Heating timer

We have a conventional gas boiler (Potterton Netaheat) with a hot water cylinder. The rather aging timer only allows us to switch on the central heating if water heating is enabled.
I'm trying to discover whether this is a limitation of the timer or whether the system itself ties central heating to water heating.
Thanks in advance for any help!
Regards, Chris
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To be able to separate them, the hot water primary must be pumped and be controlled by electrical valves. If it's just thermo circulation, it will be on any time the boiler is. In this type, the pump only works the heating, so if it's not running, no heating.
IMHO, if you do have this elderly system it would be worth converting to fully pumped - not that expensive a DIY task and well worth it - your hot water will heat much more quickly.
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wrote:

I'm not disputing the benefits of a fully pumped system but you have completely overlooked the "C-plan" system which can give independant hot water/heating control with a gravity primary system
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On Thu, 20 May 2004 08:40:15 +0100, in uk.d-i-y "Can2002"

Most probably a system limitation. If you're airing cupboard has no heating related equipment in it other than the cylinder then you're stuck with it the way it is unless you alter the pipework and add a few components. If you have an airing cupboard with at least a pump, a valve and a cylinder stat then it's probably a switch on the back of the clock that needs switching from 10 to 16, or gravity fully pumped.
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This would be the case if the h.w. cylinder coil is heated by gravity circulation. If you can trace the pipe route from the boiler to the cylinder are there any motorised valves (fist sized box attached to pipe with a cable coming out)? If not and if these pipes are 28mm OD (just over an inch) then you have a gravity system and the cylinder will always heat up when the boiler is on. If you have got one or two m.v's then the restriction is due to a link in the programmer which could probably be removed.
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Thanks for the responses guys,
I've had a good look in the airing cupboard and am not sure whether what I have meets your descriptions. I'm going to try to describe the bits in there!!
The cylinder has three entrances at the bottom, 1/3 up and at the top-centre. The pipe (22mm) coming out 1/3 the way up connection goes into a 90 degree connection and then up about 1.5 feet. it then goes into a connector that splits into 15mm (vertical) and 22mm (horizontal). The 15mm side connects into what looks a bit like a pressure release valve (brass barrel with a red plastic thumbscrew on top). The 22mm end of the connector goes into a fist-sized unit into which a three core power cable runs from the timer junction box. There are two 22mm connection leaving this box, one of which is below and feeds into what defintely looks like a pump, which is once again connected into the timer junction box via three core power cable. The other side of this pump feeds a 22mm that disappears into the wall. One other point regarding the pump is it has a switch with three settings (I, II & III - currently set to III).
Apologies if the above is complete gibberish, but does this shed any light?
Regards, Chris
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In an earlier contribution to this discussion,

It sounds as if you have a fully pumped system, using a 3-port valve (either diverter or mid-position). See whether the following description is consistent with what you have just said - albeit in slightly different terms: "Hot water is pumped from the boiler to a 3-port valve located above the pump. This valve has one inlet (connected to the pump) and two outlets. One outlet is connected to the heating coil inside the hot water cylinder. The other outlet directs hot water to the radiators." Does this fit?
If so, is there a thermostat strapped to the hot water cylinder, allowing you to control the temperature of the hot water? If so, you have all the right bits to allow independent control of hot water and central heating - it's probably just not wired up correctly.
Take a look at the different arrangements described in http://content.honeywell.com/uk/homes/systems.htm
Depending on whether your 3-port valve is a simple diverter valve (i.e. either hot water or heating, but not both together) or a mid-position valve (as above but with the added capability of having hot water and central heating on at the same time) you could potentially have either a W-plan or a Y-plan system.
Both of these give you the possibility of having central heating without domestic hot water providing: a) the programmer permits it (may be necessary to remove an internal link in the programmer) b) the cylinder stat is the changeover type - with a "hot water satisfied" contact c) (in the case of Y-plan) the programmer has a "hot water not required" connection which is correctly wired up.
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Set Square wrote:

Following it from the other direction what you suggest does indeed fit! To experiment I switched the CH off (both HW & CH were on at the time) and could hear what sounded like a small motor in the valve mechanism. I'm going to leave it a while and then see if what I now believe is the radiator feed gets colder!

There is indeed a thermostat on the cylinder.

Is there an easy way to tell? I guess one thing I noticed was that with the system running both pipes leaving the valve are very hot, which implies it's the latter of the two you describe above. The one thing I do recall when we moved in was that the thermostat on the cylinder was set very low and we found that the CH did not get very hot. When I upped the thermostat setting on the cylinder things semmed to improve, which makes me wonder!

