How does boiler interlock work?

I understand what boiler interlock is and how it prevents dry cycling but please can sombody explain to me HOW boiler interlock works when used in a system where every radiator has a TRV and one location has a room stat. (I've been told to fit a room stat as well as having a TRV on all rads to comply with regs and provide interlock).
Most sites I have looked at quote the same generic stuff from CHESS that starts with the words 'This is not a control device but a wiring arrangement to prevent the boiler firing....'

I'd really appreciate it if someone could give me an engineering explanation of exactly HOW a room stat + TRVs can prevent provide boiler interlock.

As I see it there are three situations. Asssume the boiler thermostat is set to maximum.

Situation 1
All rads have TRVs. All TRVs are set to say, 18 degrees and the room stat is set to 25 degrees. At some point all TRVs could close at the same time, turning all the radiators off while the room stat is not yet satisfied. Now the rads are off but the room stat has not turned off the boiler & pump.The boiler enters a dry cycle (or pumps through the bypass) and wastes energy. This is not allowed by part L of the current building regulations concerning the conservation of fuel and power.

Situation 2
All rads have TRVs but the settings are reversed, ie the TRVs are set to 25 degrees and the room stat is set to 18 degrees. Now the room stat could be satisfied and turn off the boiler & pump, even though the TRVs are open and calling for heat as they are set to 25 degrees. (Now it's no longer wasting heat through cycling but it's not heating the house either!)

Situation 3
If there is no TRV on the rad nearest the room stat.
If the room stat temperature is set below that of the TRVs, once the room stat is satisfied the boiler is off, regardless of the temperaure in the rest of the house and whether the TRVs are calling for heat or not. If on the other hand the room stat is set above the TRVs settings then, even if all the TRVs are satisfied, the boiler will continue (unnecessily) to heat the rad nearest it until the room stat is satisfied as well.

Essentially the system seems to be driven by the room stat with the heating in the whole house dependent upon whether the room with the room stat is warm enough or not.

Where is the flaw in this argument?

(Please consider the physics involved rather than just quoting the regs or commenting along the lines of 'that's how I've always done it'.)

From my experience, situation 3 is correct. The secret in ensuring efficiency and preventing the boiler from short cycling is in the correct location of the room stat and the balancing of the system. A room stat in a hallway (assumed to be the most central area in the property) would normally be set to achieve around 18 degrees C whilst a living room would need to be 21 degrees C. That would be achieved by proper balancing of the system.

That's my understanding and certainly works well in my house.

kbdiy said:

A room stat in a hallway (assumed to be the most central area in the property) would normally be set to achieve around 18 degrees C

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So once the hall is at 18 degrees everything else shuts down and other rooms don't get heated regardless of TRV setting. I can see that saves energy, but so would just turning off the boiler <grin>. In this situation there seems no point in all the TRVs since the hallway decides if, for example, the bedrooms get heated. The only way the TRVs can work properly is if the hallway room stat is set to max so its the last thing to shut off the boiler - but that then seems to defeat the point of boiler interlock.

No - that's where correct balancing of the system sets the temps for each room and allows the TRV's to shut down when those are satisfied.

Yes you could just turn the boiler off, but the idea is to introduce some measure of automation and eliminate human error (or rather, forgetfulness!).

Works fine for me in a fairly large 3-bed semi and plenty of others that I have seen. Like most things, it tends to be a bit of a compromise but unless you start throwing loads of money at it to fine-tune then compromise it has to be.

Nowadays with TRVs the room stat is usually placed in the living room as that's where people stay and where the temperature is most critical.

The TRVs set the temps for other rooms.

Tony

Thank you very much for replying so fast but I still seem to be missing something.

If a system is correctly balanced it doesn't mean all locations are at the same temperature. It simply means that the lockshield valves on each rad have been adjusted so that the rads further away from the boiler offer less resistance than those closer to the boiler (to account for the added resistance of each bit of extra pipework). That way you get the pressure differential across each rad and the flow rate through it to be the same. The closest rad doesn't steal all the water flow and the temperature drop inlet-outlet over each rad is the same. I checked mine using two clip on thermometers on every rad so that the system is completely balanced.

