My friends have just bought a house with a very old central heating system.
It consists of a conventional boiler, hot water cylinder, radiators, randall
103 timer and a room thermostat. The hot water tank has flow & return plumbed into where the immersion heater usually is.
They were unable to get any hot water from the tank, so the guy who serviced the boiler explained that you have to turn the room thermostat right down and the tank will heat up. This worked. Problem is, this really is an unusable system, they wouldn't be able to have the radiators warm in the morning because the tank would be warming up.
Can anybody shed any light on what the room thermostat is wired up to? and how I can improve the system to enable hot water and central heating at the same time?
It sounds as if a direct cylinder has been converted to an indirect cylinder by fitting the type of heating coil which screws into the immersion heater boss. This is likely to take a long time to heat the water, at best.
Is there a 3-port valve - with its inlet connected to the pump and with 2 outlets, one going to the coil in the hot cylinder, and one going to the radiators? If so, it sounds as if they've got an arse about face W-Plan system. [See W-Plan in
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This uses a diverter valve to direct water either to the HW circuit or to the CH circuit, but not both at the same time. The usual arrangement is to have HW priority whereby the radiators only get hot once the hot water is satisfied. This works ok if you have a fast recovery hot cylinder, where the water heats up very quickly. Your friends seem to have it set up backwards - with CH priority. That means that the HW only gets heated when CH is satisfied - so you have to pretend that it's satisfied by turning the room stat down, to get hot water!
At very least, the diverter valve needs to be turned round so that it works like a 'proper' W-Plan system. But that may not be very satisfactory because, if the water takes a long time to heat up, the CH will off for a long time after someone has a bath.
A better plan would be to convert it to a Y-Plan system, so that they can have HW and CH at the same time. This would mean replacing the diverter valve with a mid-position valve, and changing the wiring to bring it in line with the Y-Plan diagram.
[I'm not sure whether the 'wet' parts of diverter valves and mid-position valves are common. Hopefully someone here will know? If they are, you may be able to get away with just changing the actuator on the valve. Either way, you *must* have a HW-off connection from the programmer, and the cylinder stat *must* have change-over contacts, correctly connected as per the Y-Plan diagram].
I'm guessing the stat is used to give the hw/ch changeover signal, so when room heat is needed ch runs, and when the room stat's satisfied the hw runs. The resulting CH priority means no hw until the ch is upto temp, plus you've then got to wait for hw to reach temp as well. Bit crude.
If your divertor is set to midway position, hw and ch will run together. This is something you could do as a temporary measure while you sort out wiring it up properly.
A slow recovery hw tank isnt a big problem, in that one simply sets hw to come on earlier. Things only go pear shaped if repeat showering leaves you run out of hw, but you may find its not a prob in practice, so I wouldnt simply assume the HW cyl would have to be replaced, it may perform ok as is. And if your cylinder is uninsulated, I'd do some serious insulating.
That sounds like a conversion of a direct cylinder, and is frankly a bodge. The heat transfer will be poor - and even worse if old and in a hard water area so coated in lime scale.
It's normal on any heating system to have it wired to give hot water priority. Yours being old and basic suggests that when the pump is running (for the heating) the 'gravity' circulation to the hot water cylinder stops. The one time I came across one of these conversion coils for the cylinder the pipes were 15mm, so very restrictive for a non pumped system.
It probably just switches the pump on and off.
Change the cylinder to a fast recovery model and go to a fully pumped system with a 3 port valve. This will allow fully independent operation of both heating and hot water - and the control of the temperature of both.
It won't be money wasted as an interim solution, as the addition at a later date of a new boiler (and perhaps TRVs) will bring the system up to modern standards.
Not too sure about that! The motor will only work in one direction, with a spring return when power is removed. It needs to be motored to the CH position, with its home position being HW. Chances are that its home position is currently CH, and that it's motored to HW. I suspect that you'd have to physically turn the valve round to correct it.
Looking at the system again tonight, I realise it is a total bodge.
The room thermostat stops the CH pump. I guess then the hot water is heated by a gravity system, but the pipes into the coil in the hot tank are 15mm, so it takes ages to heat up.
I've recommended we rip the lot out and put a combi in!
I just assumed that was already taken care of. Thinking about it, I think it must be.
It always puzzles me when one trivial to fix issue leads people to deice to rip out everything, and pay for it all again. God knows why, I think folk are trained to be daft these days.
It isn't *automatically* taken care of in a two-pump system unless you either include a relay or design the wiring very carefully. [I think I once posted a description of how you could do it without relays, by wiring two change-over stats in a particular way - but I'm too idle to re-invent it!]
I dont see how this can be, maybe we're all cross purposes here. The present system must have this issue taken care of already, else the boiler would have been trashed years ago. What I propose is adding a pump, with its own controls, not modifying the existing control system in any way.
I'm assuming that we're going from a gravity HW and pumped CH system to a fully pumped system which has separate circuits with a pump on each one rather than a single pump plus motorised valve(s) (as in Y-Plan or S-Plan). Are we agreed thus far?
The problem with gravity HW systems (in the absence of a zone valve to convert it into a C-Plan system) is that there is no boiler interlock. In other words, whenever either or both CH and HW are on, the boiler will cycle on its own thermostat and waste energy even when the HW is hot enough and the room stat has turned the pump off.
Fully pumped systems invariably *do* have a boiler interlock - and only run the boiler when either or both HW and CH are actually calling for heat. I am assuming that when the second pump is added, it will be done in a way which provides a boiler interlock - in the interests both of energy efficiency and of complying with current building regs.
You would thus need a system where the room stat controls the CH pump and the cylinder stat controls the HW pump. When either or both pumps are running - but not unless at least one pump is running - the boiler needs to fire. It is the logic to achieve this to which I was referring.
When I last had gravity hot water with wet CH (nearly 30 years ago) I had a tank thermostat to provide a boiler interlock. If not, every time the room thermostat cut out, a slug of hot water would hit the cylinder, and end up with the cylinder overheating.
Unless you also have a motorised valve, having a cylinder stat on its own only provides a *partial* boiler interlock. It works ok in HW-only mode, and shuts down the boiler when the HW is hot enough. But in CH mode, it will continue to heat the hot water to a temperature way above the cyl stat setting (unless the boiler stat is set abnormally low).
OK, this makes more sense, we've been talking about 2 different arrangements. Youre suggesting rewiring the existing controls to a new plan, I was suggesting not touching them at all, but merely adding a _separately_ controlled pump in the hw heating line, no other change. Thus the system control stays just the same, and the boiler still thinks its operating a gravity HW system, just with a whole lot more water flow on the HW side.
In fact any time this extra pump is off, if the boiler tries to heat the DHW, it would do it exactly as it does today, by slow gravity circulation.
I'm not sure if I've been clear, but it explains the confusion.
I suggested this because the OP could do it easily, quickly and cheaply, and it would make it work properly. Although again it would be an odd plan, it would work fine.
I can see what you're suggesting - but it would be more expensive than my solution because you would need an additional programmer - and it *still* wouldn't provide a boiler interlock. My solution *would* involve a bit of re-wiring, but it would be worth it! You're going to need new wires for the additional components in your solution, anyway.
It allowed the pump /and/ /boiler/ to shut down when the hot water and CH were satisfied, thus preventing the boiler from continuing to fire until its thermostat cut out. Thus it prevented the slug of very hot water being produced, which led to the cylinder eventually boiling. It didn't prevent the HW from getting up to the normal CH water temperature though. It was effective in doing what it was installed for, preventing the tank from boiling.
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