[vhfpmr (OP)]

I'm in the process of replacing my boiler, and I note that current building regulations require that a roomstat be fitted even on systems with a TRV on every radiator. WHY?, once you have TRVs throughout the house you have full and individual control over the heat in each room, and putting a roomstat in overall control of the whole house is defeating the point of having them.

For example, with a roomstat in the hallway as recommended, I might set the hall to a cool temperature to save heat whilst wanting a warmer temperature in the lounge. So when the heating comes on, the roomstat in the hall will be the first one to satisfy, because it's set to the coolest temperature, and then at that point the whole system will switch off before the living rooms ever get warm!! When I pointed this out to the plumber quoting me yesterday, he said "well you need to turn the roomstat up to maximum so that it won't cut out". When I pointed out it's a waste of heat turning the hall up higher than I want it just to stop the lounge going cold he said "well put the roomstat in the lounge then". So I then pointed out that the same problem exists wherever you site the roomstat because it's turning off the whole house when just one room is satisfied, and he didn't have an answer to that.

I had this same argument 35 years ago when the system was first put in, and at that time I was told that if I didn't fit a roomstat the pump would fail within a year. Well, there's no roomstat, and the pump has run just fine for 35 years.

It's barmy, it's like fitting a light switch in every room bar the hallway, and then using the consumer unit to turn the hall light on and off. Anyone with a little common sense can see that you wouldn't want the whole house plunged into darkness just in order to turn the hall light off, but they don't seem to apply the same logic to the heating.

[chris]

Couldn't agree with you more. The building regulations are a sledgehammer to crack a pea. I needed a new oil boiler a few years ago. I bought one online and all I wanted was a plumber to fit it for me. They refused and insisted - "for building regs" - that I needed to comply with all these regulations which ultimately involved TRVs on every radiator, a thermostat in the hall etc etc. The cost of re-fitting my system was astronomical.

My oil bill has not gone down since we did the work, so presumably my carbon footprint has not dropped either... The only thing that has been reduced is my bank balance, by a considerable amount.

[vhfpmr (OP)]

I already have all the TRVs, so no expense there, I just don't want the roomstat because it will defeat the point of the TRVs. What I can't fathom is why I appear to be the only one asking questions like this, and why if I'm not the first one, nobody is giving me sensible answers. I asked Worcester Bosch technical dept., but they weren't interested, and told me to go and ask someone else.

[chris]

You aren't the only one; I was the same. The reason for the antipathy to these sorts of questions is because the plumbers want to sell you stuff and charge you to install it; the building regs are basically a charlatan's charter to compel you to have a heap of stuff you don't want. "Complying with building regs" is the standard reason for inflicting this on people.

[vhfpmr (OP)]

The plumber's happy to install it without a roomstat, it's me who doesn't want to get fined for a non-compliant system or give Worcester an excuse not to honour the guarantee. As I said to the plumber, I know I can stamp my foot and demand that I get my own way, I'm just trying to work out whether I'm missing the point somewhere. The problem is that the plumbers don't really seem to understand much beyond just repeating what they always do, parrot-fashion, so they just retreat into customer's-always-right mode and humour you as soon as you challenge what they say.

I've agreed that he will fit both a TRV and a roomstat in the hall, so I can set the temperature using either one or the other, and we'll see which way works best.

[Bored chemist]

The valve on the radiator can't know when the room has warmed up- because it's far away from most of the room, and very near the radiator.

[chris]

The valve on the radiator can't know when the room has warmed up- because it's far away from most of the room, and very near the radiator.

So what's the point of it then?

[vhfpmr (OP)]

The valve on the radiator can't know when the room has warmed up- because it's far away from most of the room, and very near the radiator.

The ratio of the temperature in the middle of the room to the temperature in the vicinity of the TRV is going to remain fairly constant as long as the room arrangement stays the same. TRVs are not calibrated in degrees, and are adjusted until the occupants feel comfortable anyway. Lastly, the TRV in the lounge knows the lounge temperature a lot better than a roomstat in the hall does.

The problem with a roomstat is not the location of the sensing, it's that it controls the whole system rather than independent control of each room.

Worcester Bosch give an extra 2 years guarantee if you fit their own roomstat rather than one from a third party, so I'm thinking that there's a reliability angle to all this that they're coy about discussing.

[Bored chemist]

The valve on the radiator can't know when the room has warmed up- because it's far away from most of the room, and very near the radiator.

So what's the point of it then?

It's cheap + easy.