However I can still set the TRV in the living room to shut down at a higher temperature than say, the kitchen, the rad would just stay on for longer.

But doing that is pointless though if the 18 degree room stat in the hallway shuts down the boiler regardless as soon as the hallway is hot enough.

I still can't see how such an arrangment provides for efficient boiler interlock that simply using the boiler thermostat and a reasonably high heat capacity bypass - such as dumping into the hot water coil - doesn't provide.
Unfortunately most places I ask just quote the regs without thought or explanation. Hence my post here.

You also size the radiators to suit each room.

Anyway if you have the money look at the Honeywell EVO home, a brilliant design whereas each TRV controls the room temperature and the boiler

DIYknot said:

But doing that is pointless though if the 18 degree room stat in the hallway shuts down the boiler regardless as soon as the hallway is hot enough.

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So the hallway rad is fitted with 2 lockshields...one for balancing and the other to throttle it down as necessary to ensure the hallway doesn't heat up (and satisfy the room stat) before the other rooms have reached target temperatures.

to ensure the hallway doesn't heat up (and satisfy the room stat) before the other rooms have reached target temperatures.

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But doesn't that rather defeat the purpose of central heating.

I'd like my hall to heat up just as quickly as other rooms - just not necessarily get to the same final temperature.

As far as boiler interlock is concerned though, surely throttling the rad to reduce the rate of temperature rise and make it take longer to satisfy the room stat has exactly the same effect as increasing the setting on the room stat to make it take longer to satisfy (ok the room gets hotter but that's not the point, a TRV could sort that out ) or, in the extreme case its the same as simply removing the room stat altogether so that it is never satisfied and just let the TRVs control the temperature .

DIYknot said:

in the extreme case its the same as simply removing the room stat altogether so that it is never satisfied and just let the TRVs control the temperature .

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Then you're back to square 1 - if you have bog standard TRVs, removing the room stat removes the control over the boiler and thus it will short cycle as there is no mechanism to tell it when the desired temperature is reached. Ok the TRVs will shut down but the boiler will still try to make up losses in itself. Only sophisticated TRVs will overcome that, but then there's a cost implication in installation.

Like I said earlier, it's all about compromise.

Unfortunately 95% of customers prefer to spend more on their iphone and toys than they do on a decent central heating and hot water system. So until they get their priorities right then we compromise with a room stat and TRVs. Those that want more control over room temperatures etc can spend ridiculous amounts on the overpriced top end control systems out there.

Some of the Baxis did permit TRV's on ALL the rads...the interlock to prevent short cycling was provided by a flow switch within the boiler....the boilers were though a pile of junk

kbdiy said:

Like I said earlier, it's all about compromise.

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I'm really grateful for all these comments, thank you

I thought compromise might have something to do with it.
When a fitter said 'you have to have a room stat to comply with the law regarding interlock regs and energy saving' but can't really explain 'how' the room stat did it, nor why it didn't compromise other aspects of the central heating system (as discussed above) I began to wonder whether there really was a scientific basis for having a room stat as opposed to other means of preventing a dry cycle - like a reasonably high capacity bypass.

So it looks like to comply with the law (if its true) I'm stuck with a choice of either having a room stat that cuts off the heating to all my rooms once the hall gets above 18 degrees, having my hall remain below 18 degrees for a long time just so the other rooms get hot before the room stat cuts off the boiler or setting the room stat at a high temperature and let the boiler dry cycle in the rare event that every TRV and the hot water tank stat are all satisfied at once.

Doesn't seem like any energy savings are being achieved that couldn't be achieved just as well by simply modifying the program times or turning all the TRVs down a notch. Ah well..

No. there's a third choice as well

Spend a grand on Honeywell Evo and you won't have the problem!

In reality, is having your Hall at 17 degrees for half an hour longer really going to impact on your life that much? No, didn't think so . The hall being central will usually mean heat from the other rooms (which are heating faster) will "bleed" through the walls and doors to heat it quite evenly anyway.

Gasguru said:

the interlock to prevent short cycling was provided by a flow switch within the boiler

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Interesting you say that. The boiler I am getting is a Worcester (the only manufacturer recommended by Which? magazine. They say in their small print that you need a flow switch with nearly all of their programmers. That would seem to make sense.