Boiler Thermostats

[beamer]

Hi All,

An easy question for you knowledgeable people!!

We have a elderly but functional combi boiler (in the kitchen) with a mechanical thermostat in the hallway.

The problem I have is that the range from where the thermostat switches on the heating to where it switches off the boiler is enormous. For example, we either freeze or we get roasted! Keeping a steady temperature in the house is near impossible.

I appreciate that the range of the thermostat shouldn=92t be too narrow.

At the moment the thermostat is used as more of an on off switch.

First Question: Is it possible that the mechanical thermostat is faulty?

Second question: Would replacing this thermostat with a wireless digital thermostat improve the situation? There is of course the added advantage of being able to move the thermostat around the house at our convenience.

Third question: Can it be done? Is it easily DIY able. I=92m fairly adept at these sorts of things but this is not an area I=92ve looked at before.

Boiler: Ferroli Modena 102 Thermostat: SunVic TLX

Thanks.

[Harry Bloomfield]

I don't know about the specifics, so generalisations...

There will not be much difference between the mechanical wired stat and a wireless one, unless the wired one is faulty or incorrectly wired. One mistake often made with the wired ones, which might be the cause of your problem was to omit the neutral connection. Basically when the stat turned on, it also powered a small heater within the stat, which helped to prevent overshoot - it warmed up the by-metal strip causing it to click open a little earlier than it would otherwise do, thus reducing the hysteresis. This being the difference in two temperatures, between it switching on and switching back off.

The modern wireless types respond faster and the essential hysteresis is programmed into them.

[Andy Cap]

Either change it or get it changed for a modern programmable equivalent. They are available for less than £30 on Ebay and will give you far greater control of temperatures throughout the day. Some can be set to either 0.5 or 1 degree switching range. Get one with five or six changes for each day of the week, especially if you want a different pattern at the weekend. The Danfoss TP4000 for example doesn't differentiate between the days of the week.

Most simply use the live and switched live and the neutral and earth can be left insulated and isolated i.e. in a small terminal block, tucked away at the back. Obviously make sure the power is removed from the heating system before messing about with the wiring.

Andy

[Roger Mills]

Yes, it sounds as if your stat has excessive hysteresis - assuming that it's working at all. Has it always been like this? It may not even be a suitable stat for your particular boiler. Who supplied/installed it? Mechanical stats usually have a 3-wire connection - live, switched live and neutral. The neutral connection provides a return for the accelerator heater (a small resistor which gets warm when the stat is supplying power to the boiler, and which reduces the hysteresis). But that pre-supposes that the stat is switching mains - but some combis have only a low voltage signal going to the stat, so the accelerator resistor wouldn't work even if connected.

Yes. It doesn't necessarily have to be wireless if it's a direct replacement for the existing stat - although you couldn't move it around unless it's wireless. A digital stat is much cleverer than a mechanical stat, and will adjust the ratio of on-off times to keep the temperature very close to the target. Digital stats also invariably have 'voltage-free' contacts, and don't care whether they're switching mains or low voltage. N.B they *don't* have accelerator resistors so, if there is a neutral wire on the old stat, it must be taped off safely and *not* connected to anything unless you want an expensive bang when the stat switches.

Yes, it's pretty easy - as long as you heed the warning about the neutral wire! Non-wireless ones are *very* easy - you identify the live and switched live (or low voltage equivalent) on the existing stat and wire them to Common and Demand (or however they're labelled) on the new stat. With a wireless stat, the base unit replaces the existing stat. Wiring the switching contacts is much the same, but you also need a mains feed to power the wireless receiver part.

[Ed Sirett]

The receiver unit for a wireless unit which requires line, neutral and switched is a direct replacement for a standard non-digital thermostat.

[GMM]

I too have a pretty old and inefficient boiler and had a mechanical thermostat which I swapped for a pretty simple digital (not wireless) job about 6 months ago. This has made a huge difference to the stability of the temperature around the house, so if yours is a similar situation, I'd definitely recommend it. Clearly, the wireless one would let you experiment and find the perfect location for your house. Now, if I can just find a way to stop the mother in law tinkering with it when she comes to visit....

[Mike Clarke]

Andy Cap wrote re. wireless thermostats:

This isn't the case with all of them. Some, e.g. Honeywell HC60NG (used with the CM927 wireless programmer) need a permanent line and neutral supply for the electronics in addition to the thermostat switch circuit. Depending on the design of your boiler you may or may not be able to use the live side of the existing thermostat connection for this..

A digital programmer will definitely give you much closer control of the temperature, our CM927 normally keeps the temperature within +/- 0.5C of the set point.

