Room thermostat to call pump only

[glnovice90]

I've moved into a house with very basic central heating controls (one of those timer dials from the 80s) and non traditional piping set up which is causing some complications and I want to do the minimum work at the minute to modernise it.

The way they've piped up the house the boiler sits between the hot water cyclinder and the radiators. When I just wanted hot water, the pump wasn't activated but eventually all the heat circulated to the rads. I've already modified what I could so now I have the option of heating just the hot water or the HWC and the rads.

The issue comes in where I want boost the hot water pressure but the pump temperature limit is 65 degrees. I can't wire my new 2 channel central heating system the normal way where either the cylinder thermostat or room thermostat can call the boiler because I could run into the sitatuion where the room thermostat demands could heat the whole system above 65 to heat the house (currently the system heats to around 75 which is probably limited by the boiler thermostat). I was thinking of wiring it so only the hot water can call the boiler and for the central heating to only call the pump. That way the system only uses stored HWC energy to heat the house and tops it up to 65 when it drops. Would this be a good setup? Is there a better way?

 

[Stuart981]

So what you are trying to do here is constrain the HW temperature down to a safe level and prevent convection heating the radiators in the summer. Is that right?

 

[glnovice90]

Yes. That's right

 

[MNW67]

Would a zone valve on the hot water circuit help?

 

[glnovice90]

It would require piping up a bypass circuit with another 3 port valve. The hot water side of the circuit is all 28mm dia. It's not a job I want to do.

Later down the line when I upgrade the boiler I will completely reconfigure the circuit so this is just a stop over solution.

 

[MNW67]

I have been trying to think back to how systems used to work. Wasn't your original set up common until the late 1980s? I think it was sometimes called C-Plan (in various incarnations) or maybe also sundial C-Plan. Could you use anti gravity check valves to stop the radiators heating up when the hot water is on. And if you have an old cast iron boiler, if you had a zone valve on the gravity hot water circuit, would you actually need a bypass.

 

[MNW67]

I am sorry for replying yet again But I think it has only just clicked what you are suggesting. Are you saying that you currently have a cylinder thermostat (which you will set at 65C). And your plan is that the boiler will only turn on when there is demand from the cylinder thermostat. When there is a demand for central heating, but there is no demand from the cylinder thermostat, the pump will be turned on, but the boiler will remain off. During those times the cylinder will act as a thermal store to heat the radiators. And when the temperature in the cylinder drops sufficiently, the cylinder thermostat will once again turn the boiler on.

 

[glnovice90]

Yes exactly

 

[glnovice90]

[Deleted]

 

[glnovice90]

I have been trying to think back to how systems used to work. Wasn't your original set up common until the late 1980s? I think it was sometimes called C-Plan (in various incarnations) or maybe also sundial C-Plan. Could you use anti gravity check valves to stop the radiators heating up when the hot water is on. And if you have an old cast iron boiler, if you had a zone valve on the gravity hot water circuit, would you actually need a bypass.

You are right that it's from the 1980s. I don't know what an anti gravity valve is and my boiler is more modern than that but still old. It's one that relies on a pilot light the whole time.

 

[Madrab]

Ok- you would have had what is called a gravity HW and pumped central heating - as @MNW67 suggest - what was commonly known as a C plan.

An anti gravity valve used to be incorporated into the CH to ensure there wasn't any convection bleed (Thermosyphon) into the CH if there was only a HW call and the pump wasn't running - in essence it's a single check valve. Why you are getting reverse circulation into the CH system when HW is being called will be down to the way it's piped. If you have zone valves then they should stop any circulation but the way it's using a 3 port and a 2 port is a little unusual, given the way your diagram is laid out. B port (left) is usually the HW and the A port (right) is CH but you seem to have the A port connected to the return?

What make/model of boiler is it? The older boilers with gravity HW had separate runs through the HEX for each side and they aren't interconnected so using the HW cylinder as a thermal store wouldn't really work unless you connected the primaries to the CH F&R and then controlled that via say a 2 port

 

[glnovice90]

My boiler is a concord WCF 255. I struggled to find anything about it online.

I'm not sure how the runs can be separate when it all fills from the same fill point by the cylinder and drains out through a shared drain valve.

Originally the three port and zone valve were not there. I added those in to give me the ability to force the water either through the whole system or just the cylinder, and the zone valve to heat both floors or just upstairs.

 

[MNW67]

My boiler is a concord WCF 255. I struggled to find anything about it online.

Manual here on scribd. A massive pain because of all the adverts, though.

https://www.scribd.com/document/431198062/concord-wcf-240-IS-pdf

 

[Stuart981]

It would require piping up a bypass circuit with another 3 port valve. The hot water side of the circuit is all 28mm dia. It's not a job I want to do.

Later down the line when I upgrade the boiler I will completely reconfigure the circuit so this is just a stop over solution.

Back in the day a lot of these old systems were originally connected to coal stoves and had a towel rail on the primary as a safety feature. If you have a towel rail in the bathroom might be worth checking to see what its connected to.

 

[Madrab]

One without adverts.

https://sf.surefiresoftware.co.uk/uploads/manuals/boilers/4140969.pdf

Some did, some didn't, not all of the old boiler were separate, some only had 2 separate flows with a common return too, so a 3 pipe configuration.
Even then it probably wouldn't matter in any event, the fact that the HW from the cylinder would then have to be reversed through the HEX would probably mean there would probably be a significant heat loss out through the HEX