Replacing a 3 wire stat with a 2 wire Sunvic TLX4101


Postby dazbear » Thu Oct 23, 2008 3:02 pm

Now please be gentle with me as a newbie here.

I have replaced an old 3 wire mechanical room stat with a new 2 wire Sunvic 4101. I looked on here and it seemed easy enough - bascially leave the neutral out of the equation. Trouble is - it isnt working!

When you turn the knob you can hear it click and you then hear the pump kick in, but the boiler doesnt fire.

We do have heat in the morning, but I have a feeling this is due to the boiler kicking in to heat the hot water. During the evening (hot water off) no heat at all.

This is how it is currently wired

1 = call for heat (yellow)
2 = com (red)
3 = heat satisfied (empty)

Blue/Neutral has been blocked and left out of the equation.

What am I doing wrong? Freezing here

Thanks
ps - simple answers please
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Postby plumbbob » Thu Oct 23, 2008 9:19 pm

Erm, nothing. Your wiring is correct, and as the pump is starting, this means the stat is working. You need to look at the boiler itself to see what is wrong.
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Postby dazbear » Fri Oct 24, 2008 6:53 pm

ok

I have synced the times of the hot water and the CH on the timer, so we are getting heat morning & night.

But is not controllable - the STAT may start the pump at the "click" but doesnt regulate the temp.

You say it may be the boiler? Any ideas?

Thanks

Darre
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Postby plumbbob » Sat Oct 25, 2008 12:54 pm

It's a bit difficult to be more precise as you don't mention what type of system you have and all are wired differently.

Old gravity systems needed the hot water on before the heating would operate.

As you are probably aware, you only need two wires at the room thermostat to activate the heating. The third neutral was only ever there to make the stat work more accurately by being connected to a ballast resistor. As you have not fundamentally changed anything, either you have weird wiring, or the problem is unconnected to the change. Why did you decide to change the stat?

This has definitely occurred since the change?

[quote]But is not controllable - the STAT may start the pump at the "click" but doesnt regulate the temp.[/quote]

Sorry, don't understand exactly. Can you expand?

Google "sundial plans" . Honeywell have good pdf's. Describe which fits your system the best. "S plan", "Y plan ", "W plan " etc.[/quote]
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Postby dazbear » Sun Oct 26, 2008 9:40 am

Hmm interetsing - it is supposed to act like a Y but is now acting more like a W.

Our system is an old Myson Marathon 400B(room sealed standing boiler). We did look to have it changed, but the engineer who came out said it was in perfect nick and in his opinion not worth changing.

We only changed the stat as we have redecorated and the old stat was very old fashioned.

Basically I was trying to say in my last post was, we now have heat morning and evening only because I have married up the times betweens HW and CH. We are getting heat only when the boiler is on to heat hot water.

The stat kicks in to start the pump but isnt controlling the temperature of the CH - just on or off. As a result is it quite hot (hehe)! Turning the stat down does nothing, until the click, which shuts off the pump and as a result the radiators then go cold.
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Postby plumbbob » Sun Oct 26, 2008 8:41 pm

Your last paragraph; but that is exactly what a room stat does. Set it to a temperature and when reached, it switches the heating off, and as the temperature drops below that setting, it kicks back in again. It's either on or off and does not control the temperature of the central heating just the room the stat is in. Am I missing something?

As for the other problem with needing the hot water on, nothing you have done to the stat if it only had three wires will effect this. Just a long shot, but did you have any "accidents" when wiring in the new stat?
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