Of course the wired base unit doesn't need to be located where the original thermostat is. If you need extra wiring then it's better sited near the boiler from which it should be fairly easy to run the requisite cables. But not too near or you might have problems with the radio signal, ours advised at least 30cm away from metal objects like the boiler housing - and don't use a metal back box to mount it on.

[John]

I too have a pretty old and inefficient boiler and had a mechanical thermostat which I swapped for a pretty simple digital (not wireless) job about 6 months ago. This has made a huge difference to the stability of the temperature around the house, so if yours is a similar situation, I'd definitely recommend it. Clearly, the wireless one would let you experiment and find the perfect location for your house. Now, if I can just find a way to stop the mother in law tinkering with it when she comes to visit....

I absolutely agree - a modern digital one has a closer control and having a programmable one gives a smooth transition between time periods. Best £27 I ever spent.

[Dave Plowman (News)]

Well either the thermostat is faulty with too large hysteresis - the difference between switch on and switch off temp - or it's simply in the wrong position. Personally I think it's best located in the living area - although of course this can give other problems if you have a lot of people in there etc.

[YAPH]

Yes but why would you want to put the receiver/relay unit of a wireless stat where the old wired stat was? Either fit a CM907 wired progstat (approx £55) in place of the wired thermostat or fit a wireless CM927 with the HC60 base unit fitted at the boiler, where live, neutral and switched live are available.

[BigWallop]

We have had one other minor irritating little problem with the boiler that my brain has been trying extremely hard to ignore! Would anyone have any suggestions for this one?

Combi boiler (Ferroli Modena 102) - if the heating is firing. Turn on hot tap - you get hot water. Turn off hot tap, heating also turns off

- one flashing light on the console. If you then reset the boiler the heating fires up as per normal.

Any ideas?

Thanks.

It could be caused by a change in water pressure. Try turning the Hot Water Feed to the boiler completely off. Then demand hot water from the boiler (turn a tap on). Slowly turn the water back on until the pressure is just enough to get the boiler firing up to meet the demand. Once you have it balanced on the sweet spot, it shouldn't give any more trouble. You may have to turn the feed on a bit more if you have two taps demanding water at the same time, but do it slowly until the balance is met again.

I have had this happen on a couple of friends boiler systems, and it worked for us. I haven't heard them complain about it any more, anyway. :-) So I take it that it's still working.

[dennis]

You will have to explain that one to me.. how does turning on the HW have an effect on the heating. All the boiler should do is divert the heat to the water and then divert it back when the tap is off. There must be something wrong with the diverting hardware for the problem to occur AFAICS. He really shouldn't need to reset the boiler to get the heating back.

>

[BigWallop]

Central heating is on a closed loop, which means you fill the system with water and then heat and circulate it around the house. Hot water is taken directly from the supply, heated, then sent to the taps.

He is complaining about the boiler having to be reset if the central heating is on and then he demands hot water. There are pressure difference switches in the boiler. These detect if the boiler is giving central heating or hot water. After demanding hot water from the boiler, the pressure difference switch sounds like it is not canceling the request for hot water and not allowing the boiler to change back to giving central heating.

The pressure of the mains water can cause the pressure switch to stay on demand for hot water, even if the signal has been canceled. Simply adjusting the incoming water pressure to allow the cancel signal to be detected by the boiler can cure this situation.

It is a simple test to find out if this is the cause of the problem, instead of pulling the boiler apart first to find out that it was just the mains water pressure to hot water all along.

Clear enough?

[dennis]

Its a good theory. ;-) If it happens he needs to fix the switch.

[BigWallop]

Why?

[dennis]

Because the mains water pressure varies and it should be a flow switch and not depend on the pressure?

[BigWallop]

But the problem sounds like the signal for demand of hot water is not being canceled, usually done by the pressure difference switch. The water pressure holds off the switch when no demand is being made. When the pressure falls when a tap is opened, the boiler detects this signal and begins supplying hot water. When the demand for hot water is canceled, the pressure difference switch tells the boiler to stop supplying hot water.

If the pressure difference switch is not detecting the cancel signal, because the water pressure is just a little to high even when the demand signal has been given, the boiler still thinks it has to supply hot water. The flow switch is telling the boiler that the hot water demand has been canceled, but the pressure difference switch isn't.

And all this because the mains water pressure for the hot water system into the boiler is a little high.

[Mike Clarke]

Does the heating come back on after a few minutes if you just leave it? If so this might be a design "feature". We have a Worcester 350 combi that does something similar and in the manual it states:

"The appliance will supply heat to the central heating system as required. A demand for hot water will override the central heating requirements .... When hot water is no longer required the burner will extinguish and a waiting period of about 3 minutes will prevail before returning to the central heating state and its normal mode of operation"

But I've no idea why they deem this to be necessary.