Honeywell CH zone valve question

Hmmm. What I am thinking of is energising the pump and boiler after the tank 'stat. Yes, the boiler 'stat would be the final control, but the correct setting for that could be easily determined. This would only be happening while there is heat demand from the room 'stat, so by definition there is a heat load on the system.

The mechanism is that the timer always energises the pump and boiler if it is energising the CH, there is no timer setting that calls for CH without DHW.

Maybe the pump needs to be operated but not the boiler. But the boiler temp. does not need to be very high, this is a first floor flat, so has flats above and below.

That is encouraging. What I really need is a modern timer that has the same operating characteristics as the Randall 3060. If only Danfoss would get back to me.

The mechanism must be that: The tank 'stat brings the tank to temperature, and shuts off the pump and boiler. Later, the Room 'stat closes, and opens the valve. We don't know how long it takes, but the next time the tank 'stat closes, firing the boiler and pump, there is then circulation through the heating circuit. This continues until the room 'stat is satisfied, and the system reverts to tank 'stat control. We also don't know the hysteresis of the Room 'stat, it might be quite small. I'll see if I can find anything on it. The next time I am there, I will try to do some experiments. There is no external indication of the state of the Room 'stat, so there is nothing to bring a delay to anybody's attention, as long as the temperature in the flat remains ok. It is certainly better than no heat at all.

You should have two motorized valves, one for the tank, one for the CH,

Sorry, but that ain't going to work! You *can't* rely on a demand for DHW to make the CH work - you could wait all day if you've got a well insulated tank of hot water and don't draw much off.

I've knocked up a schematic of how you would need to wire your existing hardware to make it work (almost) correctly. See

My scheme uses the microswitch on the zone valve and turns on the boiler and pump whenever either or both DHW and CH are demanding heat.

That still suffers from the problem that unless you run the boiler flow temperature artificially low, the HW tank will continue to get hotter than the tank stat set point whenever the CH is running.

As TNP has said, the proper way to do it would be to get an additional zone valve and insert it into the HW circuit. That should be fairly trivial to do - and you would then, with the correct wiring, have a fully fledged S-Plan system which would overcome the problems which I highlighted above, and would enable your flat occupant to live happily and cosily ever after!

I'll look at that schematic after lunch.

But consider this: There has never been a problem with the tank getting too hot, and the system works fine.

But there is only one, and the system appears to work. "If it ain't broke, don't fix it" is a very useful maxim. Trying to understand it, yes, but trying to find a solution for a problem that has never existed is a waste of time, energy, and money. Taking your last point, my suggestion of using the microswitch to operate the pump and boiler in parallel with the rank 'stat sounds to have the same effect as what you propose. I have found the model number of the room 'stat, I will see what I can determine about its characteristics.

Since the whole thing is controlled by the tank stat, there wouldn't be would there? The problem will be a steaming hot water tank and therefore no central heating at all....

The problem then becomes shutting down hot water supply or at least limiting the boiler heat output to - say - 60°C to avoid overheating the hot water cylinder.

The point to having two MVs is to allow *independent* operation of CH and DHW.

You cannot do it with just one. Unless its a three port device.

..... which has never since being put together in 1976 been known to happen.

I can find no information on the deadband for the Satchwell TLC2356 thermostat. Plenty of links to its wiring diagram, but no real data. Following links to Data Sheet all lead to the same wiring diagram.

Time for lunch and Snooker on TV.

But as I say, this has never been a problem. And the timer does not enable independent operation of CH and DHW anyway, if the CH is on, so is the DHW.

BUT the CH will shut down when the tank is hot enough. Unless its been set up so that the tank never in fact is hot enough by limiting the boiler outflow temperature.

If it works, fine, but don't argue that it is the correct or even usual way to do things.

I've not been following this thread particularly ... but isn't the OP describing a 'C plan' arrangement?

The 'C' Plan uses the valve microswitch to turn the boiler on and off. In my system, the tank 'stat operates the pump and boiler together, the Room 'stat opens the zone valve which feeds the CH loop. The timer always has a DHW output if the CH output is On. In theory, when the room 'stat opens the valve, if the tank is at temp. and so the pump is not pumping, there is nothing to push water around the CH loop, but it appears to work. I think that the system waits in stasis until the next time the tank 'stat demands heat, then when the pump starts the CH loop is in action, and the heat demand keeps the pump and boiler running until the room reaches temperature. We don't know for sure if this is what is happening, we don't know the length of the delay between the Room 'stat making and the next time the pump runs. But I am certain that the piping and wiring are as I describe. the wiring is as it was with the previous timer. The resident in the flat was totally happy with it last winter, and is again now it is working again, after a timer and pump replacement. The old pump was the original, installed in 1976, the timer was last replaced in 2015/6.

Hope this helps!

Your diagram is exactly what I suggested. earlier, when I was trying to find a true digital replacement for the Randall 3060.

No, because it's fully pumped whereas C-Plan is gravity HW and pumped CH. Also, C-Plan has a single zone valve in the HW circuit whereas the OP's system has a single zone valve in the CH circuit. It's closest to S-Plan but with one zone valve missing along with the usual control logic which makes use of the zone valves' microswitches.

He lived cosily and happily through the whole of last winter, and now appears to be doing so in this one as well. Installing a valve would not be too difficult, but I have seen no compelling reason to do so. Theoretical, yes, but theoretically the HW system shouldn't work, but it does, and very well.

There was no mention of automatic by pass...

Indeed. But since the boiler is 50 years old, it almost certainly has a heavy cast iron heat exchanger and large water capacity, so it doesn't suffer fron overheating when it's turned off with no over-run water flow. There's pretty certainly no pump over-run because the pump appears to be wired in parallel with the boiler.

If the boiler is ever replaced by a modern one with a low volume aluminium heat exchanger, and a proper S-Plan control installed,it will probably then need to have pump over-run and an auto by-pass.

Fair enough, but there are a few things which you perhaps ought to consider.

If the boiler is 50 years old, it's a miracle that it's still working - and it's certainly living on borrowed time! I doubt very much whether you could get spares if you needed then.

It will also be grossly inefficient compared with modern boilers, and probably has an always on pilot light rather than having spark ignition. Since how it is controlled is still not entirely clear, there may well be times when it wastes energy by cycling on its own stat when there is no demand, whereas a proper control system with boiler interlock would prevent that.

So, replacing the gas boiler with a new one (before they are banned!) and upgrading the controls to a system which complies with current building regs may be a good investment with a relatively short pay-back time and may also enhance the value of the flat should you wish to sell it. [If I were buying it, my offer would certainly need to take account of the need for a major refurb on the heating systems.]

However, I accept that the low payback time may not seem quite so attractive if the person making the capital investment is not the same person who pays the gas bill.