Rads slow to heat - bigger pump required ?

Hi all, wonder if anyone can offer any advice.

Long story, but as a result of a pump which had been installed in the wrong direction (by the previous owners), our open vented CH heating system had been pumping over for at least 2 years. Consequently, any inhibitor had long since disappeared down the overflow and a large build up of rust flakes finally blocked one of the 22m CH pipes on the ground floor.

Unfortunately, this had also taken its toll on the boiler (which had been kettling) and the heat exchanger sprung a leak.

Last Christmas, we bit the bullet and employed a plumber to locate & remove any further blockages (he didn't find any), powerflush the entire system and fit a new boiler and Magnaclean.

Now the boiler (Glow Worm Ultimate 38HXi) works great and the Magnaclean is, well, very clean (as is the water in the system) which suggests that there hasn't been any cr@p floating around for a while.

However, there is a problem. There appears to be insufficient [cooling] flow back to the boiler such that when she fires up on max burners, its a matter of a minute or two until 80 degress has been reached and the burners shut off. This means that the CH, apart from the rads nearest to the boiler, can take several hours to heat up; albeit that all radiators will get 'warm' eventually. (The nearest rads, especially upstairs, get in excess of 60 degrees, others around the 40 mark).

It might be noteworthy that the rads do tend to warm up quicker if the HW zone is switched off..

The powerflushing took an entire day (I was there) and, because we could isolate all 3 circuits (up CH, down CH and HW) using a combination of zone and gate valves, we confirmed that there was a clear flow path through each rad and all pipework.

The pump is probably 3 years old - Grundfos 15/60 - but would have a hard start in life with the vanes being clogged with rust flakes.

So, my question would be, is it worth biting another bullet and trying a higher flow rate pump - Grundfos 25/55, for instance - or am I barking up the wrong tree ?

The system comprises 15 radiators (7 doubles) in a fairly large 4 bed house. The pump and boiler live in a boiler room on the ground floor.

Thanks for reading this far and I hope I have given enough info for any advice or suggestions you can offer !

The pump is probably 3 years old - Grundfos 15/60 - but would have a hard start in life with the vanes being clogged with rust flakes.

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have you cleaned out the carp from the pump? would imagine that is your major problem along with, possibly, balancing the system.

a 15/60 pump should be big enough for your house.

how many rads do you have?

Thanks for the speedy replies chaps..

twgas said:

have you cleaned out the carp from the pump? would imagine that is your major problem along with, possibly, balancing the system.

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I removed the pump a couple of days ago and it was as clean as a whistle - no blockages, no rust, clean water etc.

TBH, I would be surprised if any muck did get in there because there is a Magnaclean fitted on the return just before the boiler (and the pump sits on the flow side right next to the boiler).

twgas said:

a 15/60 pump should be big enough for your house.

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That's why I am reluctant to blow £150+ on a 'light commercial' pump if my problem lies elsewhere.

bengasman said:

how many rads do you have?

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15, 7 of which are doubles.

let system cool down.
close all rads
call for heat.
feel if flow pipe gets hot

you say the nearest rads get hot and the distant ones don't.

sounds like the system needs balancing. Do you know how to do it?

JohnD said:

you say the nearest rads get hot and the distant ones don't.

sounds like the system needs balancing. Do you know how to do it?

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bet you a pint that either flow and return are linked or permanent bypass. possibly done by same expert who put the pump in wrong

JohnD said:

I have a feeling the system needs balancing. Do you know how to do it?

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Pretty much, I think - there are some good guides on the 'net .

And even though all my rads (bar one) have TRVs fitted, I have wound down the lockshieds on those radiators which get hottest quickest such that they are barely open. As the rads get further from the boiler, the lockshields have been opened further (trying to achieve a 20 degree drop between flow and return).

But would an unbalanced system display symptons akin to that of a low flow rate ie not enough cold water returning to the boiler to keep the temperature down (and hence boiler running for longer) ?

And if it helps explain anything further, because I have a pair of gate valves on the flow and return to the upstairs rads, I can completely isolate that circuit from the downstairs. When I do this, the problem persists. That is, even with a fewer rads calling for heat, the boiler reaches its pre-set temperature far too quickly.

yoda said:

But would an unbalanced system display symptons akin to that of a low flow rate ie not enough cold water returning to the boiler to keep the temperature down

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If the hot water can take an easy route and flow through a nearby radiator without having to go round all the others, then yes, it will take that route. And if the boiler is just heating one radiator, it will not take long to get hot.

You can close the lockshields on the hot ones, then when they cool down, put one hand on the flow pipe to that rad and inch open its lockshield until the pipe starts to get warm. It may only be half a turn or so.

Look at our plumbiung section FAQs for a better descripotion.

I tend to agree with bengadman.

Take a look at the bypass valve.
Is it a gatevalve or an actual pressure differential operated one?

If this was stuck open then the flow water would return through it to the return and show the symptoms you describe.

If it were my system, I'd be looking at that valve with some supspicion personally.

If it's a gate valve wind it completely shut. If it's an autobypass, then screw it down as far as it'll go.

bengasman said:

bet you a pint that either flow and return are linked or permanent bypass. possibly done by same expert who put the pump in wrong

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There is a bypass valve fitted (there wasn't until last Christmas, according to the plumber) between the flow and return in the boiler room - see drawing.

