Worcester Bosh Heatslave 20/25 Room Thermostat

[Incrisis]

Stumbled across this forum while searching for a query I have around my boiler.
(Sorry for the long post)

I have a Worcester Bosch Heatslave 20/25, when it was installed (about 15 years ago), the plumber didn't fit a room stat. The rads have thermostat valves fitted, apart from the bathroom one, I was told there had to be one rad without, and was also told the stats would control the boiler.
I will be honest and never really thought about it before. Over the years I have been questioning myself over this, do the radiator stats actually control when the boiler kicks in?
I am thinking this because the boiler kicks in even when the radiators are still pretty hot.
I am not sure if my logic is correct, but here is my thoughts, please correct me if I am wrong..

The hot water tank in the boiler is controlled by a thermostat?
When that water drops below a certain temperature, the boiler kicks in to heat it back up?
If there is demand for heating, the water is pumped around the system until the hot water returns to tank at the required temperature?
The stats on radiators open/close when the water in the radiators reach the required temperature?
If all radiators are closed, the hot water return is quicker, turning the boiler off quicker?
Therefore, the boiler is controlled by the temperature in the internal water tank?

If all of that is more or less how it works, would the following be correct?

My boiler is in the garage, no insulation, so is pretty much the same temperature as outside. If the boiler on/off is controlled by the temperature of the water in the internal tank, with the boiler being where it is the water will cool down quicker at this time of year, meaning the demand for heating will be more frequent, resulting in the boiler sending hot water around the system when it isn't needed.

Again, sorry for the long post, it is just that I am one of those people to like to understand how things work .... haha

So, after all that, the actual question I am wondering.

Can I get a room stat fitted?
If I can fit a room stat, will that stop the boiler sending hot water around the system when it isn't required?
Also, how would I go about fitting a room stat?

 

[stem]

What other components does your system have? For example: A programmer that has on/off time control? I believe your's is a combi boiler, so probably won't have a hot water cylinder connected to it, but some do.

Many boilers are located inside a garage, but they are well insulated, so won't be loosing that much heat to their surroundings. The boiler's internal thermostat is there to control the temperature of the water leaving the boiler and despite its name prevent the water actually boiling which would be dangerous.

Some modern smart TRV's (Thermostatic Radiator Valves) can control the boiler and they have an app that allows you to control them, but I image yours will just be valves that open and close according to the room temperature and control the flow of water to each radiator.

 

[Incrisis]

Thanks for the reply;

The boiler itself has a dual controller for water and heating, both can be programmed independently, daily, weekly, you can also select off/once/twice/permanently on.

The flow of water is controlled by a diverter valve in the radiator, basically a spring loaded valve controlled by an electronic unit on top of it. I have had to replace the spring loaded valve a few times over the years because it does have a tendency to get stuck in the open position, which usually results in hot water being sent around the heating system when running a hot water tap, even when the heating demand is set to off.

The rads have manual TRV's, setting 0 to 5, and I am assuming they are controlled to open/close (if they are all still working properly) by the water temperature in the radiator.

No hot water tank, that was removed when the heating system was fitted, hot water is by demand from the boiler.

I have no actual room thermostat fitted anywhere, which was leading me to think that the boiler was controlling when to heat the system by the temperature of the water in the boiler.

 

[stem]

You've got it. The boiler thermostat will control the temperature of the water is that is leaving the boiler and being circulated around the system, and the TRV's will slowly shut down the flow of water to each radiator as the room temperature in which it is located warms up.

I haven't had any personal experience of your boiler, or its controller, but a quick look in the manual, (If you don't have it, you can download a copy here) shows how a room thermostat can be connected to it in the top diagram of fig 20. Terminal 2 is the 'live', terminal 3 the 'switched live', and terminal 6 the 'neutral' (if neutral is required, not all thermostats need it. Some are mechanical in operation and others are battery powered)

 

[Incrisis]

Thank you.
I will have a look into room thermostats and also the manual.

 

[Incrisis]

Just so that it is clear in my head...
As there is nothing apart from the boiler to know when hot water is needed for the central heating, in the current set up the boilers internal water tank is only kept heated when the demand for central heating is switched on?
When demand for central heating is switched off, it only heats this tank when a hot water tap is opened and the flow switch activates the boiler?
By fitting a room stat, and removing the link in the diagram, this will stop the boiler keeping the internal tank heated when central demand is switched on until the room stat calls for it, and therefore only heating the water when it is needed?

I am only asking to make sure by fitting a room stat, I am not running the risk of the internal water tank being over heated.

