CH Thermostats in hallway - why?

[martinalex]

I think what I don't understand is why there is a need for an external thermostat, if there is already one in the boiler? And that if the thermostat is in the hall/coldest space it will make the boiler run more frequently/for longer and use more gas and so not save me energy.
This probably sounds really dense.

[mk1fan]

There are plenty of courses on which you can learn 'heating engineering' so enrol.

Or just let it go.

[Arthur Jackson]

No. A room stat will maintain the hall at the temperature you want. TRVs turn off the boiler INDIRECTLY. They are so far removed that the boiler needs to heat itself up before it turns off. To be honest, as hinted at earlier, a room stat EVERYWHERE would be best, but too expensive.

[Arthur Jackson]

mk1fan said:

Or just let it go.

To be honest, you could take it as read OR go your own way. I know which one I'd go with 'cos I'm paying the gas bill.
It might help your decision (and not fitting one does make the system non-compliant and un-notifiable, remember) to know that the REASON so many systems were fitted without a roomstat in the '80s and onwards was that plumbers didn't want the ball ache of running a cable to the hallway. They didn't much care how efficient it was so long as the cash was in their pocket.

[martinalex]

I don't want to become a heating engineer but I do want to understand why heating engineers recommend a thermostat in the hallway and none of the ones saying I should have one can say why - other than you have to have one!
I'm trying to get my head around the necessity for the thermostat per se - (not just specifically in the hallway) - given that the boiler has an internal thermostat activated when the water reaches the required temperature. And why this is an 'energy saving' feature?
When I hear the boiler cutting out - as Ricky says - is the pump still running thus incrring an electricity cost and a wear and tear factor?
Is it also that if I get up and switch my boiler off when I'm hot, and then wait until I feel cold before switching it back on, will the water in the system have cooled to an extent that it requires more gas to heat the water back up again than it would if I kept the boiler activated by a thermostat on a consistent temperature.
I'm not trying to catch the plumbers out - but I simply haven't used a thermostat controlled system before and am struggling to grasp the energy saving aspect.

[Arthur Jackson]

martinalex said:

... none of the ones saying I should have one can say why - other than you have to have one!
I'm trying to get my head around the necessity for the thermostat per se - (not just specifically in the hallway) - given that the boiler has an internal thermostat activated when the water reaches the required temperature. And why this is an 'energy saving' feature?

I thought we were explaining it quite well.
The boiler stat controls the boiler flow temperature. A roomstat controls space temperature. There is a big lag between these during which time your boiler will run unnecessarily. I'm not sure how else I can make this point clearer. It's a shame no-one has told you this when they have come to your house, but that's a sign of their competency. It's a simple reason which they should know. I'm no plumber/heating engineer, but I can grasp it.

[martinalex]

If I have a working room thermostat installed in my coldest room - which I don't sit in - (because it's the coldest room) - will I not just be paying to heat unused space, when I could shut off that radiator and just heat the other rooms behind closed doors?
I accept that building regs insist a thermostat is fitted - but I'm questioning if this devise really will save me money.
Would you happily assure anyone buying a new CH system that having a room thermostat will of itself save money? If that's the case, great!

[IainZ]

martinalex said:

I think what I don't understand is why there is a need for an external thermostat, if there is already one in the boiler? And that if the thermostat is in the hall/coldest space it will make the boiler run more frequently/for longer and use more gas and so not save me energy.
This probably sounds really dense.

This is always the debate. yet almost all plumbers seem to swear by having a Stat in the hall as well as TRVs. If you have a well insulated house & you keep doors closed then even the hall can get quite warm (mine does & it has a rad with a TRV). At which point the wall stat would turn the whole system off.

So if it does that & (for example) one of the kids is upstairs in a bedroom doing homework etc & getting cold, you have to override the thing by knocking the wall stat up a few degrees just to turn the system back on. At which point the hall gets too hot.

I've never understood the logic of managing the system based on the temperature in only one part of the house - and the bigger the house the less sense it seems to make.

My system has no hall stat & TRVs on all radiators with the exception of a radiator in one of the bathrooms & a towel rail in the other which are both valved down so that those rooms don't get stupidly hot. I've run it that way for over 10 years with no ill effects. I replaced the boiler last year (cos the old one was crap) & put a new pump in at the same time but the old one was working fine. I've also turned the pump speed down which makes it much quieter.

So yes the boiler cycles based on the temperature in the main circuit & the pump runs continuously when the heating is on, but the notion that this is "wasting heat" also isn't true really. What heat escapes from the main circuit pipes between the pump (upstairs in the airing cupboard) and the boiler downstairs just goes into the fabric of the house & heats the the building anyway as "background heat", it doesn't "evaporate" to nothing/nowhere.

