Central heating pump sizing

[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.