Heating woes - is it my pump or something else?

Hoping someone can offer me some guidance before I tear what's loeft of my hair out.

It's a very long short (5 years in the making) but will try to keep it short;

Property is a large one, with 17 radiators in total between the ground and first floor. Boiler is a Glow Worm Ultracom 38cxi condensing boiler. Pump in the upstairs airing cupboard is a Grundfos 15-60. All radiators fitted with TRV's.

Place needed doing up so I'd never used the system until all the rads were ready to go back on (all new rads fitted throughout). Found that the boiler would randomly cut out with an error saying the system was too restricted.

Several tradesmen later we had;

- changed the pump to a new one
- had the wiring checked
- had the boiler service
- had the system power flushed

The last decent tradesman was a great guy and knew his stuff. He suggested changing the system to a sealed one which we did, and fitted a Magnaclean system too. It didn't fix it so baffled, and as a last resort, he ripped up some floorboards to find what was going on. Someone had fitted isolation valves at multiple points to both the flow and return pipework - he stripped them out and the system seemed to work okay for a while.

Recently re-done the bathroom and ensuite so had to take three rads off (was able to isolate them so didn't drain down the system). Put them back on just before November and refilled the system. Initially there seemed like there was no problems but realised over Xmas that while the top of rads get red hot, the bottoms are nowhere near the same temperature (I'd expect some difference but this is really noticeable).

-Bled all radiators, no sign of any air in any of them

- Turned all the rads off except one at a time. The radiator heats up uniformly from top to bottom (so they're not clogged with crap - water is clear and Magnaclean is only slightly dirty)

- I've tried balancing the system but it made no difference

Now the system is struggling and seems to be unable to keep the heating on unless the hot water is also on. The boiler isn't kettling (which it was right at the very start of all this) but it does start to sound like it's struggling to shift the heat (the fan is going like crazy) before it cuts out.

To my mind, it's almost like, without the hot water on, it has nowhere to dump the heat so the fail safe kicks in. From all the times I've tested it's never cut out with the hot water on the same time.

This weekend there was the horrendous sound of trickling water near the Magnaclean pipework but checked again for air and no sign of it anywhere (rads, pump, auto air valve or Magnaclean bleed point).

I'd originally been told to pressurise the system for 1 bar but it desperately sounded like the system was short on water. Put it up to 1.5 bar and its settled down quite a bit and running quieter, but the radiators still won't warm up uniformly. The boiler still cuts out if you ask it for heating only.

Appreciate that a diagnosis over the Internet is tricky but does anyone have any ideas?

I spoke to the last decent plumber again who suggested the only thing he could think of was the pump was undersized. The next size pump up is around £400 from what I can see (a 25-80 would be needed ?) so wanted to gather some ideas and views before shelling out.

Have you checked the magnaclean valves?, I've had these badly blocked up before now.

Do you have one or two motorised valves?

ollski said:

Have you checked the magnaclean valves?, I've had these badly blocked up before now.

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Yep, no sign of them being blocked (got a bit wet in the process of checking but the water seemed to be flowing fine).

dilalio said:

Do you have one or two motorised valves?

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Two - one on the run just before the hot water tank and the other on the run just before the pump. Forgot to say both were replaced as part of this (probably about two years ago)

boilers oversized, pump is undersized, have the boiler down rated and perhaps another 15/60 in series with the pump you have will stop the boiler overheating/cycling.

Having had a similar problem on a different type of boiler it turned out to be the low pressure on the boiler expansion vessel, it was pumped up to one bar and all running as sweet as a nut.

Just had a look at the instructions and your boiler has 30kw output to the central heating (if it is a cxi) so not so oversized as I thought, still reckon you need another pump and is your boiler hxi ?

What's the boiler...surely not a combi in a house that size...38 Kw is 5/6/7 bed house?

Does it really need 38 Kw? A lot of boilers are way oversized.

TRV's on all rads....take some off, you should have at least one zone where the room stat is located that does not have TRVs.

I'm assuming it's an Hxi. There's no mention of an auto bypass valve. This is essential on these models to allow sufficient water to flow around the boiler and for pump overrun once the system shuts down.

What are the fault codes exactly? Have you pulled up D40 and D41 to check the flow and return temperatures? AFAIR D99 will give you the status...might be doing an S53/54 and spending most of it's time waiting to dump the heat.

Apologies, I had bed brain last night when posting - the boiler is the Ultracom 38 Hxi (condensing boiler not a combi). It was a six bedroom house (but some very small rooms have been knocked into one now, so it's a 4 bed).

Also should have said I've already taken the TRV's off the two radiators in the hallway - fault codes I've looked at are the standard ones for F22 (system restricted). I didn't realise there are other status codes tucked away in that Service Menu so will need to take a look.

I don't think there's an auto bypass fitted - I can't see any sign of anything like that being fitted. There is a bypass gate valve just before the pump in the airing cupboard - the last engineer told me to close this and then open it back up a quarter turn but it hasn't made any difference to how the system runs.

Have attached a photo of the setup to make things clearer

Looks like the bypass is a gate valve fitted below the pump on the right hand branch...I doubt its even open with just a quarter turn. A simple gate valve is a poor substitute for a proper automatic bypass valve (they are spring loaded and respond according to differential pressure ie. TRVs shut down it opens more or both zone valves shut allowing pump overun). You could try shutting all the TRVs down and opening up the bypass until you can feel heated water passing through it. Then faff about with it to see if there's any improvement. If you pull up the flow and return temperatures on the boiler you can monitor the temperature drop....you want 15 to 20 degrees. I'm assuming the radiators are properly balanced? Ideally you need a proper bypass eg. a Honeywell DU145 perhaps set to 1 or 2.

Aside from the bypass I would down rate the boiler to 24kw (a guess at your buildings heat loss) and see if the pump can cope with that, its in the instructions how to do it.

Thanks all - lots of stuff to try there so will give it a go

picasso said:

Aside from the bypass I would down rate the boiler to 24kw (a guess at your buildings heat loss) and see if the pump can cope with that, its in the instructions how to do it.

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You sure you can range rate this model? Is there an unofficial way?

Page 25 of the instructions shows how to range rate the 38kw, I tried to paste it on here but failed miserably

picasso said:

Page 25 of the instructions shows how to range rate the 38kw, I tried to paste it on here but failed miserably

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Screen shot? That would work