 

[ericmark]

With mine I was told because it does not have any cool down sequence there needed to be some where for the heat to go when it switches off, so domestic hot water is thermo-syphon so there is some where for heat to go when switched off. Not sure if right or wrong, but as a result domestic hot water is thermo-syphon with no valve or thermostat for domestic hot water.

However all radiators supplied with pumped water have TRV fitted, however the pump is on the return, so no by-pass valve.

The boiler is switched on/off by the Nest heat link, but since the boiler does not modulate, it will switch on/off all the time as it produces more hot water than the radiators can dissipate. The hall where the thermostat is does not heat up fast, and then it tends to over shoot, so in the morning I increase wall thermostat temperature by 0.5°C an hour, over three hours to give chance for the rooms to catch up.

Thermostat was wired to hall when we bought house, but not connected, may be it would be better in the living room? The problem is the boiler does not modulate, it is designed to switch on/off, it works, but not perfect, and that is with a wall thermometer and 9 programmable TRV heads.

 

[Incrisis]

I am guessing if the house/room is cold, the boilers internal tank will continue to be controlled by it's thermostat, to prevent over heating, so switch on/off dependent on the water temperature returning to the tank before the room stat tells it to switch off, the radiators will get hot and the boiler will switch off but the room stat will still be trying to call for heating.
I guess it would be best to put the stat in the room that gets the warmest first, that way it will be controlling the heating, rather than the heating still being controlled by the boiler.

 

[Terrywookfit]

A room stat prevents the boiler cycling unless there is a demand for heat.

 

[oilhead]

A room stat prevents the boiler cycling unless there is a demand for heat.

This applies only to the central heating, when the space set temperature is achieved.
The boiler will still maintain the heatstore temperature to the level set by the hot water stat as long as hot water is selected on the programmer, so it will effectively cycle in the DHW mode

 

[stem]

I guess it would be best to put the stat in the room that gets the warmest first.

No! just the opposite, the thermostat should keep the whole heating system running until everywhere has warmed up. Whilst it's doing so, the boiler thermostat will look after the temperature of the water leaving the boiler and heading out to the radiators. Then, when the whole house is warm enough, and finally the room in which the thermostat is located reaches the temperature you have selected on it; it will switch the heating off completely (in the same way that the time switch or programmer does)....until the temperature in the area where the room thermostat is located drops and then it will start it up again.

Be very careful when selecting where to site the room thermostat. Get it wrong and you will regret installing it. Fitting a room thermostat when installing a new boiler is really simple. So I have to wonder why it wasn't done. Maybe there isn't a really ideal location. [Building regulations now dictate that all new boiler installations must be fitted with a boiler interlock (ie a room thermostat) that switches the boiler off completely]

So here are some tips as to where you should locate a room thermostat:

The room thermostat should be in an area that is not effected by other sources of heat such as an open fire, cooking equipment, or direct sunlight etc. It should not be somewhere draughty or too close to a window, especially one that is opened in the winter.

It should be in a cool room, ideally the last room to warm up. That way the other rooms will be warm enough, (but not overheated if fitted with TRV’s) before the room thermostat switches the entire heating system off.

At least one radiator in the room with the thermostat installed should not have a TRV fitted. Otherwise the TRV could prevent the room thermostat from working properly.

The thermostat should be mounted about 1.5 metres from the floor, and not close to, or above a radiator.

Preferably it should not be mounted on an outside wall, (in newer properties with well insulated walls, this is not quite so important.)

It should be in an area where air can circulate around it freely, not in a corner, or hidden behind curtains, or furniture.

As there is nothing apart from the boiler to know when hot water is needed for the central heating, in the current set up the boilers internal water tank is only kept heated when the demand for central heating is switched on?

The boiler's internal controller has two channels. With one you set the time for the heating and the other the hot water. The Hot water store inside the boiler has a thermostat fitted which will switch the boiler off when the tank is hot. Unless of course the central heating still requires the boiler to be on.

 

[Incrisis]

I am not sure if 15 years ago there was the requirement to fit a room stat, I guess because my system was fitted by a friend doing it as a favour over the weekend, instead of going through the books, maybe it was something he over looked.

 

[stem]

I seem to think it could be longer than that, but it's irrelevant to this thread really you haven't got one fitted and that's that

 

[Terrywookfit]

I am not sure if 15 years ago there was the requirement to fit a room stat, I guess because my system was fitted by a friend doing it as a favour over the weekend, instead of going through the books, maybe it was something he over looked.

2006 Part L building regs

 

[stem]

There you go then 14 years