DHW cylinder thermostat and cut-out.

Philip Saunders · Mar 11, 2019

Hi again ......

I have inherited a central heating system which is oil fired and ancient (mid 1980's), but works perfectly so up until recently I had no reason to go looking for issues.

The only ''new" part is the unvented DHW cylinder which, although it works, has NOT been installed in accordance with the manufacturer instructions.

The current setup is that the manufacturer installed cylinder thermostat and overheat cut-out are not connected at all and never have been (2009 installation date). Instead, the 3-port valve that controls flow through the heating coil is controlled by a pipe thermostat on the hot outlet (recirculating system so this is at cylinder temperature AS LONG AS THE PUMP IS RUNNING - which is why I'm looking at this now, because it's on a timer), and a pipe overheat cut-out, both mounted on the pipe within 150 mm of each other. Both devices are SPDT switches, with 3 core cables

My question - in order to put this system back into compliance, I need to connect the cylinder mounted thermostat and overheat cut-out in place of the existing devices. Problem - the manufacturer fitted devices are SPST devices, which is probably why they weren't used in the first place.

The 3 port in question requires DT switching, as it's positively driven in both directions.

Any suggestions?

TJ Anderson · Mar 11, 2019

Thats dangerous, but i think you already know that.

The reason the stat and safety cut out are NC devices is because they are in series and it should have a 2 port zone valve on the primary to that cylinder to ensure it shuts off when cylinder satisfied or if thermal cutout opens.

Philip Saunders · Mar 11, 2019

Yep I'm aware that it's dangerous which is why I want to fix it.
I know it's supposed to have a 2 port to shut off the coil - the 3 port installed does that, except instead of simply cutting the flow, it diverts it straight back into the boiler return.
In the meantime I've got the secondary circulation pump on 24/7 to ensure that the pipe is at cylinder temperature.

Wilko · Mar 12, 2019

Hi - as per @TJ Anderson - I think the problem started when the unvented cylinder was fitted onto a 3 port valve, instead of having its own 2 port. I would look to getting that part right as it's the recognised standard. Perhaps raise a thread in the Plumber's Forum next door. @Last plumber ?

Last plumber · Mar 12, 2019

Philip Saunders said:
Hi again ......

I have inherited a central heating system which is oil fired and ancient (mid 1980's), but works perfectly so up until recently I had no reason to go looking for issues.

The only ''new" part is the unvented DHW cylinder which, although it works, has NOT been installed in accordance with the manufacturer instructions.

The current setup is that the manufacturer installed cylinder thermostat and overheat cut-out are not connected at all and never have been (2009 installation date). Instead, the 3-port valve that controls flow through the heating coil is controlled by a pipe thermostat on the hot outlet (recirculating system so this is at cylinder temperature AS LONG AS THE PUMP IS RUNNING - which is why I'm looking at this now, because it's on a timer), and a pipe overheat cut-out, both mounted on the pipe within 150 mm of each other. Both devices are SPDT switches, with 3 core cables

My question - in order to put this system back into compliance, I need to connect the cylinder mounted thermostat and overheat cut-out in place of the existing devices. Problem - the manufacturer fitted devices are SPST devices, which is probably why they weren't used in the first place.

The 3 port in question requires DT switching, as it's positively driven in both directions.

Any suggestions?

Hello Philip.
Do you know how the system is piped, i.e. is it piped as a fully pumped system should be?
I ask this due to your description. It sounds like someone in the past has maybe avoided the work to make the alterations to a fully pumped system or at least tried to avoid alterations to controls and wiring.
It should ideally be an 'S' Plan. My apologies if you already know how these things are wired, I don't want to teach my Grandma to suck eggs but a lot of sparks do not get involved with Heating. If you are one that does please let me know!
Which make and model of three port do you have?
When you say the three port diverts back to the Boiler return, is that definitely how it is piped? Normally there would be a flow into the valve, a flow out to DHW (Domestic Hot Water) and a Flow to CH (Central Heating), on a Domestic job.
On Commercial we do use POPC (Power open, Power Close) valves to do all sorts of things with flow. We use them on Domestic obviously but not normally piped as per your description. On domestic it would be unusual to see that. Well, it would be unusual in my part of the world anyway.

Philip Saunders · Mar 13, 2019

@,Last plumber - you may well be right about avoiding work ....

I should probably have said that the system was installed when the building was commercial - although it's residential now.

If there is call for DHW heat, the coil for the cylinder is fed directly from the boiler flow, (pumped) then through the 3 port and back to the return.

If there is no demand for heat, the water flows from the flow, through the other port and directly back to the return.

Seems bonkers, but it does avoid the pump pumping against a closed valve, as it's never actually closed as far as the pump is concerned.

Changing any of this is not trivial, as it's all plumbed in 2 inch iron pipe, except for about a foot of 22mm pipe where the connection is made to the cylinder.

The valve actuator is an ancient Staefa A204.

The thermostats (control and overtemp) are mounted on the hot water outlet, which has a secondary circulating pump. They are SPDT devices.

I suspect the original DHW cylinder was a vented gravity fed cylinder (based on the fact that there is a still connected but decommissioned one still on site)

A complete mess I know, and given the building's previous owner and use it's incredible that it was done at all, but somehow I need to fix it.

Last plumber · Mar 13, 2019

Philip Saunders said:
@,Last plumber - you may well be right about avoiding work ....

I should probably have said that the system was installed when the building was commercial - although it's residential now.

If there is call for DHW heat, the coil for the cylinder is fed directly from the boiler flow, (pumped) then through the 3 port and back to the return.

If there is no demand for heat, the water flows from the flow, through the other port and directly back to the return.

Seems bonkers, but it does avoid the pump pumping against a closed valve, as it's never actually closed as far as the pump is concerned.

Changing any of this is not trivial, as it's all plumbed in 2 inch iron pipe, except for about a foot of 22mm pipe where the connection is made to the cylinder.

The valve actuator is an ancient Staefa A204.

The thermostats (control and overtemp) are mounted on the hot water outlet, which has a secondary circulating pump. They are SPDT devices.

I suspect the original DHW cylinder was a vented gravity fed cylinder (based on the fact that there is a still connected but decommissioned one still on site)

A complete mess I know, and given the building's previous owner and use it's incredible that it was done at all, but somehow I need to fix it.

I obviously haven't seen this so you'll need to put up with me thinking out loud. The image of the system I have in my head, going off your description sounds like it could do with a contact set?

I'm thinking that your switched live (SL) from whatever calls for heat through the over heat and cylinder stat and SL from that to pull a contactor in which will use the NO side to put SL on to open the valve and obviously once the cylinder is satisfied, the stat will open and the contactor will drop out and the NC side will put live onto the close terminal in the valve. You can also use this method to turn the Boiler on and off if need be? In some systems I include a timer for a pump over run.

If this is miles off what you're aiming at and I have misinterpreted the situation, I'll think of something else I'm sure. Let me know either way.

Philip Saunders · Mar 14, 2019

Thanks for that - I spent a couple of hours last night buzzing cables out - and they don't go where I thought they did!

The whole system is cabled in pyro which makes tracing a nightmare - but it appears you're right. There is an octal relay that is energised by the DHW programmer which is nothing more than a single channel timer. I need to spend some time working out what EXACTLY happens when the thermostat says 'go', and then wire in the cylinder stats accordingly. There is no pump overrun timer. As far as I can tell, the primary pump has been running continuously for about 30 years. There is a switch on the panel that swaps between primary and backup pumps, but no timer.

Since you obviously know this kind of system, please let me know if you're ever in the north Highlands!