Thanks for all your help!
Regards, Chris
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In an earlier contribution to this discussion,

If the valve has a model number on it, someone can probably tell which type it is.
If you select CH and DHW at the same time, and turn both stats up, and if both lots of pipework get hot (too hot to touch!) at the same time, it's very likely that you've got a mid-position valve (Y-plan).
If the radiators only get hot when the hot water stat is calling for heat, I'd put my money on a wiring error. There *must* be 3 wires (in addition to any earth wire, if applicable) connected to the cylinder stat. If there are only 2, it is definitely wrong.
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Despite what others have said, it is possible to have a fully pumped system with a motorised valve and yet a controller which only allows HW or HW+CH. I know; I've just ripped one out of mum & dad's house after some 36 years.
It was a mechanical timer (with pegs) and two rocker switches. One set off/on/timed, the other HW only or HW/CH. The primary reason for ripping it out was that the motor had suddenly become very loud and clicky. The secondary reason was that it hadn't actually worked for <n> [30>n>10] years. Instead mum & dad would get up to a cold house, go downstairs to the "Water Heating" switch in the cupboard under the stairs and turn it on. HW had priority, so if the cylinder was cold it took 30 minutes before the radiators started warming up.
I replaced the thing so that HW and CH were separately timed. It involved a complete rewire of the system and (oddly) turning the motorised diverter valve around 180 degrees. I also had to add a relay because the two timers were SPCO only and the two "on" situations are pump only and pump plus valve.
Because it is a diverter and not a 3-position, HW still has priority, but with both systems timed it is possible to stagger the on-times in order to ensure you get heat when needed. I used 1 HW timer, 1 timer/stat, 1 "wiring centre" (bit of chocky block in a box) and 1 240V DP relay in a box. It's not the system I'd install if I were doing this from scratch, but it was a relatively cheap and simple upgrade and has made a *big* difference to the ease of use of the system.
Hwyl!
M.
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On Fri, 21 May 2004 13:53:27 +0100, in uk.d-i-y Martin Angove

I'd just like to make it known that that's what I said, in different words.
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I am planning a similar installation, sort of, with a programmer controlling my immersion, and my electric central heating (bizarre, I know). What relays did you use? i was looking at some contactors, but this would entail adding them to the consumer unit, and I am not that brave.
Ro
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Well, your immersion can be controlled from a immersion heater timeswitch, these are rated at 16A usually. What is the rating of the electric heating? You still need to add the relays to the wiring somewhere, a relay and a contactor are, basically, the same thing. If you are unsure of how to connect up a contactor then a relay isn't going to help you much, if anything they're probably more involved in that you need to make your own enclosures for them.
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What I'm planning to do is use a 3 channel programmer with 3 contactors or relays to control the immersion, the heating unit (rated 20A) and a heated towel rail.
Currectly are all controlled separately with timeswitches, but they don't have advance or boost facilities, and I have had a few big bills attributed to having left either one on for a prolonged time.
I would be happy to use the relay option, but reckon the the contactors wired directly in the comsumer unit, in series with the MCB for each of the 3 circuits would be a more professional, and probably safer option that making ny own enclosure with relays.
I am a bit worried about messing eith the consumer unit. I would need the electricity supply cutoff prior to adding the contactors, wouldn't I?
Ro
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Definitely. I would fit DIN rail mounted contactors and the associated control circuit wiring into a seperate enclosure, like a blank consumer unit.

Er, yes, but not all of it. Are you sure you should be doing this?
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I'm not really. I am pretty proficient at most DIY electrical stuff, and have wired lots of bits like new sockets, lighting etc, but have not ventured to the consumer unit yet. I assumed that any work done on the consumer unit would necessitate the supply being turned off? If I take the cover off, there will be an exposed live feed, I would have thought?
I'm confident I would have no problems with the actual wiring, but I would be worried about working on it while at least some of it was live. Is there a safe way to fiddle with the MCB etc on the same board as the main switch?
I'm probably going to get an electrician to do it for me.
Thanks,
Ro
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Not usually, unless it's quite old. Most modern-ish units have all the permanently live stuff quite well insulated - that is the incoming before the big red switch, and certainly with most MCB units even the live after the switch has a certain amount of protection such that you really have to *want* to touch a live part to be able to.
In other words, switching off the big red switch will usually (but take a look to check first) isolate the supply sufficiently for you to have to try very hard to touch anything live with the lid of the unit off.

As above. Usually simply turning the main switch off does the trick. Just try not to stick a bare metal object into the screw terminals holding the incoming cables eh?

If you're in the Caerphilly area I know a good one :-)
Hwyl!
M.
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Thanks for the offer. I'm actually in Ireland.
I had a look at the consumer unit with the cover off. It looks less scary now. I can see the rather large fat red wire bringing in the feed, and the main switch.
The rest is pretty straightforward. i just need to wire each contactor in series with the appropriate MCB. There's actually an old mechanical timeswitch with pegs, (pegs missing) which I can get rid of also, which will simplify things a lot.
Just ordered the contactors. Will give it a go as soon as they arrive.
Ro
On Mon, 24 May 2004 23:04:18 +0100, Martin Angove

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My parents are all gas, so the relay was only switching a couple of amps for the pump, the valve and the gas valve on the boiler. I bought something suitable with a plug-in base and put it on a bit of DIN rail in a little box, mounted next to the wiring centre.
If I had heavy electrical heating loads I would certainly be looking at contactors as they are almost bullet-proof, though there are undoubtedly relays available which will cope with 16A or so - one per heating device perhaps. It'll all have to be mounted though.
Why do your contactors have to go into the consumer unit? Can you not wire up a separate "contactor box" somewhere nearby? An empty CU with 100A switch, and split the incoming (from the meter) with the appropriate connector box perhaps?
Hwyl!
M.
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