output, and it is quite normal then to exceed the target temperature, normally by 5C, before it trips as you can't have the target temperature and the cut out temperature the same except it is a non modulating boiler like a oil fired boiler, some gas fired boilers then seem to have a problem in modulating down fast enough to avoid again cutting out after ignition especially with very low heat demand, which of course was the reason for tripping in the first case.
You have your boiler set to 7kw, this shouldn't prevent it modulating to 5 kw if this is its minimum output. You could consider opening the TRVs fully on another few rads to enable the boiler to run continuously to see how it performs.
The boiler shouldn't have the trip temperature set only 2C above the target temperature as there is bound to be a bit of overshoot after coming up to its target temperature.

 

[SJB060685]

So John. Are you saying if there's a restriction on the return to the boiler this would cause pumping over on a system we originally thought was a traditional V,C,P? I can only picture this causing a lack of flow through the boiler and the delta t widening.

 

[John.g]

I would term a old traditional vented system as one where the pump was on the boiler return with the cold feed sometimes before the pump and the vent a continuation of the cylinder coil flow supply ( or a variation of this) which could be very difficult to get right even with a perfectly clean system, there are dozens of posts re pump over/air ingress problems just because of a simple pump change out so I would think that any restrictions anywhere would cause problems with this type of system and I often wondered how it worked at all without either pump over/back or pulling air in through the vent.
The combined vent and cold feed prevents all this but obviously cannot or should not be used with a solid fuel system whereas the VCP can as the vent is before the cold feed.

 

[Reddwarf4ever]

You have a combined vent and cold feed, mine was installed like this 48 years ago and works very well to prevent air ingress etc so even if you have restrictions it won't pump over. The 4 rads, if doubles, should emit ~ 5.5kw total at 75C.
The normal reason for a boiler cutting out is because the heating load is less than the boiler minimum output, and it is quite normal then to exceed the target temperature, normally by 5C, before it trips as you can't have the target temperature and the cut out temperature the same except it is a non modulating boiler like a oil fired boiler, some gas fired boilers then seem to have a problem in modulating down fast enough to avoid again cutting out after ignition especially with very low heat demand, which of course was the reason for tripping in the first case.
You have your boiler set to 7kw, this shouldn't prevent it modulating to 5 kw if this is its minimum output. You could consider opening the TRVs fully on another few rads to enable the boiler to run continuously to see how it performs.
The boiler shouldn't have the trip temperature set only 2C above the target temperature as there is bound to be a bit of overshoot after coming up to its target temperature.

Thanks that’s interesting, I can’t change the trip temperature can I ??? It’s obviously a Delta T trip, % of range, but can’t se a setting to alter it......when the boiler fires up 90% of the time, it fires hard to begin with, cuts back a little, then continues until it trips.....when the 5.3 / 5.4 cycle finally ends and it actually fires normally, the modulation can clearly be heard, tweaking up and down a little to control the temperature, sometimes it can go this way for maybe an hour, but then for some reason......maybe a TRV shuts back or opens, it trips on over temperature......only on very very cold days do I need to increase the KW to around 7-10 as it is in 5.4 but struggling to fire.....unfortunately as soon as I increase the KW, it fires up, leaving the inlet temperature hardly changed and goes to 5.3 Delta T > 30oC......

I will note the TRV values and open 2-3 up fully tomorrow and see what happens.....

this is really a pain, my Wife is disabled and feels the cold, need to get this stable and running virtually continuously.....thanks

 

[John.g]

Edit: Are you actually getting over temperature flagged up or it as I describe, below?

Just to be clear on the "trip" or better to term it burner off maybe as this is quit normal when the boiler flow temperature exceeds its target (set point) value by a certain margin, normally 5C I think, and the burner will come back on at the target temperature -5C, I would be very surprised if it were less this, might be worth asking the manufacturer.
The deltaT trip at 30C is probably to protect the heat exchanger from thermal stresses and I wouldn't expect this to be user settable, or the above, either.

Forgot to ask you carry out the easiest check of all ( if not already done) now that we know its a vented system, ensure that there is water in the feed and expansion tank.

 

[Reddwarf4ever]

Edit: Are you actually getting over temperature flagged up or it as I describe, below?

Just to be clear on the "trip" or better to term it burner off maybe as this is quit normal when the boiler flow temperature exceeds its target (set point) value by a certain margin, normally 5C I think, and the burner will come back on at the target temperature -5C, I would be very surprised if it were less this, might be worth asking the manufacturer.
The deltaT trip at 30C is probably to protect the heat exchanger from thermal stresses and I wouldn't expect this to be user settable, or the above, either.

