CH pump runs 24/7

[Mike]

Actually I'm fast coming to the conclusion that a pump over-run is what I want and my boiler doesn't provide it. Is there a recommended timer module for doinf this ?

[Dave Plowman (News)]

As I said in an earlier post, I made one when I realised it would make an improvement.

However, others have suggested using a thermostat on the output pipe on the boiler - you can adapt a tank one to fit. It will need change over contacts as you need it to switch off at the set temperature and on when above this - ie the opposite of the normal application. But most I've seen have this.

I've not actually used this solution, and you'd have to check the circuit diagram of your installation to make sure doing this wouldn't interfere with normal operation. Which might involve adding a relay to make sure the overrun circuit is only active when required - but that's only a surmise.

[BigWallop]

Using a thermostat on the output pipe isn't all that simple unless the switching has double throw on it with the pump connected to the common terminal. One way it is activated by the boiler until it reaches the desired temperature. The other way, when hot, it is running under it's own separate power supply.

It isn't a simple case of inserting a thermostat in between the boiler and pump power supply, because when the stat' is cold its switching is open and the pump won't run.

[Dave Plowman (News)]

Yes - I'd guessed it would involve perhaps up to a couple of relays too. My home built electronic timer does, but then only ever comes into operation when both the room and water stat are satisfied. There might well be a more elegant way of doing it - I have a BE boiler.

However, it's worked well for so many years I can't even remember the exact circuit. I used a Temic U6046B which is a very useful little timer IC which will drive a relay direct and allows all sorts of options for triggering, and a wide timing range - ie a few seconds to several hours - by selection of just one R & C. Don't think it's available anymore, though. Its original application was car heated screen timing. So has a useful toggle input - ie a pulse from a simple push switch will alter its state, or it can be made to start its timing cycle from power up.

[Set Square]

It's not that complicated! All you need is a thermostat with single-pole change-over contacts. Connect the pump live feed to COM, the boiler switched live to NC and a permanent live to NO. The pump will then *always* run when there is a boiler demand, and it will also run when the boiler demand is removed but the outlet temperature is above the stat set point. What more could a maiden ask for?!

[Mike]

Just what I'm after - many thanks.

A suitable part number and supplier ? :-)

Don't worry, I'll find one later in BES or Screwfix.

[savedelhi]

Hi Set Square,

I've checked and there is no bypass valve.

The system has two motorised valves,one labeled "HW" and other "CH". I checked the Vaillant ( Ecomax Pro 618E) installation guide and it says that a bypass valve is not necessary QUOTE " unless ALL the radiators are fitted with thermostatic valves OR the system controls could allow the boiler and pump to operate when there is no flow"" UNQUOTE Any views on this ??

Only half the rads have TRV's. So I guess there is a chance that the system controls (could be stuffed. Maybe it got stuffed when the boiler was installed as it had been working perfectly when it was doing the timing for the original Potterton Netaheat boiler. OR, the possibility still exists whreby the wiring has been done incorrectly. On this front,I'm still trying to locate a competent installer.

I pushed the levers on the motorised valves and both moved against the springs and returned OK.

[Set Square]

On an S-Plan system with two motorised valves, the controls MAY WELL allow the pump (but hopefully not the boiler) to operate when there is no flow - other than through a by-pass valve. This happens when the boiler controls the pump - via a pump over-run stat - and requires water to keep circulating for a while after the boiler has stopped firing, in order to prevent the residual heat from causing the boiler to overheat. My view is that you NEED a by-pass.

As stated earlier, the boiler should only fire when either or both zone valves have been driven to the open position by the programmer and room/cylinder stats. If it fires at any other time, you have a wiring fault.

[John]

OR a short-circuited limit switch in one or both of the drive heads OR a short circuit overrun thermostat

[IMM]

When are you due back the clinic.

[IMM]

< snip inane babble >

[IMM]

You haven't actually done anything have you! The inane babble you read on here...........

[Dave Plowman (News)]

You're back are you? And still not offerening any useful advice - as if you could on matters electric.

Stick to quoting websites. That's about the only thing you manage half decently.

Oh - merry Xmas. Although it sounds like you've made an early start.