How do room thermostats work

[morriseysquif]

I'm a bit embarrassed to be asking this but here goes.
We have a boiler with a timer and the heating goes on and off with the timer. In each room are thermostats on the radiators with numbers 1-5. I assume these regulate how hit the radiators go, so they are if upstairs most of the winter as we prefer cool bedrooms.

In another thread somebody mentioned having thermostats and the heating comes on depending on the temperature in the house. This blew my mind. 😆

How does this work? Can anybody help ?

A thermostat is an on/off means of temperature control. It goes off when the temperature reaches the value that it's been set to and it's on when it's below this temperature. There's a band of about 1 degree where the thermostat may be on or off, for example, if the temperature is going up in the room, it may switch off at 21C and come back on at 20C. This is called hysteresis.

The thermostats on the radiators are called TRVs and control that radiator only. They close the valve when the temperature is reached. The numbers on the TRV are a relative value so 5 is a higher setting (temperature) than 4. 4 is higher than 3 etc. If it's set to 1 or even * you want a fairly cool room.

There is usually a separate thermostat in a room/hallway that has a radiator without a TRV. This controls the switching on and off of the boiler and has an adjustable temperature setting. When the temperature in this room is reached, the boiler switches off until the temperature drops by enough girls it to come on again.

For, not girls!

Yeah my thermostat is in the hall and is set to around 19⁰C normally. If the heating comes on (controlled by timer) then it will heat until the hall has reached 19⁰C and then switch off again until the temperature drops below that again. If the temperature in the hall is already 19⁰C or higher then the heating will not come on and the heaters stay cold. That is separate to the TRVs on the radiators as that just controls the individual heat setting in the room

Thanks, I'm a bit clearer. All the radiators have those thermostats on them though? and settings of 1-5. Which setting would make the heating come on if we wanted it on around 20 degrees?

Is your boiler controlled by a thermostat?

You cant be that specific with TRVs. You could try each setting on the radiator and see which one gave the required temperature. Is your heating either on or off with no way to control what temperature the house reaches (apart from TRVs)?

the TRVs don't trigger the heating. Your timer triggers the heating and then the radiators heat until you turn the heating off again, unless you have a separate thermostat linked to the boiler.

I'm so pleased someone asked this question. I need more of this!

Look for a room stat somewhere on the wall. The old ones are wired bimetal types commonly about 4" square and an inch deep with a knob on the front. Newer ones can be wireless and tend to use thermistors/thermocouples and often have a digital display. Its easy to add a room thermostat if you really don't have one.

You're talking about two totally separate things. A thermostat controls the temperature of the house, it's a small box attached to the wall usually in the hallway. This will trigger the heating to come on or off depending on the temperature of that room. For example have it set to 20 degrees and it will click the heating off if the room reaches that temperature. That's if the timer is set for the heating to be on. Without a thermostat the boiler would pump out heating continuously while the timer said so, and it would get too hot.
The dials on a radiator are so you can have some rooms cooler than others, so you are using those correctly but they aren't a thermostat.

@morriseysquif

Thanks, I'm a bit clearer. All the radiators have those thermostats on them though? and settings of 1-5. Which setting would make the heating come on if we wanted it on around 20 degrees?

There are some systems that have a TRV on each radiator. If there is demand from a single radiator, that will trigger the boiler to run (if it's within the time schedule). I've not seen a system like this since 1992, so I don't think they're very common. Most systems have a single rad without a TRV in the same area as the control stat.

It’s a different devise

We have a boiler set on a timer abs radiators with 1-5. If it gets too hot we have to turn it off. If it’s too cold outside the timer we switch it on.

The wall thermostat - if you have one - we don’t will come on and off at set temperatures.

So if the house dips below freezing overnight for example the heating comes on.
You can leave it like this for holidays so your pipes don’t freeze.

So, if you want your living room to be 21C, you can't set it to 21, you just have to set the rad valve to give you a comfortable temperature, and adjust all the others to give the temperature that you want in their locations. For instance, you'd want a cooler temperature in bedrooms as you're mostly not in them, and when you are you're under a duvet. It's trial & error.

My trvs in previous house were number 1 =16c, 2 is 18c, 3 20c, 4,22c and 5 24c. The line in between each one was 17c, 19c etc.

The temperature sensor for a TRV is contained within the valve casing (unless it's a remote wifi system) and so is influenced by being close to a heat source (the radiator) and so doesn't give an accurate value of the room temperature. The only way you could set the room temperature to be 21C is by having a sensor by you, and not picking up any heat from you.

We use to have a wall thermostat but when we had the house wired the electrician cut the connection. We didn't realise but called the plumber when the heating didn't work and he said that was what he had done so he put these thermostats on all the radiators.

I like the idea of the heating coming on in the house intuitively when it gets cold but still not sure how to achieve that!

What are your bills like post-valve installation compared with pre-installation & thermostat in place?

We got a Hive thermostat a few years ago and have zero regrets. It's so easy to control on an app on my phone.

@morriseysquif
One of two things have happened. Either the plumber has looped the thermostat connection in the wiring centre (it's just a closing contact usually) and left you with TRVs only, or you have wireless control heads on the TRVs that feed a demand signal back to the boiler. The wireless control heads are quite expensive (or were) so you would know and they typically have a scaled temperature setting on them.
I think it unlikely that you have wireless control heads, your best bet is to buy a cheap wireless thermostat (screwfix have a selection). In the wiring centre try to identify the demand circuit, I'd start at the boiler as it's usually marked in the terminal box. The boiler demand is usually looped through the diverter valve(s) so that the boiler demand only operates when one or other of the CH or HW loops is open. The room thermostat usually drives the CH Diverter valve ( with the timer contacts in series). Once you have tracked it down you'll probably find a loop of wire in the wiring centre that when open shuts off the heating and when closed (with the timer on) switches it on. That's the loop to wire the thermostat closing contacts to. This is all in general terms as there are may different boilers and installation practices.

Hive works in a similar fashion but replaces the room stat and timer allowing control over the web.

Eng123 que?

@Eng123 I'm not too sure that would be understandable to most people, including plumbers! OP, you need a boiler person who's au fait with modern technology.

Its much simpler than it sounds, it's just a couple of closing contacts. If a plumber can't grasp it... they are not a plumber:)

As a minimum I would have a programmable thermostat fitted

This is a thermostat but rather than have one fixed temperature you programme it to have various temps during the day/ night