How TRVs actually work?

[berty3000]

@berty3000

Is your heating s plan, y plan or something else? This will determine if you need a bypass external to the boiler.

No idea! House is a bungalow with combi boiler. There are different pipe branches in the attic going to the different radiators.

In any case, does the towel rail and non TRV radiator act as a bypass?

 

[berty3000]

If you've got the Flow Temp set to 55C then the boiler will burn sufficient gas to heat the returned water (at whatever temp it's at) to 55C, so when the water is only going through 2 rads the boiler will burn less gas as the returning water will be at a, relatively, high temp.

I assume you mean flow temp

Does this then reduce the opportunity for condensing though as higher temperatures are being returned by the two non TRV radiators, or is the boiler able to adjust itself accordingly?

Yes, I meant flow temp is 53C.

 

[berty3000]

It sounds like it might be worth tightening lockshields a bit to reduce flow rate. Presumably this allows more heat to release from radiators and then lower return temperatures?

Will it be necessary to go through the whole balancing process from scratch, bearing in mind that the radiators all heat up fine and fairly consistently as they are or, for the purposes of increasing temperature differences across radiators, would it be fine to just try a quarter turn clockwise on each one and monitor?

 

[Mister Banks]

No idea! House is a bungalow with combi boiler. There are different pipe branches in the attic going to the different radiators.

In any case, does the towel rail and non TRV radiator act as a bypass?

With a Combi you are ok without a bypass as long as you leave a radiator without a TRV and leave it turned on.

 

[Mister Banks]

Does this then reduce the opportunity for condensing though as higher temperatures are being returned by the two non TRV radiators, or is the boiler able to adjust itself accordingly?

Yes, I meant flow temp is 53C.

You need to have the return temp below 55c for the boiler to condense, the lower the better for boiler efficiency.

 

[berty3000]

You need to have the return temp below 55c for the boiler to condense, the lower the better for boiler efficiency.

Thanks.

For the bypass does the non TRV radiator need to be fully open or just open, so heats up.

Also, I've noticed sometimes that even though flow temp is 53C, sometimes the reading on the boiler will go to 56 or 57. Presumably hot water being returned, maybe from TRV'd rads rejecting it? How do I prevent these over runs?

 

[Mister Banks]

It’s the return temp that matters, the flow temp can do what it wants as long as the return temp is under 55c

 

[ericmark]

My towel rail is thermo syphon, and even if pumped, too far from boiler to act as a by-pass.

Basic idea is as the TRV's close, the by-pass opens allowing hot water back to boiler, which in turn reduces output of boiler, first my modulation if that type of boiler, then the mark/space ratio, some boilers have anti-cycle software which increases off time if water returns hot after boiler auto restarts, all this is by-passed if a wall thermostat switches it off, aim is to have anaguloge control when ever possible, so TRV rather than on/off thermostat.

But closer the by-pass is to boiler the better, the idea of one radiator without a TRV was before we used by-pass valves and modulating boilers. Fitting a TRV on mothers hall radiator was the biggest improvement of everything I did to her central heating.

However the two thermostats wall and TRV needs to either be carefully matched, or linked, and I found the TRV is always a lower temp to wall, so not the same temp set, I found TRV around 2 degs C lower to wall thermostat.

I was slow to fit a TRV to hall radiator as it seems I was reading outdated books which said don't fit TRV in room with thermostat, but when I considered it, in the hall the TRV makes sense, as it allows a quick recovery after front door opened, but then slows down before turning boiler off prematurely so rest of house gets cold.

However does not work too well with this house, as hall cools too slowly, the lock shield can adjust heat up speed, but not cool down speed.

 

[berty3000]

It’s the return temp that matters, the flow temp can do what it wants as long as the return temp is under 55c

Yes, but I have it set to 53C but the boiler will go higher than this from time to time. Why is that and how do I prevent it?

How would you recommend getting return temps lower? Tightening lockshields a bit?

 

[ericmark]

I found with gas modulating boiler easy way to set lock shield valve was look at the current and target temperature shown on the TRV heads software. The current should not exceed target, if it does close the lock shield a little.

Mothers house once set the wall thermostat only job was to turn off heating on warm days, or when the programs set on all the TRV heads asked all rooms to reduce heat, clearly if not linked then TRV programs in room with thermostat needs to match.

I found half hour before TRV was going to go to higher temp I would reduce wall thermostat by 0.5 deg then back up on change to ensure boiler runs.

 

[JohnD]

So closing the lockshield a little on all the radiators will keep more heat in them?

No.

By slowing the flow of water through the rad, the exiting water from the return end will be cooler, and the average temperature of the radiator will be lower, so the heat given out will be less. The radiator dies not "keep more heat"

Also, if you reduce the amount flowing through your hottest radiators, more of the water will be forced to flow through your cooler ones, making them heat up quicker, and get hotter, thus warming their rooms better. In many cases this is considered an advantage.

 

[berty3000]

No.

By slowing the flow of water through the rad, the exiting water from the return end will be cooler, and the average temperature of the radiator will be lower, so the heat given out will be less.

Also, if you reduce the amount flowing through your hottest radiators, more of the water will be forced to flow through your cooler ones, making them heat up quicker, and get hotter, thus warming their rooms better. In many cases this is considered an advantage.

So what's all this about adjusting lockshield to get a 12C temperature drop across the radiator from flow to return? Doesn't that involve heat being given out to the room and then cooler water returning to the boiler to assist the condensing process?

From what you're saying, tightening lockshields will just make the radiator and room cooler with only other benefit being that some other radiators get more heat.

I don't have a problem with some rads being cooler than others, I just want to achieve a bigger difference between flow and return to ensure more efficient heating system. Currently only 5 or 6C difference.

 

[JohnD]

So what's all this about adjusting lockshield to get a 12C temperature drop across the radiator from flow to return?

That's what you said you wanted.

Do you want it, or don't you?

 

[Mister Banks]

Yes, but I have it set to 53C but the boiler will go higher than this from time to time. Why is that and how do I prevent it?

How would you recommend getting return temps lower? Tightening lockshields a bit?

The flow temp will change due to the radiators releasing cooled water as their TRV opens.

 

[BlueLoo]

So how do I reduce the flowrate?

Why bother?

The boiler efficiency con is exactly that. It's damn hard to keep a boiler in it's condensing range whilst coping with all the other variables in play.
As you are finding out.

Stop hobbling the rads and crippling the output of the boiler to satisfy outdated rules of thumb for long since extinct setups. (Balancing).

Assuming your system is ok (and **** me, that's a hell of an assumption given the state of the trade) just open up all the rad LS valves full. Set the trvs to how you want them and enjoy a responsive and full powered heating system.

Unless you have a 10k btu rad in a shoebox room, i fail to see the issues.

Set the boiler temp to get you in the condensing zone when starting from cold (full load) and leave it at that unless there is something obvious.

If you really are mega arsed about efficiency, go fit a xplan system or similar where the boiler treats hw and ch seperatly then turn the boiler to min for CH.

You'll likely find in doing so you probably have to open up the LS valves and up the pump speed to get enough heat throughput.