But this pipe doesn't get as hot as the flow from the boiler which suggests that, if there is a link between flow and return, that is not it.

JohnD said:

If the hot water can take an easy route and flow through a nearby radiator without having to go round all the others, then yes, it will take that route. And if the boiler is just heating one radiator, it will not take long to get hot.

You can close the lockshields on the hot ones, then when they cool down, put one hand on the flow pipe to that rad and inch open its lockshield until the pipe starts to get warm. It may only be half a turn or so.

Look at our plumbiung section FAQs for a better descripotion.

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I'll certainly give that a go - thanks for the suggestion.

toasty said:

Take a look at the bypass valve.

Is it a gatevalve or an actual pressure differential operated one?

If this was stuck open then the flow water would return through it to the return and show the symptoms you describe.

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Its an auto-bypass [pressure] valve and the indicator is wound about 1/4 of the way down from the top.

You must have posted whilst I was still typing, but the bypass pipe into which the valve sits does not get at all hot (it is the same temperature as the return pipe, which is only luke warm).

That said, I'm keen to try anything so I'll wind the valve right down when I get home tonight anyway.

yoda said:

Glow Worm Ultimate 38HXi

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No such boiler! Do you mean the Ultracom 38HXi (GC no 41-019-06) or the 38Hxi (GC no 41-047-71)? If so then the pump is not up to the job.

At 38kW output, the water flow through the boiler has to be 1600 litres/hour with a 20°C temperature drop. According to the Mfrs literature, at this flow rate the boiler has a resistance equal to a 4m head. Assuming that the radiator circuit has a resistance of 2-3 metres means that the pump has to be able to deliver 1600 litres/hour against a heat of 6-7 metres. The Grundfos 15-60 pump can only only deliver 1600 litres/hour against a head of 3.5 metres.

Even assuming that the total head is 5 metres, the highest flow rate will only be 600 litres/hour. This would require a temperature drop of 54°C to deliver 38kW

This explains why

you said:

its a matter of a minute or two until 80 degress has been reached and the burners shut off.

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The water flow is not fast enough to remove the heat generated by the burners

You then said:

is it worth biting another bullet and trying a higher flow rate pump - Grundfos 25/55, for instance

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Yes, but the 25/55 is still too small. You would need something like a Grundfos Magna 40-100F.

Final thought

I have assumed that you need a 38kW boiler. But

you said:

The system comprises 15 radiators (7 doubles) in a fairly large 4 bed house.

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That's unlikely to be more that 22-25kW, so why do you need a 38kW boiler?

Great post, many thanks..

D_Hailsham said:

yoda said:

Glow Worm Ultimate 38HXi

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No such boiler! Do you mean the Ultracom 38HXi (GC no 41-019-06) or the 38Hxi (GC no 41-047-71)? If so then the pump is not up to the job.

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Yes, my mistake, sorry. Its an Ultracom; this one:

D_Hailsham said:

At 38kW output, the water flow through the boiler has to be 1600 litres/hour with a 20°C temperature drop. According to the Mfrs literature, at this flow rate the boiler has a resistance equal to a 4m head. Assuming that the radiator circuit has a resistance of 2-3 metres means that the pump has to be able to deliver 1600 litres/hour against a heat of 6-7 metres. The Grundfos 15-60 pump can only only deliver 1600 litres/hour against a head of 3.5 metres.

Even assuming that the total head is 5 metres, the highest flow rate will only be 600 litres/hour. This would require a temperature drop of 54°C to deliver 38kW..

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Very well put; that makes so much sense and was effectively what I was thinking but was unable to put into words..

D_Hailsham said:

This explains why

you said:

its a matter of a minute or two until 80 degress has been reached and the burners shut off.

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The water flow is not fast enough to remove the heat generated by the burners

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Absolutely..

D_Hailsham said:

You then said:

is it worth biting another bullet and trying a higher flow rate pump - Grundfos 25/55, for instance

Click to expand...

Yes, but the 25/55 is still too small. You would need something like a Grundfos Magna 40-100F.

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I've had a quick a trawl around the 'net - some places are selling that pump for over a grand .

I'll keep on looking (found one for £429 excl VAT) but, in the meantime, any suggested sellers who might be more 'reasonably' priced or alternative pumps which might do the job ?

D_Hailsham said:

Final thought

I have assumed that you need a 38kW boiler. But

you said:

The system comprises 15 radiators (7 doubles) in a fairly large 4 bed house.

Click to expand...

That's unlikely to be more that 22-25kW, so why do you need a 38kW boiler?

Click to expand...

'Twas what the plumber thought was the same size replacement for the behemoth which was installed previously.

Did he do bad ?

For starters as has been mentioned the boiler is too big for the system.
To enter the diagnostic menu:
a) Press and hold the ‘MODE’ button for 5 seconds.
The display will change to flashing ‘0’.
b) Use the ‘+’ or ‘-’ buttons to scroll to 96 (This is
the installer level access password).
c) Press ‘MODE’ to confirm

-it will then show you d0 and 38
-press mode and the 38 will flash
-adjust the 38 to 24 for now and hold down mode to log it and exit diagnostics

see how this does for you.