Room-stat v. TRVs

[bri160356]

http://www.hpmmag.com/asktheexpertitem.asp?articleID=406

Apart from the requirement to meet building regs these days, I have yet to read a convincing argument as to why the mandatory room-stat, that shuts off the heating to the entire house!, is even necessary when all the radiators are fitted with a TRV.

According to the above article the room-stat should NOT be placed in a living-room or kitchen but should be in a hallway or landing.

In the article the paragraph that begins “Research shows that.....” really illustrates the quandary of the ‘room-stat’ in a hallway (with a TRV fitted) and in essence it’s saying that if TRVs are fitted to all radiators then a room-stat is effectively competing with the TRVs to accurately control the temperature of the entire dwelling................which seems a bit bonkers to me.

The ‘room-stat requirement’ is basically an attempt to control the average ambient temperature throughout the entire dwelling and is obviously an excellent concept for homes that have radiators without TRVs.

However, TRVs can (and do!) control the temperature within individual rooms very effectively.

For instance, my home is a well-insulated detached property, with good quality double glazing/doors etc.

Each radiator has a TRV fitted; I know for a fact that the TRVs, during cold days/nights, will quite happily keep any room to within a temperature of +/-1 degree centigrade of any desired setting.

I do not have a room-stat anywhere in my property and I see little value in having one,....., but maybe there is something I’m missing;........ are there energy efficiency benefits that a room-stat/TRV combination will have?

I am quite happy to be re-educated on this (or any) subject.

 

[Dan Robinson]

Read up on interlocks.

 

[bri160356]

Read up on interlocks.

Thanks so much for that tip about interlocks, very enlightening, my friend.

However, my boiler switches off itself and uses no gas when there is no call for heat from any of the radiators i.e. if all the TRVs have closed and the water in the ‘hot water’ tank is also up to temperature.

When the boiler switches itself off does it do it in a different way than the method used when a room-stat shuts the boiler off?

Cheers

 

[Dan Robinson]

It is not switched off though.

It is cycling, and your pump is dead heading.

Unless you have one of those POS Baxi's with an integral flow switch on the primaries.

 

[bri160356]

It is not switched off though.

It is cycling, and your pump is dead heading.

Unless you have one of those POS Baxi's with an integral flow switch on the primaries.

Fascinating Dan, thanks so much. I’m learning lots today!

Why would the pump be dead-heading? ...I assume (??) dead-heading means the pump is unable to move any water through it because all circulation routes are shut-off ;...... but would the by-pass valve prevent that happening and is the circuit to the hot water tank not always open anyway?

 

[Dan Robinson]

If the by-pass is ... by passing ... then the boiler will be firing periodically for as long as the timer is calling, meaning you are needlessly heating a loop of pipe work.


Constantly starting and stopping your boiler is not going to be good for it.

 

[kbdiy]

You say that you think the TRV's can happily control the heat but what happens when the temp drops cold? A standard TRV cannot signal any heat demand to the boiler unless the boiler is set to run 24/7 and is continually pumping. This is very inefficient and a room stat overcomes this by turning on the boiler when heat is demanded and off again when the ambient temp is reached.

There should not be a TRV in the same location as the room stat so there is no competition between them.

 

[bri160356]

You say that you think the TRV's can happily control the heat but what happens when the temp drops cold? A standard TRV cannot signal any heat demand to the boiler unless the boiler is set to run 24/7 and is continually pumping. This is very inefficient and a room stat overcomes this by turning on the boiler when heat is demanded and off again when the ambient temp is reached.

There should not be a TRV in the same location as the room stat so there is no competition between them.

Thanks for that info kb; when I said the TRVs quite happily control the temp in each room they most certainly do........but only when the system is in ON and in full swing so to speak.

I’ve read your comment a few times and I think I’ve understood what you mean ???? (that’s down to me....not your comment, by the way!)

So, (correct me if I’m wrong!) you are saying that if the CH is left in the ON position then the room stat effectively acts as a switch to bring on the heating as the internal temperature starts to drop as the late afternoon approaches, rather than leave the CH off altogether and have it switching on automatically at , say, 5pm.

Have I understood you correctly?.......... that certainly sounds like plus-point!

