Separate CH & HW control on an old Worcester 24i RSF..?

[SpikeyMike]

My son has recently bought a house that’s fitted with a (fairly old) Worcester 24i RSF combi boiler. This has a strange setup for hot water and central heating control which makes no sense to me. When I downloaded the manual, it seems to confirm that there is no electrical connection that would allow separate CH and HW demand. Its either all on or all off.

If the boiler is ‘on’, then the CH pump runs and regulates to the set circ temp (with TRV’s on the rads), unless the system detects flow on the HW intake when it all flips over to HW (CH pump off).

I can’t find any wiring that would allow HW but no heating of the CH loop, so no way to fit a proper CH timer/stat to the system (cos you would end up with a cold shower as soon as the rooms warmed-up). It also means you need to plan your showers around the system timer, or leave it heating the CH loop 24/7 for no reason, which right now would lead to stratospheric gas bills.

Just wondering if anyone on here had come across one of these dinosaurs before..? Is my understanding correct, or did I miss something..?

 

[denso13]

If the boiler is ‘on’, then the CH pump runs and regulates to the set circ temp (with TRV’s on the rads), unless the system detects flow on the HW intake when it all flips over to HW (CH pump off).

That's normal.

I can’t find any wiring that would allow HW but no heating of the CH loop

It's a combi, if the heating is off and you turn on a hot tap the boiler fires up and heats the (tap) water. The CH shouldn't get warm.

 

[SpikeyMike]

Here's the diagram. The board that controls CH & HW is powered off by the stat/timer, so it can't see any HW flow to warm up. Then if it's on, it does both. I can't figure it out...

 

[denso13]

The board that controls CH & HW is powered off by the stat/timer

The inputs at the top tell the boiler what to do. But as it is combi, the HW isn't controllable per say, it heats up when a hot tap is opened.

The CH is controlled by the room stat and is seperate from the hot water, except that HW has priority.

I have the 28i version and that's how it works. Hot water works whether CH is on or off, like virtually all combi's.

 

[JonathanM]

I have the 28i version

Looking through the manual for the OP's boiler, I noticed it only has one heat exchanger which does both CH & DHW, rather than a separate plate heat exchange for the DHW. Was that common on older combis? I know Intergas use a single heat exchanger but I got the impression that most others now use two (that might be wrong!).

 

[ianmcd]

Looking through the manual for the OP's boiler, I noticed it only has one heat exchanger which does both CH & DHW, rather than a separate plate heat exchange for the DHW. Was that common on older combis? I know Intergas use a single heat exchanger but I got the impression that most others now use two (that might be wrong!).

It is called a Bi-Thermal gas to Water heat exchanger, no diverter valve , no Plate heat exchanger, simply when there is a demand for HW the burner stays on and goes to high flame and the pump stops so all the heat now heats the HW side of the bi-thermal heat exchanger, when the HW demand ceases and there is a CH demand the pump will now run and the heat will now be transferred to the radiators

 

[SpikeyMike]

Yes, I believe that it is bi-thermal, bit like an early attempt at a combi boiler...

Apart for a couple of confusing references to the room stat, the system behaves the way the wiring diagram says, with no separation of CH and HW control. The wiring diagram shows the transformer for the controller board, gas valve, fan, everything being powered from the one source (X2 connector). And the X2 connector is little more than a bigged-up terminal block...

Wondered if there was some sort of mis-print in the manual, or if anyone had got over this issue. I have this idea to put an additional flow switch outside the boiler on the cold DHW feed, with maybe a short run-on timer. I can use some volt-free contacts and wire this in parallel with a new timer/stat and into X2. That way, the CH can be controlled by something more 21st century and any HW tap can fire up the boiler any time from whatever it is doing.

Really would like to know if I have missed something first tho. They don't live nearby, so not easy to go over there and twiddle... certainly would be easier if there was a simple 'HW' connection...

 

[oldbutnotdead]

Is there a room thermostat and controller installed anywhere? Are there links in X2 between 2&3 and 5&6.
Recent house purchase- wonder if previous owners had Hive or Nest or something and took it with them?

 

[denso13]

certainly would be easier if there was a simple 'HW' connection...

The hot water flow is simply detected by a simple flow switch and the boiler fires up.

You have posted a functional flow diagram but it doesn't say for what. Could just be the CH

This is the wiring diagram, water flow is detected by the flow switch and the boiler heats the water to the hot taps. Wheather the CH is on or off.

 

[JonathanM]

My son has recently bought a house that’s fitted with a (fairly old) Worcester 24i RSF combi boiler.

Have you used a combi boiler before? Do you know how they work?

I can’t find any wiring that would allow HW but no heating of the CH loop

As a test, turn the temperature dial on the boiler all the way anti clockwise to the sun symbol. Then turn a hot tap on full blast and see if the boiler fires up and produces hot water.

 

[Londoner2]

1) its 78% efficient
2) Non condensing boiler.

Pencil in a replacement over next 2 years.

I had the 240 RSF it was okay did its job, it was fitted in 97, i assumed ownership in 04 and was replaced in 19, (08 diverter valve replaced, 15, chnaged prv n loop as it was passing)

It had on off button but this related to heating only, hot water was not affected by this switch.
It had a temp rotary dial like yours to control heat flow, i had set it to 2 and never changed that, this heated up rads but never used gas excessively.
I hardly serviced it every year.
I gather many engineers hated working on them as they were difficult to work on.

But it lasted a long time.

 

[ianmcd]

Yes, I believe that it is bi-thermal, bit like an early attempt at a combi boiler...

Apart for a couple of confusing references to the room stat, the system behaves the way the wiring diagram says, with no separation of CH and HW control. The wiring diagram shows the transformer for the controller board, gas valve, fan, everything being powered from the one source (X2 connector). And the X2 connector is little more than a bigged-up terminal block...

Wondered if there was some sort of mis-print in the manual, or if anyone had got over this issue. I have this idea to put an additional flow switch outside the boiler on the cold DHW feed, with maybe a short run-on timer. I can use some volt-free contacts and wire this in parallel with a new timer/stat and into X2. That way, the CH can be controlled by something more 21st century and any HW tap can fire up the boiler any time from whatever it is doing.

Really would like to know if I have missed something first tho. They don't live nearby, so not easy to go over there and twiddle... certainly would be easier if there was a simple 'HW' connection...

There is absolutely no need, there is nothing wrong that is exactly how the boiler works as @denso13 has explained to you correctly twice