Flow/Return Joined Via Valve Near Pump & Mid-Point Valve

[Burleigh]

I've recently moved and I'm trying to understand the pipe layout of the central heating system.

Just after the pump and the 3-way valve, the heating feed is connected to the heating flow return via a 15mm pipe with a stop valve. The valve is set to fully open.

I don't understand and am curious as to why there's a direct connection between flow and return like this and how should the valve be set ? Fully closed is my instinct, but then why install this pipework in the first place ??

I'm in the process of flushing out the system and then balancing the radiators, but I think I need to understand what this valve is for before going too much further.

 

[Nozzle]

A recirc pipe. If the system has all of the TRVs closed (house is hot) but the room stat is no satisfied, then where does the flow go? It'll either dead head the pump or this pipe is there to bleed off some of the forward flow to protect the pump. If it's too far open it'll mean the radiators would be bypassed instead, so it should be cracked. I'm no plumber though.

Nozzle

 

[Dan Robinson]

It's called a by pass. Not a recirc pipe. .

 

[Burleigh]

Dan, Nozzle - thanks for your time in replying. What you say makes sense, but I'm afraid that I'm cursed with an inquisitive mind......

As there's a stop valve on the joining pipe, does this imply there's a need to adjust the flow using the valve ? I guess there's already a slight restriction on the flow as the pipes go down from 22mm to the 15 used for the bypass.

If there's no need for adjustment, why have the valve ?

Isn't the pipe which circulates the house good enough as a bypass if none of the radiators tee'd off are calling for heat ?

 

[Dan Robinson]

What happens if they are all turned off and the boiler is still running?

 

[Burleigh]

With the bypass closed.....

.....there'll always be a 'circuit' for water to travel (ie no dead-ends) as the 3 way valve is open to either tank, radiators or both.

.....the radiators warm up quicker and get hotter. I've now switched down the pump speed and boiler water temp and still getting good warmth through the house.

I guess with the bypass closed, hot water can no longer take the 'easy route'.

Is it possible the bypass was part of the originally installed central heating system (bypass pipe work is relatively old) and now obsolete over time with the addition of TRVs and mid-point valve ?

 

[Dan Robinson]

And what happens when you have heating only and all the rads are off?

By passes are there for a reason.

 

[Burleigh]

Dan,

As before, if TRVs are all closed and valve is heating only, water circulates around the pipes from which radiators are tee'd off. Alternative is that valve is set to water and there'll still be a circuit for the water to follow via ht water tank.

In both these cases, as soon as the return to the boiler gets up to temp, the boiler thermostat will stop calling for heat and power-off the boiler.

The question you've posed is the one to be answered : why is the bypass there in the first place ?!?

 

[Dan Robinson]

Read what I have written again.

Why would there be a BT pass valve if the flow and return pipes are joined under the floor somewhere?


We don't make this stuff up you know.

 

[MrTherm]

Burleigh,

If you can verify that your central heating pipework system is of the permanent loop type, then a manual by-pass is not needed and can be left closed or even removed.

I have fitted many systems over the years with by-passes that you have, i installed them under contract to British Gas and it was within their install specification to fit such a by-pass back then. The adjustment procedure was with the valve closed, to crack it open slightly 1/2 to 1 full turn (with a three or four turns to full open valve). This was required even with cast iron heat exchangers, which over the years have evolved to ever lower water content designs. In many cases at that time boiler manufacturer were not openly giving by-pass install information as we have today.

Because of the advent of D.I.Y'ers installing their own systems for several decades now, the permanent loop system has become almost extinct through lack of knowledge to such a degree that boiler manufacturers evolved into fitting auto by-pass methods integral to their appliances to compensate for this ignored pipework system exclusion - to safely protect the boiler.

With permanent loop systems any auto by-passes fitted, boiler integral or not, would never function, they would be there doing nothing because there would be no flow pressure progressive increase to activate them in a correctly designed and calibrated system.

Further more, in strategically designed pipework layouts, especially where regular boiler service was involved, the said by-pass valve could assist in being one of the component isolation valves - by isolating say two or even three valves, the pump, three port valve, two two port valves etc' etc' could be replaced quickly without the need to fully drain the system.

Does this give a clearer picture?

 

[Burleigh]

MrTherm.....

Thanks for your contribution. All you say makes sense and I now not only understand the pipework I have, but also appreciate some of its history!

There's a radiator which is not fitted with a TVR (in vicinity of the room thermostat), so the manual bypass is now closed and will remain so. There'll always be a flow and no dead heads, whatever the conditions.

Thanks again to MrTherm, a model answer and all questions addressed.