[chris]

Be interesting to hear what @Tim the Plumber thinks about this

[vhfpmr (OP)]

The valve on the radiator can't know when the room has warmed up- because it's far away from most of the room, and very near the radiator.

So what's the point of it then?

It's cheap + easy.

Not really. Roomstats start at about £16, which is a lot less than 8 TRVs at £7, and a cheap roomstat will require the inconvenience of a wire to it unlike TRVs.

Attempting to control the whole house from a single thermostat which is isolated from all but one room when multiple rooms have radiators is crude and ineffective, but it prevailed in early central heating systems because it is cheap and simple. TRVs emerged as wealth increased because they are far superior, which begs the original question: why bugger up a good system with a roomstat?

[Bored chemist]

You need some sort of valve on the radiator anyway, so you need to subtract that cost. Then there's the cost of fitting a roomstat, then, as you say with a roomstat you need extra hardware to make it do anything.

[Colin2B]

I think it's a catchall. Not all boilers have the ability to sense when all the trvs are shut which can lead to what is known as dry cycling where the boiler just keeeps heating water in its own loop usually around a bathroom radiator with no trv or a bypass loop. However, the room thermostat is rarely in a position to stop this happening.
Problem is, if the regs don't explain the detail of why, and what you need to do in order to prevent problems then they aren't helpful. I suspect someone looked at the problem and decided it is a lot of writing to explain all the options, so they don't change it!

[Tim the Plumber]

Be interesting to hear what @Tim the Plumber thinks about this

Yes. It's a bit mad.

But then you need some sort of circute for the hot water to go around when all the radiators have reached the cut off point so that the boiler can sense that the returning water is still hot and should not be heated again.

This is normally done by having the radiators in the room with the room stat not having TRV's so that the whole system still works and the boiler does not pump against a system with all the valves closed and the boiler does not boil the water in it.

But yes the regs are a bit historic...

[chris]

thanks @Tim the Plumber

But then you need some sort of circute for the hot water to go around when all the radiators have reached the cut off point so that the boiler can sense that the returning water is still hot and should not be heated again.

This is normally done by having the radiators in the room with the room stat not having TRV's so that the whole system still works and the boiler does not pump against a system with all the valves closed and the boiler does not boil the water in it.

On the system I have, which has a Grant oil boiler at the centre and then an obligatory open loop through the water tank and a valve-controlled diversion to radiators, the boiler sees the temperature in the water jacket and turns off when the return is coming back at the same temp as the flow. The thermostat on the wall in the hall, right next to the front door, in a draft, triggers at a much higher temperature for the rest of the house, so you need to set it very carefully to avoid overheating the rooms. And because heat rises, and the TRVs are at floor level, the rooms can get very hot before the TRVs shut off the radiators. I'm not saving any money since the enormous shelling out for an updated "build regs compliant" system, so the paypay period for the "investment" is infinitely long...

I think this all needs a bit more thought...

[vhfpmr (OP)]

Not all boilers have the ability to sense when all the trvs are shut which can lead to what is known as dry cycling where the boiler just keeeps heating water in its own loop usually around a bathroom radiator with no trv or a bypass loop. However, the room thermostat is rarely in a position to stop this happening.

My 35 year old system was installed with no TRV on the bathroom to prevent the pump trying to push water into a dead end, but auto bypass valves were introduced to prevent the need for this. Nowadays boilers like the one I'm getting have them integrated.

I currently have a boiler for space heating and a separate multipoint for hot water, so I'm replacing them with a combi so that I won't need to build an airing cupboard for a hot tank. With having a multipoint, I'm already used to waiting for the tap to run hot when I turn it on, but a lot of people who don't understand how combis work complain about it. In order to address this, the combi I'm having has been deliberately designed to 'dry cycle' so that the water jacket remains hot in order to reduce the time it takes for the tap to run hot when you turn it on.

With heating though, if it takes x watts to warm the house, dependent on the difference between the inside and outside temperatures at any given time, and the boiler power is y, then the duty cycle of the boiler stat will be x/y, and the frequency that it cycles at will be determined by the hysteresis on the stat. If you already have TRVs to set the room temperatures, then there isn't really anything to be gained by fitting a roomstat in between. For example, if a roomstat was operating at a duty cycle of 50%, then the duty cycle of the boiler stat (whilst the roomstat is on) would just rise to 2x/y in order to maintain the same mean power overall, and hence the same space temperature. The end result though would be that the air temperature in the room will cycle up and down by an amount equal to the hysteresis on the roomstat, whereas without it the temperature variation due to the boiler stat hysteresis tends to get smoothed out by the thermal inertia of the water and radiators.