positioning a 2 port valve

[jobloggs]

Diagram shows my heating system. I want to control CH by putting TRV's on all radiators and the HW by a thermostat on the cylinder controlling a 2 port valve.

I intend to arrange pipework at boiler by running a new vent pipe back to the flow side of the boiler and repositioning the pump on the flow side of this. I also intend to situate a bypass valve just after the pump linking the flow pipe to the return.

Having done the above:

Can I site the 2 port valve adjacent to the cylinder on the return pipe or does it have to go closer to the pump?

What alterations have to be made to the feed pipe arrangement? Does this need to be run right back to the return pipework at the boiler or can it stay linked into the HW circuit as it is now. If so where would link in be relative to 2 port valve

Would replacing the existing vent from flow side of cylinder by an automatic air valve allow filling/emptying of HW circuit when necessary? I assume leaving it as it is would result in pumping over when TRV's and 2 port valve close.

Thanks in anticipation.

 

[chrishutt]

What you propose sounds broadly OK, but it would help if you could also do a diagram (since you're good at doing them) of the proposed layout.

You can put a 2 port valve anywhere, as long as it doesn't obstruct the vent pipe from the boiler. But why not use a 3 port to control the heating circuit as well (or 2 x 2 ports)?

The location of the feed and expansion pipe connection needs thinking about. Ideally it would be just upstream of the vent connection to avoid any risk of pumping over, although other locations could be satisfactory.

 

[ChrisR]

What you propose sounds like you would have no "interlock" on the CH side. You have to have a way to turn it off - a thermostat and zone valve. This is now part of the building regs (Part L). You still have to have TRV's on at least the bedroom rads.

Convention, for good reasons(!), would have:
Boiler flow > Vent > feed (close to vent)> Pump > Automatic bypass > Motorised valves ( 1 x mid-position 3 port or 2 x 2 ports) >>Cylinder and rads.
This is about what you were thinking, if I read you right. If you use 2 x 2 ports, the cylinder one can go anywhere on the relevant circuit.

A risk, with the circuit as you have drawn it, is "pumping over". This occurs because the water pressure in the vent pipe is higher than that in the feed. Unless you have a huge upward loop on the vent pipe, you make water flow around the vent > f/e tank > feed > pump> vent, loop. Even if that doesn't happen you have alot of movement when the pump starts and stops, entraining air/oxygen which supports corrosion.

 

[jobloggs]

Thanks Chrishutt & ChrisR

This is what I was thinking of. I was trying to avoid the need to run the feed back to the boiler, but if that is essential it will just have to be done. (What is recommended size?) If not essential to run feed back to boiler is it on the right side of the 2 port valve?

As the existing vent has to be removed from the flow side of the cylinder coil to avoid pumping overhow how do you drain down the HW circuit and release air on refilling. Will an automatic air valve do the trick?

I see the sense of going for full control of CH but I wanted to keep costs down and avoid a second 2 port valve and either a wireless room thermostat or a lot of redecoration with normal one. Most radiators have TRV's already.

Again many thanks for your kind suggestions.

 

[chrishutt]

Diagram is basically OK, but you seem to be ignoring the advice about going for full control and boiler interlock with a 3 port mid position motorised valve (doesn't cost much more than a 2 port). There are good energy efficiency reasons for doing this. Check the Honeywell site for their Y plan diagram.

As for the cold feed connection, put that the other (downstream) side of the MV (if you insist on just the 2 port). You can also combine the feed and vent pipes to avoid pressure conflicts, although it's not ideal. In that case run the feed and exp. across to join the vent pipe near the F&E tank.

You will of course need an AAV at the top of the flow pipe to the cylinder. That will release air and admit air when you drain down. It will need maintenance so put it somewhere easy to see and get at.

 

[slippyr4]

And all of the valves get dirt cheap if you buy them as part of a control pack - like this one.

This gives you a cylinder stat, room stat, programmer and a valve, for less than 70 quid. Solves the interlock needs, gives you good control. Only extra you may need is a bypass valve.

