Warmflow CH & DHW heat exchanger/s

24 Nov 2004
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United Kingdom
I am doing some work in an old house which originally had a solid-fuel CH and DHW installed in the 70s. about 1990, this was replaced by an oil-fired system (a Warmflow boiler). I believe the boiler only was changed out, so the existing plumbing would have remained as-was for the original solid fuel system.

Here's an overview of the system:
I think it's "gravity HW, pumped rads."

large cold water storage tank in loft supplying the cylinder (on the firstfloor) and also cold water to the bathroom.
The cylinder is vented back into this tank.

small F&E tank in loft supplying water to the boiler. (the feed pipe tees off to the cylinder on its way down to the boiler)

The boiler has 4 pipes connected which I have identified as follows:
Main feed AND return from HW cylinder
flow to HW cylinder
flow to radiators
return from rads

There is a 2-channel digital programmer wired up to the system, and is functioning in itself OK.

The rads circuit has a brand new Grundfos 1650 on the return leg next to the boiler. (I have just replaced the old seized Euroma pump)

The room stat for the house controls only this circulating pump. (when it gets cold, the pump starts) This stat will not cause the boiler to fire.

There is a frost stat and separate room stat for a vulnerable extension to the house which is isolated by the only zonevalve in the entire system. When the frost stat operates, the TRV opens, the boiler fires and the pump runs. When this remote roomstat operates, only the pump runs.

The boiler will not fire if the CH is turned on at the programmer - only the pump will run.
The boiler will fire if DHW is turned on at the programmer.
There is a stat on the front of the boiler which governs temperature of the DHW (and, I suppose, eventual rad temperature)

I am a bit confused as to how this all hangs together. Apparently the system has been worked on by a number of different engineers in its history. In particular I am perplexed by this:

Lets say its getting abnormally cold in the house in the middle of a sunday, then you might go to the controller and press advance on the CH side. The stat's set for 20C and its 15C in the house, so that is ON. all that happens, though, is that the pump runs and circulates cool water in the rads.

If you are in the know, then you advance both CH and DHW, and then the boiler fires and you start getting heat out of the rads.

Its as if there is a dual channel controller where a single chanel would do...

Also I am perplexed that there is no explicit feed line for the rads. There must be an interconnection between the 2 heat exchangers at the back of the boiler (?)
I have the warmflow manual but there is F.All in it about the exchangers.

Would appreciate professional clarification on the thermodynamics of this system - 'ow duz it work?
I should add - rads are piped in 15mm, DHW is 22mm and other larger sized.
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Because its not plumbed up/wired up to allow it.
(The programmer shouldn't allow it either, and has been set up incorrectly for a gravity system, if you tell me/us the model of your clock, we can advise on how to correct it.)

How it works.

Gravity system work by hot water being less dense then cold so it gets heated in the boiler gets displaced by colder water feeding in and it pushed round the system by gravity acting on the colder water... which is then heated.. and displaced by the colder water behind... repeat.

So unless you plan on stopping the force of gravity acting on the water, when the boiler is on the dhw is on.

the water is pumped round the heating circuit so you can choose when it's on or off, the restriction of the pump/smaller pipework/radiators/etc means the system requires pressure to work.
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The programmer is a Channel plus XL NLH21.

Thanks for explaining the principles. As you say, I cant have the rads on without the dhw being on.
Is there a "soft fix" for the programmer that would fire the boiler with only the CH side of the programmer lit up? I realise that I could modify the wiring behind the programmer but I feel that I shouldnt have to do this.

A separate problem is this - one of the g/floor rads won't heat up. I feel it's airlocked.
the 15mm rad pipes come vertically up from the boiler into the 1st floor void and are teed there. one side of the tee leads to 8 of my rads g/floor and first floor, all 8 of these are working. the other side of the tee has only the bathroom rad.
I have tried shutting off all 8 of the hot rads to "force" the hot into the bathroom but no use (boiler starts grumbling a bit).
There is a draincock in the return line of the problem rad, and if I crack this with the pump running, I can hear air sucking into the water. (so the return line is clear)
all the rads are full and I have had a good go at bleeding the circuit with vent plugs in the first floor void. Are there any other tricks?
not up on that programmer, but its usually a jumper on the back of the programmer to switch it between gravity and fully pumped. Then

Check the valves are both open on the rad, you could have a blockage on the flow valve.
Cheers for that. Unfortunately now at sea but my brother has taken over work on the place. Now you mention it, I do recall a switch on the back of that programmer, and I think when I fitted it, I flicked it to pumped, just cos I saw a pump in the pipework.
So you can have a circulating pump for rads, yet the system is still nominally "gravity"?

as for the rad; could well be a blockage. I'll get round to it eventually!

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