DIY Doctor

Upgrading to S Plan and Auto Bypass Valve (do I need one)

Postby daniel foss » Fri Dec 04, 2009 11:38 am

Hi All,

I intend to update my central heating controls as the boiler is staying lit far too long on the dhw side of things. Heating is fine

When I moved to this house it had an old randall rotary timer, a 2 way motorised valve, a thermostatic cylinder valve, radiator Thermostatic valves and a gate valve fitted as a bypass (this i feel is probably half the trouble as its letting too much flow)

18 months ago I done away with the old Randall time clock and fitted a Drayton Tempus 7 programmer with a drayton 3 wire room stat to do away with the trv's so I had better control over the heating.

However the thermostatic cylinder valve has now failed and is fully open all the time and as a result the dhw is close on boiling, the boiler only stops for a very short time due to the boiler stat cutting in (spending a fortune on gas!!!!)

A replacement valve is around £80 so I am thinking of going the S plan route instead?

I reckon I would only need another 2 way Motorised valve a tank stat and a few fittings and a bit of wiring and I would be in business! I do have a couple of questions though:

Should I do away with the gate valve?

Do I need an Auto bypass valve in its place?

Can I fit the new 2 way valve with 15mm reducers to connect the cylinder coil?

Any suggestions as always would be greatly appreciated.

Regards, Daniel
daniel foss
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Postby plumbbob » Fri Dec 04, 2009 7:11 pm

Sorry, but I am not quite sure that what you plan to do will work as you expect, and you may damage the system or stop it working properly.

TRV's are fitted to give individual room control and make the system more controllable and efficient. Not fitting them (or I suppose removing them) actually contravenes current regulation and absolutely should not be done!

I am not actually convinced the valve you refer to is actually a bypass?? Are you sure it is not the cylinder lock shield???

"S" Plans must always have a bypass fitted (usually a cracked open gate valve) to prevent serious damage to the pump and boiler!

Can you explain your description of "a thermostatic cylinder valve" and explain how it differs from a "motorised valve"?

"I reckon I would only need another 2 way Motorised valve a tank stat and a few fittings and a bit of wiring and I would be in business!" You do realise this entails significant alterations to the system wiring??

"Can I fit the new 2 way valve with 15mm reducers to connect the cylinder coil?" Sorry, don't understand what you mean.
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Postby rosebery » Fri Dec 04, 2009 7:53 pm

You MUST have a bypass with S plan. Stick with the gate valve.

Why would you want 15mm reducers on the DHW circuit? It should be piped in 22mm minimum. What size are the tappings on the cylinder.

BTW why do you think removing TRVs will give you more control over the heating?

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Postby daniel foss » Sun Dec 06, 2009 9:58 am

Hi Guys,

I have had trouble getting this reply onto the forum. having written it out a few times and loose it! This attempt 3.

Firstly, thanks for your input I really appreciate your time and help.

I will try and describe the system I have as best I can, what I have done so far and what I would like to acheive at the end. Please bear with me.

The original heating system when I moved in this 3 bed semi is as follows: Ideal Concord WRS boiler with no pump overun circuit, Randall rotary time switch 2 on 2 off cycles per day, on the base of the timer there is a single manual rocker type switch when in the off position DHW only. When turned to on this selects CH and operates the 2 way motorised valve, to let flow to the rads. when turned off it valve motors back to DHW. When the system times "on" the pump runs continuously, as a result water is constantly pumping round the DHW which is losing heat that’s why I think the boiler is on too long

The DHW is controlled by a 15mm ( faulty/stuck open and hot water way too hot 80 degrees ) Danfoss thermostatic cylinder valve this mounted on 15 mm pipe to the top feed side of the cylinder coil. the temperature of the cylinder is detected by a bulb and capillary tube and this the only thing connected to the cylinder. Between the top and bottom of the cylinder heating coil is a gate valve which is now leaking. The heating coil fittings are 22mm by the way.

I will describe the system as it appears in our upstairs airing cupboard.

22mm copper pipe comes up through the floor about 8 inches above that sits the pump on iso valves, outlet of the pump is 22mm pipe another 8 ins long then goes into a 15mm reducer which carries on up vertically about 2 feet, on top of this is an auto bleed valve.

Just before this reducer a 22mm tee takes the pipework horizontal, to this horizontal pipe all the valves are fitted in the following order:

1. Gate valve tee'd off this pipe in 15mm this goes down to the cylinder heating coil lower fitting and tee's into 22mm pipe again this goes into an elbow and down through the floor.

2. Honeywell V4043 motorised valve tee'd off downwards and mounted vertically on 22mm pipe after the valve this pipe also goes down through the floor

3. Danfoss T Stat valve on 15 mm reducer mounted horizontally and feeding the top feed coil in 15 mm.

As mentioned in my previous post I have done away with the rotary time switch and fitted a Drayton Tempus programmer and Drayton room stat. This I wired in accordance with the "W Plan" giving DHW priority and from a CH point of view works fine.

By the way, I dont intend to remove the rad TRv's I have just turned them fully up in the living room and the coldest bedroom is that not right?

I thought what I needed to do is get the DHW sorted out by replacing the faulty cylinder valve with a motorised one and wire it for S plan, is that not the right way to go?

With S plan wiring, when CH and DHW is not calling for heat should the pump be off?

The only other thing is the gate valve. As this is leaking and needs to be replaced how would i set up new one? what is the purpose of this valve I kind of thought that it just allowed the pump to circulate water when all the valves were closed to avoid pressure build up and hot spots.

Kind Regards, Daniel
daniel foss
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