Forgot to ask you carry out the easiest check of all ( if not already done) now that we know its a vented system, ensure that there is water in the feed and expansion tank.

Hello, yes the FE tank is fine, only recently gave the system a clean & new inhibitor.

0845 24/12/2020
i haven’t changed a thing so far. This morning was rather cold, boiler fired up as usual and as usual the outlet temperature ( target 72oC ) quickly exceeded the inlet temperature by 30oC + and the boiler went into its ‘ holding state’ of 5.5.3....( it stays at 5.5.3 regardless if the Delta T has been satisfied or not, must be a timer ) after some time, 5-10 minutes it then went to 5.5.4, but as it was set at 5KW, there wasn’t enough heat to raise the temperature above 59oC. I raised it to 6KW & the temperature climbed to 63oC with a Delta T of 21oC......then raised to 6KW outlet temperature rose to 68oC Delta T 21oC, then increased to 7KW. Temperature increased to 70oC Delta T 22oC then the outlet started modulating between 70oC & 73oC Delta T 23oC....

obviously it’s not ideal to have to tinker with the settings in this way, just to get the boiler to ‘ behave’ OT sure if I have proven anything, except presently it’s working fine...if the outlet should increase to 74oC the burner will go off and the cycle will begin again.....will keep an eye on it today......will defo ask vailiant about the burner off setting, as 3oC above target temperature does seem low


UPDATE 1010 Boiler just went OFF outlet temperature 74oC now it’s back to its fire up hard, exceed Delta T 30oC enters 5.5.3 holding state....etc...etc.....this goes on for ages, until the inlet temperature finally heats enough, to allow it to stay on 5.5.4
mayne if I reduce it back to 5KW it will climb slowly and allow the Delta T to stabilise, but I shouldn’t have to do that.....will give it a try nonetheless...

UOFATE 1050 Dropped to 5KW, but boiler still going OFF at 74oC.....really feel that as the outlet temperature nears the target temperature it should start modulating, for some reason during the initial firing, it stays at the same firing intensity right until it reaches 74oC and goes OFF......so frustrating, been this way since new, wish I had insisted stronger, while it was still under guarantee to get to sorted, but had air ingress issues, which had to be resolved first.......also found on a forum that the 418 has narrow channels in its heat exchanger, not sure if that’s true or not, as lots of stuff on the internet is garbage, unfortunately

 

[John.g]

The Hx blockage possibility did cross my mind.

You had a deltaT of 21C at a fairly constant boiler output of say 5.5kw which means the flow rate was only 3.8 LPM, the flow rate through any rad with both valves fully open should be 5 to 6 LPM, you have 4 fully open and while you won't get 20/24 LPM, you should be getting at least 12 to 15 LPM with a boiler deltaT of ~ 6C so I think it pretty certain that you have a flow rate problem either from system restriction, Hx blockage, or a defective circulating pump.
The only quick test you can do on the circ pump is get hold of a energy monitor and measure the power, on speed 3 it should read 65 watts, this won't tell you at what rate its pumping at but at least you know its running at full speed. If it is running normally then it means that its running at nearly its full head of 6.5M to give that tiny flow rate of 3.8 LPM and IMO a complete waste of money buying a higher head pump, mechanically you could isolate the boiler, shut the pump isolation valves and remove the pump head and inspect/clead the impeller and the pump ports.

That SP/cut out hysteresis is definitely too low at 2C/3C, also, when it re fires it will do so at the SP-3C so only has ~ 6c to modulate from initial firing whereas it should be 10C.

The only thing i can suggest now is to open the isol valves fully on another few rads and close off the remainder, you should be able to nurse it along until you decide what route to go.

 

[SJB060685]

Through some quick maths i estimate based on the above you have a flow rate of around 3-3.5 lpm. You mentioned you had severe air ingress, which as I'm sure you know causes the build up of black magnetite (sludge). Pumps when spinning act as a magnet, which collects and stores this magnetite. It is possible that the pump is clogged and struggling, which myself and others mentioned some way above.
I suggest whipping the pump head off and having a look. I think you need to prove or disprove a flow problem before looking at the electronics side of things.

 

[Reddwarf4ever]

The Hx blockage possibility did cross my mind.

