CH System - what is this gate valve for?

Standard stored hot water gas boiler CH system:
Following the balancing of my system, I decided to upgrade my pump (all done) but notice a gate valve in a funny place. Any idea what it's for? My ASCII art is rubbish so I'll describe:
Hot feed from boiler to pump, from pump to diverter valve. Diverter valve to HWC and CH system.
Now, from the pumped output prior to the diverter valve there is a 15mm T off through a gate valve to the return pipe of the HWC which presumably returns to the boiler. The gate valve when opened would then feed pumped hot water directly back to the boiler bypassing diverter and HWC coil.
The system was originally installed with a condensing boiler (not combi) which I since replaced with a "standard" boiler. Is the valve to balance the hot water side of things or to allow the pump to work OK on the overun when the CH and HW are satisfied? Surely the diverter valve will just open to the HW side when CH is satisfied and tank stat simply switch off boiler when satis?
What is the gate valve for and how should it be set?
Colin
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It is called the bypass. It gives the water somewhere to go when no heating is called for. This would happen if all the TRVs shut off and the diverter is on heating. It happens more easily on S-plan when all the circuits can be turned off.
It is set through voodoo magic. Alternatively, replace it will an automatic bypass valve, which only opens when it detects too large a pressure differential.
Christian.
P.S. Why did you replace your condensing boiler with a less efficient one?
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Christian
I had a Potterton Envoy that was installed by the builder when the house was new 6 years ago. Problems began six months down the line and after the warranty ran out, I tried to get a BG service contract (yes I know!), but BG condemned the boiler and confirmed in writing that they would not take on a contract.
Maintenance costs were horrific with boiler chamber seals apparently being a routine service item!
As I travel a lot, it was becoming very inconvenient coming home to a frozen house as the boiler had shut down days ago and so I swapped it for a standard Potterton Profile almost 3 years ago. My gas bill might be slightly higher, but my house has been warm since and I am practising the no service money saving tactic. The boiler was around 500 from B&G, about 100 to my CORGI mate to fit it and no costs since. If it lasts another year, I will have broken even on running costs compared to the condensing heap of junk I threw out.
Colin M
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I'm surprised you went for another Potterton. The problem with the old one was that it was a Potterton Envoy, not that it was condensing.
Christian.
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It sounds like a classical by-pass circuit - which you may or may not need.
Whenever the boiler stops firing (either on its own stat, or as the result of a room stat, cylinder stat or timer going off) it needs to keep the pump running for a little while until the boiler's internals have cooled down. [If you look at your wiring, you'll probably find that the pump is controlled by the boiler].
The water pumped during this "pump over-run" period has to have somewhere to go - even if all the zone valves are closed - hence a by-pass circuit.
In your case, if you have a mid-position diverter valve, there *will* always be somewhere for the water to go - UNLESS you have TRVs on all your radiators AND they are all shut OR all the rads are turned off manually - so you probably don't need a by-pass. [My system is like that, and I *haven't* got one]. If you're brave enough, try completely closing the gate valve. If the boiler trips on its over-heat stat, and won't re-start until it's been manually re-set, that will show that you *do* need a by-pass - in which case you will need to partially open the gate valve again. Have it open as little as possible, consistent with preventing boiler trips. Better still, replace it with a spring-loaded automatic by-pass valve which *only* opens when necessary.
HTH, Roger
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Roger
Thanks for that. I have thermostatic valves in most of the house but a couple of rads without. They are set with an 11 degree drop (per balancing technique in FAQ) and are therefore providing a path for pumped water at all times.
Funny thing was the gate valve was almost fully open (and the radiators not balanced) so I wonder how much pride house builder plumbers really take in their work.
Colin M
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If you shut the by-pass, you may get slightly more water going through the rads - resulting in a lower drop - but that shouldn't matter. It shouldn't affect the balance - but it may be as well to re-check it.
Roger
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Guess what I am already doing this evening!
Colin M
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