Purpose of Gate valve on hot-water tank

[canonpyon]

I have a hot-water tank with an integral Central Heating header tank. The 22mm pipe from the pump feeds to a 3-way valve to select hot-water or central-heating. Between the pump and the 3-way valve is a 15mm take-off which goes to a gate-valve and then feeds through the floor and has a take-off that feeds into the bottom of the hot-water tank.

I would like to know what is the purpose of this gate-valve. It appears to be about half a turn open - is this correct or should it be closed?

Thanks
Ian

 

[doitall]

Sounds like a crude by-pass valve and should be cracked open

 

[andy.budgell]

Doitall, Is that fully open then? Cos I've got the same thing on my HW/CH.

You say 'Crude', does that mean there should be a better way of doing it?

 

[DarkInferno]

Doitall, Is that fully open then? Cos I've got the same thing on my HW/CH.

No. it should be cracked open by the installing engineer who should remove the head from the gate valve so that it can't be tampered with. Open it too much and you will loose a large percentage of your flow through your rads/cylinder, close it and increase the possibility of kettling and damaging the boiler.

You say 'Crude', does that mean there should be a better way of doing it?

There is a better way of doing it, its called an auto bypass and (when set correctly) it opens when required, to the degree required.

 

[andy.budgell]

Yeah ok I think I've got that. Is the idea to remove the 'height' of water
such that the pressure is released so you dont get water creeping up into the header tank when on HW only for example?

The other gate valve, on the return from the hot water cylinder, how much should that be open then? Obviously you want it open enough so that it heats the water at a reasonable rate, and not too much that it takes too much pressure from the rads when both needs are required, and not too little such that the back pressure causes water shooting into the header tank?

 

[felix]

The bypass valve allows a little water to flow through your boiler when all other paths are shut. In these conditions the boiler will cycle on and off on its own internal stat but the heat input is huge and the volume of water is small. There's a chance that the water will boil and trip the overheat stat up on the top BEFORE the main stat can respond. For this reason your boiler will have a pump over-run stat (or maybe a timer) as well. Have you ever noticed that the pump comes on in time with the boiler but runs on a while after the boiler goes off? It's keeping the water moving while the boiler cools down. It can't do this if there's nowhere for the water to go and that brings us back to that bypass.

With only CH on demand and all radiators off, that valve should be open just enough to stop the boiler from kettling. Opening it too far is pointless as Darkinferno says.

The valve on the cylinder heating coil return serves no real purpose, except maybe to isolate the coil for maintenance. You COULD use it as you describe but what's the point. Surely it's better to get your hot water up to temperature as quickly as possible then your rads can have the boiler's full attention.

There should be NO settings of ANY valve that will send water up your vent pipe. This cannot happen if the pipes have been laid out properly. In a fully pumped system, feed and vent pipes must join the circuit very close together and (thinking back to an old post) without any venturis!

 

[kevplumb]

The valve on the cylinder heating coil return serves no real purpose, except maybe to isolate the coil for maintenance. You COULD use it as you describe but what's the point. Surely it's better to get your hot water up to temperature as quickly as possible then your rads can have the boiler's full attention.

 

[andy.budgell]

Cheer guys thats really useful. I will open the hot water return gate valve right up, seems to make sense. As for the other gate valve, I guess it must be ok as my boiler doesnt kettle.

 

[bradleybear]

Noooooooo!
You must set the balancing valve correctly to;
a) correctly warm your cylinder and rads equally at the same time
b) stop the water whizzing round your cylinder too quickly, hence, not heating as quickly.

Water naturally takes the path of least resistance, and correctly balancing a system will mean that it works more efficiently, hence saving you money which can't be bad.

Normally the gate valve is set at half open (or half closed whichever way you look at it). Count the turns to fully open, and divide by two, then crank it back to the desired setting.
When commissioning systems, the desired flow and return differences must be achieved by correctly balancing a system. Without measuring temperatures, the basic Jo Bloggs from Acacia Avenue can do it the Half turn method, however, the correctly qualified Energy Efficient Engineer can adhere to the recommended flow and return differential as stated in the Manufacturers instructions., and as required by Document L.

 

[kevplumb]

 

[moosehead]

Why the whistling and laughter kp? Do you not agree? The reason I ask is that i have asked a similar question (purpose of return flow gate valve elsehwere). If you have a different opionion to the above would be interested to hear. Tks.

 

[kevplumb]

the gate valve is there to balance the flow if hw and ch are both selected all the heat runs to the cylinder ie path of least resistance
its not there for maintainance

it atcually should be a locksheild

it also stops to some extent the heating curcit using the cylinder as a thermal store when the system is cold

 

[felix]

I can see that some people might want their radiators to get a bigger share of the boiler's output when HW is also demanded but ---

it also stops to some extent the heating curcit using the cylinder as a thermal store when the system is cold

You've lost me there.

 

[andy.budgell]

Lost me too!

I can see why some people have a preference to putting it and half open, but this efficiency thing I cant buy. Water running through the HW cylinder will dissipate heat into the hot water regardless of the speed its travelling, ok more energy will be dissipated per volume of water if its travelling slower.
But the boiler doesnt need to reheat the water so much etc..
I'm sure we could go on and on and on!

On a different note, what causes overpumping?

 

[andy.budgell]

Dont worry, I did a search and found overpumping info..