You had a deltaT of 21C at a fairly constant boiler output of say 5.5kw which means the flow rate was only 3.8 LPM, the flow rate through any rad with both valves fully open should be 5 to 6 LPM, you have 4 fully open and while you won't get 20/24 LPM, you should be getting at least 12 to 15 LPM with a boiler deltaT of ~ 6C so I think it pretty certain that you have a flow rate problem either from system restriction, Hx blockage, or a defective circulating pump.
The only quick test you can do on the circ pump is get hold of a energy monitor and measure the power, on speed 3 it should read 65 watts, this won't tell you at what rate its pumping at but at least you know its running at full speed. If it is running normally then it means that its running at nearly its full head of 6.5M to give that tiny flow rate of 3.8 LPM and IMO a complete waste of money buying a higher head pump, mechanically you could isolate the boiler, shut the pump isolation valves and remove the pump head and inspect/clead the impeller and the pump ports.

That SP/cut out hysteresis is definitely too low at 2C/3C, also, when it re fires it will do so at the SP-3C so only has ~ 6c to modulate from initial firing whereas it should be 10C.

The only thing i can suggest now is to open the isol valves fully on another few rads and close off the remainder, you should be able to nurse it along until you decide what route to go.

Its now 1345 and the boiler is still doing it’s ‘ usual cycle routine ‘ when it goes OFF it shows code 5.5 then immediately 5.7.....in. A few minutes will open up some more TRVs fully, maybe the flow will be sufficient

 

[Reddwarf4ever]

Well not sure what to do, can’t shut back or open any rads permanently.

Through some quick maths i estimate based on the above you have a flow rate of around 3-3.5 lpm. You mentioned you had severe air ingress, which as I'm sure you know causes the build up of black magnetite (sludge). Pumps when spinning act as a magnet, which collects and stores this magnetite. It is possible that the pump is clogged and struggling, which myself and others mentioned some way above.
I suggest whipping the pump head off and having a look. I think you need to prove or disprove a flow problem before looking at the electronics side of things.

Hello, will do as suggested, will remove the pump and check. Won’t be able to until the warmer weather comes, as can’t afford anything to go wrong, my Wife being severely disabled, relies on keeping warm. Can you give the calculation you use to determine the flow, because what I found gives 2.4 l/min currently modulating ( but staying at a fairly constant firing, so assume it’s at 5KW ) 19oC DT 72oC target 5KW, problem I have is as soon as it gets to 74oC it goes off.....then when it restarts, assuming it doesn’t overshoot again, which it usually does several times, it then goes into ‘hold‘ 5.5.3 as the Delta T of 30oC is exceeded. If only it could go to at least +5oC above target without going off, think most of the problems would be solved. waiting reply from Vailant on this.
I actually phoned a local ‘long established’ Central hearing engineering company, listed the issues, the person on the phone, hummed and mumbled, but said couldn’t really think what might be wrong, suggested I contact valiant as it sounded technical

been to this website

Volume Flow Rate Q Calculator

Volume Flow Rate Q Calculator Shown below is a volumetric flow rate calculator that can be used for the purpose of central heating pump sizing and for other uses. Formula: Φ x 0.86 = Q (tF -tR)

myboiler.com

gives me 3.77 l/min....( 0.2263 m3/Hr ) looking at the pump chart from Wilo, it’s at the very bottom, assuming I am reading it correctly....if I can borrow a power meter to check the power used by the pump, will it clip over the outer cable, or does it have to be on the live cable only ? Kind of hoping the pump is faulty
also if I decide to get a more powerful pump, which I can always turn the speed down on, any suggestions ?

 

[SJB060685]

My maths was done quickly and using mass flow rates equation. Similar to Ohms law it uses a magic triangle and if you know two factors you can work out the third. Output in KW at the top triangle, with flow rates in litres per second bottom left and DT times specific heat capacity on the bottom right, specific heat capacity usually taken as 4.2.
So 5kw with a DT across the boiler of 30°c means the flow through the boiler will be about 2.4 litres per minute as you say but 5KW output needs 3.6 litres per minute.

 

[Reddwarf4ever]

My maths was done quickly and using mass flow rates equation. Similar to Ohms law it uses a magic triangle and if you know two factors you can work out the third. Output in KW at the top triangle, with flow rates in litres per second bottom left and DT times specific heat capacity on the bottom right, specific heat capacity usually taken as 4.2.
So 5kw with a DT across the boiler of 30°c means the flow through the boiler will be about 2.4 litres per minute as you say but 5KW output needs 3.6 litres per minute.

Does the calculation on this website make sense then ? How do I use the f,ow rate with the pump curve chart ? Looks right at the bottom of the bottom axis

one other question, the Grundfoss ‘standard’ pump has quite a higher specified Power rating than my Wilo pump, does that imply it may have, ‘ potentially have a higher fow rate ?

