Central heating pumping over after alterations

Hi all,
We have a fully pumped Oil fired CH. (S Plan I think, 2 separate motorised valves one for heating, one for hot water.
I am told the pump is in the return.
Some years ago, we had some alterations (mostly addition of a shower), and the plumber said the system was pumping over (hadn't seen this before), and he extended the pipe that goes up in the air (higher) and drops down into the F&E tank.
Since then I think all has been well.
Fast forward a few years. A boy (didn't look old enough to shave), came and did some modifications last week. Added some radiators, replaced another, firred thermostatic valves on all the radiators he'd installed, replaced the pump.
Announced the system was pumping over, and that if I made sure I always turned on the hot water at the same time the heating was on, we wouldn't have any problem (my immediate thought was - what happens when the HW stat says "no more heat thanks").
Generally, he was rigt, but on one occasion it has pumped over even though both Heating and HW were on (perhaps as the syslinder stat was no longer calling for heat).
I noticed after he had left that he had the 3 position Grunfoss pump set to it's highest head/pressure/speed and I wound it down to "1" , which according to the stuff on the pump, is lover / slower than "1" on the old pump. But this made no difference.
Questions............
What might have caused it to start pumping over?
What is the cheapest way to fix it (I really don't want to have him back, his plumbing of the Radiators was awful! (but I could get the builder who supplied him to supply another plumber who MAY be better)).
Could the top of the inverted U in the loft be taken even higher (there is plenty of height up to the ridge)?
Could the system be converted to a sealed system (what MIGHT need to be replaced? - Boiler (one of the original Wallstars)? Cylinder? Anything else?
I suppose I could fit a Calor Combi (but there lies MUCH capital expense and disruption).
Any other suggestions?
TIA
Chris
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Do you have a room stat somewhere and are the radiators in that room fitted with TRVs because you may have a situation where the stat is calling for h eat but the TRVs are set below that temperature and are shutting off the fl ow to the radiators. You should also have at least one radiator with no TRV s usually the choice one is the bathroom radiator this acts as a bypass in a situation where every TRV has shut down.
If you have the above situation a temporary solution is to turn the TRVs in the room with the stat to full on and let the stat control the temperature .
Richard
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Hi, we only have TRVs on a few rads,(and it on those near the roomstat and I’ve tried setting them to 6 and it hasn’t fixed the proble m.
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On 11/12/2017 17:28, Tricky Dicky wrote:

Get a CH engineer to fit a bypass to the system. It does the same job as leaving a TRV open
Malcolm
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On 11/12/2017 16:45, snipped-for-privacy@o2.co.uk wrote:

Had this problem after a replacement boiler was fitted. The cure was to tee off the vent pipe (the U shaped pipe) Higher up the fed from the feeder tank than it was. About 6 inches was enough in my case
Malcolm
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Surely with a longer pipe run more water in the system equals more expansion and contraction, so the brain, though a little rusty might suggest raising that pipe to accommodate the expansion?
Brian
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Surely the tank accommodates the expansion, and the height of the overflow just accommodates the pressure differential due to the pump?
--

Roger Hayter

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On Tuesday, December 12, 2017 at 9:33:08 AM UTC, Roger Hayter wrote:

Hmmmm,
Some interesting comments. SOME of the new rads are actually replacements (in new positions) for old rads which have been removed), but overall, yes, there is more water in the heating circuit, so perhaps that is the issue as Brian suggests.
How to solve it? I'm not yet sure. As I say, there are plenty of rads still left at the moment without TRVs, so I't's (sadly) not a question of simply fitting a bypass.
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