 

[ChrisR]

The controls we're advising aren't jut good sense, they're the law now!

The feed: the thing to avoid is having the pum p between the feed and vent. If you leave it connected where you have it shown in your last pic, that isn't the situation so you should be OK. I'd do that; if there's a problem you can always convert to a "combined feed and vent" unless your boiler instructions preclude it. Something to look up .

If you do remove the air escape route from the cylinder supplies, yes you add a vent, either auto or manual.

 

[PaulAH]

Chris - The www isn't exactly awash with info on "combined feed and vent". Am I right in thinking that you can join the vent to the feed at a point below the tank, so expansion occurs upwards within the body of water?

What's the point in having a vent at all, then?

A lot to learn in this game!

 

[chrishutt]

The feed: the thing to avoid is having the pump between the feed and vent. If you leave it connected where you have it shown in your last pic, that isn't the situation

I'm afraid it is, Chris. When the motorised valve (MV) is closed, the feed connection is effectively just downstream of the pump and the vent just upstream.

Even if the vent is too high to allow air to be sucked into the system, the movement of water down the vent and up the feed pipe every time the pump switches on (when MV closed) will increase oxygenation of the system water and promote corrosion. That's why I suggested making the feed connection downstream of the MV.

 

[ChrisR]

Chris - The www isn't exactly awash with info on "combined feed and vent".

THEN DO A SEARCH ON THIS SITE THERE ARE 23 REFERENCES!!!!!!

 

[ChrisR]

Even if the vent is too high to allow air to be sucked into the system,

you didn't mean that...

the movement of water down the vent and up the feed pipe every time the pump switches on (when MV closed) will increase oxygenation of the system water and promote corrosion.

yes , I did say ....

I suggested making the feed connection downstream of the MV.

You're right I was wrong - , I missed the MV. The feed needs to be connected to the boiler return with no valves in the way. It can be up by the cylinder.

 

[chrishutt]

ChrisR doesn't like it when someone spots an error in his posts - fortunately it's a rare event.

Quote:
Even if the vent is too high to allow air to be sucked into the system,

you didn't mean that...

Well, yes I did. Did I not express it well, or am I wrong too?

What I meant was that, given the situation where the pump sits between the vent and feed connections (as in the diagram when the MV is shut) the pump would suck air all the way down the vent pipe if the negative head (suction) of the pump was greater than the static head created on the feed side by the lowering of the vent pipe water level.

In the case in point the static head created will be much greater than the negative head crated in the vent pipe by the pump, so air will not actually be sucked into the system, although the water level in the vent pipe will drop by several metres, depending on resistance through heating circuit (TRVs shut, etc.).

 

[PaulAH]

THEN DO A SEARCH ON THIS SITE THERE ARE 23 REFERENCES!!!!!!

Chris, I may be stupid but please don't shout.

Done that. Trying to find a definitive reference to combined vent and feed is far from easy. For instance, having spent ten minutes downloading and trawling through an Ideal installation manual (a recommended source), all I found was "Also suitable for combined vent and feed arrangement" (or words to that effect - closed it now).

Isn't that why stickies were perceived as a good idea?

 

[ChrisR]

Paul, if I give a phrase to look up
and I get a sarcastic answer saying it's difficult to find
and I find 23 references the first time I look

then I am likely to shout.

The first one (Ideal) I check shows the answer - in about 10 seconds

For textbook information - buy one!

 

[ChrisR]

ChrisR doesn't like it when someone spots an error

I don't mind at all - we all make mistkaes.
I thought "You're right I was wrong - , .." was a reasonable response.

with the feed connected below (as we look at it) the MV all that's between the feed and vent is the boiler and a length of pipe - 22mm I assume. Any balancing (eg gate type) valve would be between the pump and the cylinder. With that arrangement I don't think there would be sufficient head available across the boiler and length of pipe, to lower the level in the vent very much. I haven't come across one doing it yet. Boilers aren't usually that resistive.

I'd already pointed out about movement of water - I didn't need reminding! It's just a matter of inhibiting...