Meanwhile (and especially when the cold weather comes, the TRV's are cycling the radiators as needed & just maintaining the required temperature in each room, and heat is available anytime/anywhere its needed just by cranking the TRV up a notch.

At the speed its running at the pump uses around 45W - i.e. 0.4p per hour roughly; so the idea that turning the pump off saves significant amounts of electricity is just not true either - if you ran it 24x365 it would only consume about 30 quid a year. I don't run it constantly though, its on a timeclock, so it runs for a few hours in the morning & then its on all evening at the moment. When it gets very cold I'll stick it on for an hour or two in the middle of the day as well.

In the summer , when the heating is off, the stat on the HW tank does just turn the whole system off when there is a tankfull of HW & that's the way that needs to work.

Edited by IainZ on Friday 4th November 22:19

[martinalex]

Thanks Arthur - I understand that you're saying the space temperature measured by the room thermostat will cut off my boiler long before the boiler's internal thermostat - that makes sense, that will save me money. Would it not make more sense then to have the room thermostat (in the absence of one in every room) in the room I use most often rather than an empty cold space. Surely, I would then be able to set the thermostat to more accurately match with my body comfort and set the thermostat to cut off even sooner - saving more money.
Do building regs. insist on a particular thermostat location?

[martinalex]

Thanks Ianz - I was beginning to think I was speaking a different language - I understand exactly what you're saying - you've made the point much more succinctly than I.

[-Pete-]

Arthur Jackson said:

-Pete- said:

..as I now have to have the roomstat up around 20 to stop the hall radiator turning the whole thing off too quickly.

System is poorly balanced. That hall radiator is flowing too much of the total load.

Oh how obvious, I wish I'd thought of that - Thanks! Hall rad has lockshield on inlet and manual valve on exit, I've closed that down to a quarter turn and it's slowed things down nicely. I guess it shouldn't have a knob, just a lockshield cover so it can't accidentally be turned up or down?

Also, while you're at it, my gravity fed Vaillant lets me set HW and CH temperatures, I have them on 58 and 68 respectively. Is that about right? Would there be any need to vary the CH temp depending on the season?

[IainZ]

martinalex said:

Thanks Ianz - I was beginning to think I was speaking a different language - I understand exactly what you're saying - you've made the point much more succinctly than I.

I can sort of see the theory behind having a Hall/central stat but I bet in practice the vast majority of them are just cranked up higher to stop the boiler turning itself off. To get it to work right it would have to be set "just so" & balanced against what the TRV's do (which most people don't seem to understand anyway!). In smaller houses I can also see the sense in it - in fact in very small houses its almost not worth having TRV's - just have a central Stat & leave doors open for the air to circulate.

As to balancing the system (or rather the flow to individual radiators), it seems to me that practise pretty much went south when TRV's came in because the resistance in the system is changing and moving about from one part of the system to another as different valves open/close. I re-balanced mine last year based on temperature difference instead. Basically I went round each radiator in turn & cranked the thing fully open at each end & then closed the lockshield down gradually till I got the correct temperature difference between the inlet pipe & the exit pipe (used an IR thermometer I got off e-bay) I.e. you slow the flow of the water down such that you are taking a specific amount of heat out of it before it leaves the rad to return to the boiler. I can't remember what I used but I think it might have been a drop of about 15 deg F - I found the number on the web somewhere. The theory, as I understand it, is that that with the right temperature drop the thing runs much more efficiently & gives out maximum heat when asked to do so. With each one I then closed the T'stat down & moved on to the next one before going round & resetting all the TRVs at the end.

It seems to me the one thing you don't want in any house is temperatures that yo-yo between too hot & too cold & that's what you will get when a system is slow to react I think. The way mine runs the temperature is where I want it in each individual room (and the hallway) pretty much all the time when the heating is on because the TRV's are managing it, not some "thing" in another part of the house.

Whether what I'm doing is inefficient or not I'm not sure - but the house is comfortable & even if it is inefficient what kind of percentage are we talking about? Not much I think. I bet in most houses you'd achieve far more in terms of cost savings with draught proofing & insulation and (certainly in this house!) by getting kids to turn the darn lights off when they leave a room!

The only other question mark I have over the way I run it is that the boiler does cycle & often as not is running at a fairly low burn (i.e. not flat out) because its just "topping up" the heat in the main circuit most of the time. Whether, in the long term, its better for the boiler (or more fuel efficient) to run flat out for slightly longer periods is something I don't know, but if its doing that it would seem to be because the temperature in the heating system (and therefore by implication the air temperature in the house) is varying by more than I would want. Constant temperature is good (in my book) & I think actually allows you to keep the place a notch or too cooler overall - which must use less fuel in the long run.