 

[John.g]

If that circ pump is not A rated which i don't think it is and is like the old traditional 3 speed ones then even if working perfectly the power at any given speed will vary very very little between no flow and max flow, all the meter will show IMO is that it is running at its requested speed/head which at speed 3 is 65W.

The other calculations are very easy to do from first principles......ie
(Flow in LPM) X 60)) X deltaT (deg C) / 860 = Heat demand in KW
ie LPMX60XdeltaT = KW
Example. boiler output is "known" to be 5kw, deltaT is 20c, find flowrate in LPM
LPM = (5*860)/(20*60) = 3.58 (LPM) and so on.

 

[SJB060685]

Johns method and mine seems to differ slightly but still arrive at the same answer and your method you mentioned seems to add up as well.
Pumps will supply a set flow rate at a given head generated, that is correct yes John?

 

[Reddwarf4ever]

If that circ pump is not A rated which i don't think it is and is like the old traditional 3 speed ones then even if working perfectly the power at any given speed will vary very very little between no flow and max flow, all the meter will show IMO is that it is running at its requested speed/head which at speed 3 is 65W.

The other calculations are very easy to do from first principles.***..ie
(Flow in LPM) X 60)) X deltaT (deg C) / 860 = Heat demand in KW
ie LPMX60XdeltaT = KW
Example. boiler output is "known" to be 5kw, deltaT is 20c, find flowrate in LPM
LPM = (5*860)/(20*60) = 3.58 (LPM) and so on.

Hello
so as this problem has been present since fitting my new valiant 8-9 years ago, with the new pump, I don’t think it’s pump sludge. My old boiler a potterton netaheat, which I believe was 60% efficient, which never had issues, most likely as the tech was very low, may have had a lower pressure drop....

So all I need to research is an A graded pump ? Or is there anything better ? Thanks

just saw mention on a website that a CH getting too hot may need changing, obviously the pump contents are between 55-75oC so will be hot anyway, but if I touch the area around where the flow control knob is, it’s too hot to touch, is this normal, was told not to insulate the front ( obviously not the electrical connection box )

thanks

 

[John.g]

yes, correct in principle, if one takes my example above of 3.58 LPM or 0.21 m3/hr then thee above pump if running in speed3 provide this flow rate at a head of 5

Johns method and mine seems to differ slightly but still arrive at the same answer and your method you mentioned seems to add up as well.
Pumps will supply a set flow rate at a given head generated, that is correct yes John?

yes, correct in principle, if one takes my example above of 3.58 LPM or 0.21 m3/hr then the above pump if running in speed3 will provide this flow rate at a head of 5.3M which means that IF the pump is performing normally then there has to be a big restriction somewhere as there is no way that any domestic system should require this sort of head with such a low flow rate.

 

[John.g]

Hello
so as this problem has been present since fitting my new valiant 8-9 years ago, with the new pump, I don’t think it’s pump sludge. My old boiler a potterton netaheat, which I believe was 60% efficient, which never had issues, most likely as the tech was very low, may have had a lower pressure drop.***

So all I need to research is an A graded pump ? Or is there anything better ? Thanks

just saw mention on a website that a CH getting too hot may need changing, obviously the pump contents are between 55-75oC so will be hot anyway, but if I touch the area around where the flow control knob is, it’s too hot to touch, is this normal, was told not to insulate the front ( obviously not the electrical connection box )

thanks

If the new Valiant was installed without a system clean/power wash then it would quite likely foul the Hx very early on as this acts as a perfect filter with a dirty system so, hopefully not, the boiler may require a new Hx and magnetic filter plus system power wash.
That boiler Hx (clean) requires a head of 1.2m at the required flowrate of 13.33 LPM at 18kw output so a 6M pump should be quite adequate for your 10 rad system.
Just ensure that whatever pump you choose that it displays the power in watts.
I have a 10 rad system (OF boiler) and my 6M Wilo yonos Pico provides a flow rate of 11.5 LPM @ ~ 3.5M head to give me a deltaT of 25C through my 20kw boiler

 

[Reddwarf4ever]

Any thoughts on the heat I mentioned of the pump front ? If I paid for a power flush, would a differential pressure figure be given before and after ? To see if cleaning helped, would. A power flush clean the HX ? and or if there is a problem.....how do you determine the amount of head required ? Is it from the boiler or pump curves not familiar how to do this.

if I pay to hire a power meter, will that be of any benefit ? Thanks

Also, could piping configuration, particularly changes made during the extension build be responsible for this restriction or could it only be solids build up ?

 

[SJB060685]

What changes were made?
Any idea of pipe sizes etc?