I totally get the point about the pump running continuously when all the TRVs shut off, as opposed to the room stat switching off the boiler/pump completely. That aspect has to be more efficient.

However, a room-stat just samples the temperature in ONE location (hallway or landing etc) and that is used to control the ‘average’ temperature throughout the entire dwelling.

It surely has to be seen as something of a ‘compromise’ solution even though it certainly has it’s undoubted merits.

For example, what happens when there is a shift in wind direction, and one or two bedrooms suddenly need more heat. The TRVs may open but they are effectively over-ridden by the room-stat that is nice and cosy (and off!) in the well protected hall-way.

A system whereby the temperature in each room is able to be controlled individually , is more conducive to a comfortable lifestyle than a system that attempts to control all the temperatures from a single source. It must always be a balancing act.

The premise seems to be that the other rooms will never get too hot because the TRVs will ensure they don’t, but the other rooms, may, from time to time be too cool, because the room-stat has switched the boiler off for a while.

In a way that’s the ultimate energy saving ploy.............. i.e. switch the heating off completely!

In effect the heating system has become a part-timer!......it only delivers heat when the room-stat gives it permission,...... not necessarily when a bedroom demands it.

In some instances, the room-stat switching off may allow other rooms to become too cold; in which case the room-stat would have to be cranked up a little, but that may in turn mean that the hallway or landing in which the room stat is sited gets too warm and effectively wastes energy.......... is that an acceptable compromise or does that scenario never happen in practice?

I guess the room-stat, sited in an area that has a radiator without a TRV, is obviously the best solution but it’s not without its flaws and is certainly not a Panacea.

It seems to me that a room-stat situated in a hallway is analogous to the story of the ‘aged’ piano player in a brothel.......

....likes the atmosphere downstairs,... but isn’t really sure what’s going on upstairs!

P.S. please feel free to dissect and correct anything I’ve posted, but if you do tell me I’ve posted a load of sh*t (which is quite likely!) please tell me where I’ve got it wrong.......

...I’m keen to learn from you guys. Thanks.

 

[kbdiy]

I understand your comments about a room stat being a compromise but if the entire system is well balanced then it is easy to set, say the living room, at a comfortable temperature whilst the stat controls the overall ambient temp. That has certainly worked for me over the years and I have never been cold indoors!

Clearly you could zone every room independently with individual room stats or more sophisticated TRV's which I'm led to believe can create their own call for heat. However that would involve considerable upheaval and cost, by comparison with a more 'standard' installation. Plus, in my personal view, it creates much more fault potential and requires engineers with a far better understanding of control systems rather than plumbing. That view is somewhat reinforced by many of the comments on here about users with faulty 'standard' systems, let alone anything more complex.

 

[Costeta]

Hi,
I am thinking of installing LightwaveRF TRVs to individually control the temperature and schedule of my radiators. I also plan to leave the boiler permanently on and not rely on the general house thermostat.
How does your suggestion of using the TRVs to control the heat rather than the house stat affect the boiler and pump?
I think this scenario is the same as yours.
Thanks
Regards
Costeta

 

[Dan Robinson]

Oh sweet Jesus. . April is months away.

 

[Costeta]

Can I ask you to also look at this post
//www.diynot.com/diy/threads/lightwaverf-trvs.420531/#3265468

 

[Newboy]

You need to understand the control aspects of your existing heating system before removing the interlocks and controls.

 

[Dan Robinson]

Can I ask you to also look at this post
//www.diynot.com/diy/threads/lightwaverf-trvs.420531/#3265468[/QUOTE]

Already seen it. . Those controls look like shyte.

 

[Costeta]

Hi,
sorry to annoy you. I am a novice.
I don't see the difference with what I want to achieve to that which bri160356 outlined at the beginning of this post in that we are both using TRVs, leaving the boiler on permanently and not using the house thermostat (bri160356 says the house thermostat conflicts with the TRVs).
A similar system is Heat Genius http://www.heatgenius.co.uk/ which uses wireless TRVs that give individual control of temperature and schedule to allow zoning of the radiators. A modern control system.
However I'm not sure of the implications of TRVs being the main control of heat output with the boiler on permanently. I was hoping bri160356 would offer his experience as he advocated the control I am looking to replicate albiet with LighwaveRF TRVs rather than traditional TRVs.
Regards
Costeta