CH/HW WITH SINGLE MOTORISED VALVE:

[RONCHAM]

Hi - I've got a fully-pumped CH/HW system with a single MV on the CH - the HW tees off upstream of this MV. All radiators and the hot water cylinder have TRVs although I think the bathroom radiator also acts as a bypass. So presumably the only mechanism for switching off the boiler & pump is the boiler stat? Is it risky or dangerous to be entirely reliant on the boiler stat??

Could I install a room stat and just set it a bit low with the TRV in that room set high? Could I also install a HW thermostat linked to boiler/pump live supply (and orange wire from MV)?? I believe I can only programme for CH + HW, or HW only. Am I overlooking something blindingly obvious? Advice appreciated.

 

[ColJack]

not even close to the right forum...
try plumbing...

 

[RONCHAM]

Apologies for that! Haven't used forum enough to know my way around.

 

[gas4you]

If I have understood your post properly, I would say the programmer is operating the boiler and the pump for CH and HW, whilst the room stat is controlling the zone valve only for the CH.

You can install a room stat and then leave the trv set on max, or remove the head completely.

The best way to achieve your goal of full control would be to remove the HW cylinder tap stat, fit a standard cylinder stat and another zone valve where the tap stat body is fitted in the pipe work.

You will probably need a new 'full programmer' such as a Danfoss FP715Si, to achieve full independent control of heating and HW.

It sounds complicated in words, but practically it is very simple

 

[RONCHAM]

Hi - thanks for your very helpful reply.

You will appreciate there is no room stat at present. The boiler is 10-15 years old and not of the modulating type. When CH and/or HW are satisfied during a programme on-period, surely the only mechanism that can switch off the boiler/pump is the boiler stat?? I understand full control could be achieved with a 2nd MV or a 3-port MV but I was hoping to minimise plumbing!! I think my existing programmer is suitable.

Would the room stat and HW stat I suggested work with just the present single MV??

Regards.

 

[gas4you]

With your present set up, if you remove the tap stat and just fit a cyl stat, you will be able to electrically control the boiler/pump on/off to a certain extent, but because you have no MV for the HW, this would still get heated whenever the ch came on.

You need to fit another 2 port valve for the HW side, then you can have ch, hw, or both on depending on your programmer set up.

 

[RONCHAM]

Thanks again. Of course, you're right, HW would still get heated (that's the blindingly obvious I was worried about overlooking!). 2nd MV or 3-port MV sounds the sensible solution. I don't tend to programme HW only as boiler must give out far more heat than HW tank could handle right from start (pipe to HW only 15mm which seems unusual?).
Am I right that boiler stat does eventually turn off boiler/pump? - and presume it would be dangerous if boiler stat failed?? Expect my system was done for cheapness? - but at least MV is a Honeywell.
Regards, Ron.

 

[gas4you]

Depending on which boiler you have, the boiler stat will probably just turn the boiler off. Some boilers control the pump from the boiler so there is 'pump over run', but not all.

If it is a system boiler then the pump will also be linked to the boiler operation in some way.

Again, depending on what boiler you have, it may also be fitted with an overheat stat as well as the standard boiler stat.

 

[RONCHAM]

Hi - thanks yet again.

For info. boiler is an Ideal W (RS460) which I bet is as basic as they make 'em? - do you know if it fails "safe" if its stat fails?

Am I right that HW only (42x18 tank) with just 15mm inlet, wouldn't take much from full boiler output? I have rad TRVs turned down to minimum in summer and always programme CH and HW - still wasteful I know, but don't care to programme just HW only.

Regards, Ron.

 

[gas4you]

From memory if the stat fails it fails to how it likes

Often on basic boilers such as this, the first sign of a stat failed is the boiler 'boiling' and possibly the header tank spilling out of the overflow pipe due to the plastic ball valve being melted in the hot water in the header tank.

15mm to the cylinder indirect side is ok. It will carry approx 4Kw of heat comfortably and as most systems were/are designed to allow 3 Kw for HW re-heat, it is ample.

 

[RONCHAM]

Thanks gas4you, boiler stat failure sounds a bit grim! Could you (or another helpful plumber) indulge me one last time. I have a wiring diagram which I believe is from a reputable source. With a single MV on CH and roomstat plus cylinder stat, it shows live side of cylinder stat connected to orange wire from MV together with lives to boiler and pump. This presumably means with HW only programmed on, cylinder stat will control boiler and pump, and with CH + HW programmed on, both have to be satisfied before boiler and pump are switched off. Perhaps in the coldest days of winter HW temperature may become higher than normal (HW usually takes about 2 hours to get to "normal" temperature). I know this would not be the ideal arrangement but do you consider it a safe and acceptable option? This seems a simpler job for me to contemplate compared to an additional MV or 3-port MV. Thanks for any advice.

 

[gas4you]

I think what you are decribing is a 'W' plan.

This is a tried and tested wiring system from Honeywell.

Their website has all the diagrams and info you need for wiring ch systems.

 

[RONCHAM]

Hi Dave - thanks for usual quick response. Not sure if I got the wiring diagram from Honeywell although I've got a few of their downloads - it was presented as a wiring option if only a single MV (on CH). It seems to me it would work with just the possibility of very hot water on a very cold day. But I'm so mean I seldom have CH/HW on continuously!!
I believe W-plan uses V4044C diverter valve which I presume is their 3-port MV (mine is a 2-port on CH circuit, HW tees off before it)?

Regards, Ron.

 

[gas4you]

It might be a 'C'plan then and you are right about the 'W' plan. I'm not to good with names of things or memory

 

[D_Hailsham]

With a single MV on CH and roomstat plus cylinder stat, it shows live side of cylinder stat connected to orange wire from MV together with lives to boiler and pump.

This presumably means with HW only programmed on, cylinder stat will control boiler and pump, and with CH + HW programmed on, both have to be satisfied before boiler and pump are switched off. Perhaps in the coldest days of winter HW temperature may become higher than normal (HW usually takes about 2 hours to get to "normal" temperature).

You have hit the problem with this arrangement on the head. During the Summer, when the motorized valve is permanently closed, the boiler will be effectively controlled by the thermostat on the side of the cylinder - set to,say 60C.

In the winter, the hot water cylinder thermostat will only control the HW temperature when the valve is closed. At other times, when the heating is on and the valve is opened, the water temperature will depend on how long the boiler keeps running before the room thermostat operates. Under these circumstances the Hot Water could easily rise to 80C, which is dangerous.

The cost of a second Honeywell V4043 Zone Valve, replacing the mechanical thermostat in the cylinder circuit is about £45 and the cylinder thermostat about £10. This will give you safe HW temperatures all year round.

What timer do you have (Make and Model)? It may be possible to set it for separate HW and CH control.