 

[John.g]

You can't determine if a pump is operating correctly by its temperature.
Don't think you will be provided with any differential pressures before/after a power wash.
Others will inform you but IMO a power wash will not clean a blocked Hx.
Don't know if you can actually hire/get a clamp on power measurement meter that gives you watts , it may only give the current in amps which is only of limited value.

Because you didn't or still don't really know if you have a pump or a restriction problem you were able to determine reasonably well IMO from the boiler output and the deltaT across it that the flow rate (by calculation), appears to be between 3 and 4 LPM, say 3.5 LPM, this information, on its own does not tell you that you may have a restriction or not as the flow depends on the pump head which in turn can be read off the pump curves.

Flow rate is 3.5 LPM or 0.21 ms/hr (3.5/16.666)
To read off the pump head, go to 0.21 on the horizontal (X) axis and "draw a line" vertically upwards until it meets the various speed curves and the corresponding head is on the vertical (Y) axis. You can see that IF the pump is running on speed1 the head is 2.9M, if running in speed2, 4.6M and running in speed3, 5.5M.
You havn't definitely confirmed that the pump is running in speed3 (you might do so) but even if it is only running in speed1 you will still only get 0.29 ms/hr, 4.8 LPM @ ~ 5.4M head on speed3 because flow is proportional to the sq.root of head so again this would point to a restriction somewhere IF the pump is operating correctly.

We can talk about it until the cows come home, if it were my system I would just go away and install a 6M A rated pump that displays the power in watts and you will know almost instantly where the problem is and while it will not fix it if the original pump was OK, at least you now know where the problem may be.

Any thoughts on the heat I mentioned of the pump front ? If I paid for a power flush, would a differential pressure figure be given before and after ? To see if cleaning helped, would. A power flush clean the HX ? and or if there is a problem.***.how do you determine the amount of head required ? Is it from the boiler or pump curves not familiar how to do this.

if I pay to hire a power meter, will that be of any benefit ? Thanks

Also, could piping configuration, particularly changes made during the extension build be responsible for this restriction or could it only be solids build up ?

 

[Reddwarf4ever]

You can't determine if a pump is operating correctly by its temperature.
Don't think you will be provided with any differential pressures before/after a power wash.
Others will inform you but IMO a power wash will not clean a blocked Hx.
Don't know if you can actually hire/get a clamp on power measurement meter that gives you watts , it may only give the current in amps which is only of limited value.

Because you didn't or still don't really know if you have a pump or a restriction problem you were able to determine reasonably well IMO from the boiler output and the deltaT across it that the flow rate (by calculation), appears to be between 3 and 4 LPM, say 3.5 LPM, this information, on its own does not tell you that you may have a restriction or not as the flow depends on the pump head which in turn can be read off the pump curves.

Flow rate is 3.5 LPM or 0.21 ms/hr (3.5/16.666)
To read off the pump head, go to 0.21 on the horizontal (X) axis and "draw a line" vertically upwards until it meets the various speed curves and the corresponding head is on the vertical (Y) axis. You can see that IF the pump is running on speed1 the head is 2.9M, if running in speed2, 4.6M and running in speed3, 5.5M.
You havn't definitely confirmed that the pump is running in speed3 (you might do so) but even if it is only running in speed1 you will still only get 0.29 ms/hr, 4.8 LPM @ ~ 5.4M head on speed3 because flow is proportional to the sq.root of head so again this would point to a restriction somewhere IF the pump is operating correctly.

We can talk about it until the cows come home, if it were my system I would just go away and install a 6M A rated pump that displays the power in watts and you will know almost instantly where the problem is and while it will not fix it if the original pump was OK, at least you now know where the problem may be.

Ok, as the pump is rather old, it will not be a bad idea to replace it, of for nothing else than as an insurance against possible early failure....you said “6M A rated pump that displays the power in watts” can you suggest 1 or 2 examples to get me started ?
in your opinion, would you suggest a power flush in he first instance as a good starting point ? ( I have flushed through myself bi annualy and treated with cleaner & then inhibitor but never had a power flush since new in 1987 ) then fit a new pump as suggested and take it from there. P.S. it is on speed 3....

some of this is a little bewildering, I guess everything is until you understand it.....when you refer to “5.4M head on speed3” does pump head mean how high the pump will pump ? Or what head pressure it needs ? may sound a daft question to someone whom such a thing is so obvious, but then hopefully I will have a better understanding, thanks for explaining the pump curve chart

thanks so far for everyone’s input and advice, has really helped a lot.

P.PS. S there an easy way to find out what all the numbers mean on CH pumps ?