[GMLeo]

Hi martinalex

Couple of things.
1) I think you've getting too caught up in the "coldest place in the house" and taking that as literal. It's not. It's just the hallway ticks a lot of boxes for location in general, but not always the best place.
What Drayton Controls say is "The best location for a room thermostat is in a regularly heated room with a free flow of air around the unit. This could be the hall, lounge or main bedroom. The thermostat must be wired to interlock with the boiler and pump to prevent the boiler from firing when there is no demand for heat."

2) No one has mentioned the main reason for the boiler thermostat, in today's modern boiler it's all about condensing.
Some engineers understand this better than others, the gas guy who installs the boiler isn't necessarily a heating engineer too.
The "dew point" for condensation is around 53 degrees, depends who you talk to, some say 54 degrees.
This is important to know.

What's also important to know is that condensing boiler don't always operate in condense mode.
When not in condense mode, it loses efficiency.
You want the boiler to work in condense mode as much as possible, on the heating system.
Worth noting that most old Combi boilers (older than 2013) don't operate in condense mode on hot water.

Old school thinking took an 11 degree variance between the flow and return at the boiler (ideally balance the rads to the same variance too).
So to get a return of below 53/54 degree (for condensing) you'd want to add 11 degrees to that figure for the flow temp.
Some say 65 degree flow temp was good, to get the 54 degree return temp.

The bigger the temp' variance the better for the boiler to condense. Imagine a cold system, the flow leaves the boiler at 65 degrees and returns at 40, that's better for the boiler, more latent heat to recover and it's operating in condense mode.
The more the return temp catches up to the flow temp, the closer it gets to that dew point of 53/54.

If your boiler is cranked up to 66 degrees, and your return is coming back in a 57, you're not in condense mode, you're spending money throwing heat out the flue and in turn, throwing out higher emissions.

The boiler stat, the lagging on the pipes, the TRV's, the room stat, the programmable timer, they all work together on a properly balanced system to deliver the most efficient heating system available today.

It's not perfect no, but it's better than no room stat at all, which seems to be your solution to a problem that you don't fully understand.

Don't forget, for most installers it's a conveyor belt of jobs, the more thrifty the client, the less likely they are to get good service.
If money is no object, most engineers will take the time to design a bespoke system to every property, because every property is different.

[Blue Oval84]

I've just had a new boiler and roomstat fitted to my flat.

Before this I had TRVs only, and no roomstat. It was a fking nightmare, constantly having to turn the boiler on and off when I got too hot, or too cold. TRVs weren't all that helpful as I found getting a decent temperature quite difficult. Coming from a house with a roomstat the was centrally positioned and effectively set the temperature for the whole house I found it most unacceptable.

Anyhow, the new system is so much more efficient, the boiler is running a lot less, and the roomstat in the hallway (which actually has no radiator, I rely entirely on heat circulating around the flat) is keeping things in check very nicely.

The thing I don't understand about the "no stat and lots of TRVs" approach is that it seemed to assume I wanted all the rooms in my flat at different temperatures? In reality I just want the whole place at 20-21 degrees, all of the time.

I'm a happy boy now

[dave_s13]

I've guy that stat in the hall with the radiator directly under. That rad does have a TRV but it's set at the max setting.... Does that matter or should it not have a TRV at all?

[Blue Oval84]

dave_s13 said:

I've guy that stat in the hall with the radiator directly under. That rad does have a TRV but it's set at the max setting.... Does that matter or should it not have a TRV at all?

That will never work, as soon as the heating comes on the radiator throws heat straight onto the thermostat, which sees it is hot so switches the heating off straight away.

If you really have a stat above a radiator, you either need to move the stat, or the radiator, or switch the radiator off permanently.

[Paul Drawmer]

GMLeo said:

..just thought I'd resurrect a really old thread.

[Spare tyre]

I have a wireless one

In the lounge in the evening, in my office during the day

My home office is small so heats up quickly, so only heating what I need

[dave_s13]

Blue Oval84 said:

dave_s13 said:

I've guy that stat in the hall with the radiator directly under. That rad does have a TRV but it's set at the max setting.... Does that matter or should it not have a TRV at all?

That will never work, as soon as the heating comes on the radiator throws heat straight onto the thermostat, which sees it is hot so switches the heating off straight away.

If you really have a stat above a radiator, you either need to move the stat, or the radiator, or switch the radiator off permanently.

Oops my mistake, it doesn't have a TRV on it.

Thing is, if I turn it off then surely the boiler will run more than it does presently. As it is at the moment I've got the stat set to 18.5 and the house is comfortable at that. Maybe it could be set up more efficiently though?

One thing, the rad is quite small for the size of the hall/stairs landing.

[Blue Oval84]

Do you by any chance find that your heating is going on and off in a fairly quick cycle? As soon as that radiator comes on it's going to throw warm air all across the thermostat.