How TRVs actually work?

[Harry Bloomfield]

That would seem to make perfect sense, especially if all TRV's when sitting 'fully open' do exert some force on the pin and close the valve to some degree, not sure they all do though? I haven't definitively tested that fact but when fitting some TRV there is a feel that the pin isn't being depressed by very much. I suppose it's gaining an understanding as to just how much that actually effects the flow through the valve. i.e. how closed does a valve need to be before it starts to affect the flow.

Common sense, suggests that even wide open, that a TRV will be a major restriction on the flow. Much more so than a lock shield.

 

[Madrab]

Common sense, suggests that even wide open, that a TRV will be a major restriction on the flow. Much more so than a lock shield.

What common sense Harry? Unfortunately I don't think common sense actually comes into it. Without knowing when the max flow through a TRV or the lockshield valve is reached then an open TRV could theoretically reach max flow just as quickly as an open lockshield. There are obviously a number of variables involved in that - circulator head at the valve - heights, distances, resistances etc

When using lockshields to control the flow through a rad, the valve can be opened 1/2 a turn and the flow starts, the flow can then reach it's max within another full turn, that by direct observation. Depending on the temp in the room a TRV that's then opened, whether it is @ 1 or 5 can allow that rad to warm up equally as fast, again that's using direct observation, timings and temp monitoring.

It will obviously close down faster depending on the temp setting, so it comes down to - how open does any valve actually need to be to reach max flow. That I guess would need a test bed, numerous number of different valves, pressure and flow metering, pumps and settings etc

 

[fixitflav]

TRV are best considered as a secondary control function where you want cooler rooms eg bedrooms) than the central part of the house, or in any room with additional heating.

No, if the rad is correctly sized for the that room and for its desired temperature, you wouldn't need a TRV in that room either.

 

[fixitflav]

What common sense Harry?

As Voltaire said, the trouble with common sense is it isn't very common

 

[Mister Banks]

No, if the rad is correctly sized for the that room and for its desired temperature, you wouldn't need a TRV in that room either.

But only if you used weather compensation?

 

[fixitflav]

EPH are of the same view "By turning to a higher figure setting will not heat the room any faster" I have used their TRVs for years but afraid I can't agree with this, I came across a test I did a few years ago using a new TRV and using a digital thermometer and a vernier calipers to measure the valve opening.
With head removed the maximum (stroke) opening of the valve was 3.48mm, with the head attached and set to index 2.5 and left in a room at 14.9C for over a hour the valve opening was 1.42mm, when placed in a room at 20.9C it fully closed after ~ 18 minutes. I find that a setting of 2.5 on these TRVs gives me a very consistent 20.2C/20.5C.
The room would/will definitely heat up faster if the head was fully rotated to allow a opening of 3.48mm but simply cannot heat it up in the same time with a opening of 1.42mm, it will also start throttling down as the air temperature starts rising, but these certainly arn't on/off controllers.

You've clearly put some thought into this, and I'm sure you're right. As I acknowledged in #10, a TRV doesn't give a sharp cut-off. And as it's a proportional-only control, when the weather's colder so load higher, the room temperature must fall, though it doesn't seem to make much difference in practice.
To take advantage of the higher TRV flow (if any) when fully open (compared with being at a lower setting but the room colder), what does the OP do? Go round all the rooms and when up to required temperature turn the TRVs down? Several times a day?
I have Honeywell TRVs which were a few years old when I moved in 25 years ago. They work OK and I don't touch them (or the roomstat) from one year's end to the next.

 

[fixitflav]

But only if you used weather compensation?

No, I don't think so. The room with the stat sees varying load as the weather changes, just like the other rooms, so in principle the system will still be balanced.

 

[Johntheo5]

You've clearly put some thought into this, and I'm sure you're right. As I acknowledged in #10, a TRV doesn't give a sharp cut-off. And as it's a proportional-only control, when the weather's colder so load higher, the room temperature must fall, though it doesn't seem to make much difference in practice.
To take advantage of the higher TRV flow (if any) when fully open (compared with being at a lower setting but the room colder), what does the OP do? Go round all the rooms and when up to required temperature turn the TRVs down? Several times a day?
I have Honeywell TRVs which were a few years old when I moved in 25 years ago. They work OK and I don't touch them (or the roomstat) from one year's end to the next.

Yea, same here, havn't touched some TRVs in years also, the only practical thing to do if he has access to the pump is to increase the head which will give a increase in flowrate.
I have a kitchen rad on the end of a very long run, I have my pump set to PP4.7 which gives me the required 3.6M with all TRVs calling, as they throttle down then the head falls so the kitchen rad is a bit slow to get to its full temperature, in very cold weather I change the pump to a CP (constant pressure) setting of 3.5M which does help it heat up faster. I could leave it like this as the TRVs are not noisy at this setting (or any CP setting up to 4M) but I just like to see the pump power falling on PP mode.

 

[Harry Bloomfield]

What common sense Harry? Unfortunately I don't think common sense actually comes into it. Without knowing when the max flow through a TRV or the lockshield valve is reached then an open TRV could theoretically reach max flow just as quickly as an open lockshield. There are obviously a number of variables involved in that - circulator head at the valve - heights, distances, resistances etc

Well, you have a radiator, which offers almost zero resistance to flow, a 15mm pipe which has by comparison to a TRV, little resistance - so your only resistances to flow are the TRV, and the lock shield. Of the two, I would suggest the TRV, has by far the greater impact on flow, when both are wide open, but then I have never opened up a TRV to check the internal design..

 

[fixitflav]

The solution isn't to kill all the other rads. The solution is to speed the slow one up.

But how do you speed the slow one up, other than by throttling the faster ones? (without redesigning the whole system)

 

[Harry Bloomfield]

But how do you speed the slow one up, other than by throttling the faster ones? (without redesigning the whole system)

I agree. The flow through pipes and radiators, compares very well to a mixed series, parallel resistor network, which can become extremely complex indeed. Whether by good design (not mine), or luck, my system works fine without any need for any balancing - all radiators heat up at a similar rate, as soon as the boiler fires.

The TRV's simply work to limit the room temperatures.

 

[Madrab]

Well, you have a radiator, which offers almost zero resistance to flow, a 15mm pipe which has by comparison to a TRV, little resistance - so your only resistances to flow are the TRV, and the lock shield.

yes, agree with all of that.

so your only resistances to flow are the TRV, and the lock shield.

Everything in the system will reach a max flow rate that it's design allows. There is no reason why TRV's or Manual valves wouldn't be designed to flow at the very least the max flow rate a normal system circuit would normally run at, with room to spare, to obtain a certain level of efficiency (Deltas)when fully open, otherwise the largest radiators may never be able to obtain the flow they need to heat up properly. I can't be certain of course and will need to browse through piles of datasheets but that flow rate would be in the order of low L/Min.

 

[BlueLoo]

But how do you speed the slow one up, other than by throttling the faster ones? (without redesigning the whole system)

You actually mean: "how do you divert the flowrate from some zones to others?"

Does throttling all rads (including the poorly behaving one) seem a good idea (balancing)?
No. Of course not.

Do you look to make all rads work equally bad?
No. Of course not.

Does this approach work with modern CP/PP control pumps?
No.

Does this approach work where other rads in the system need the fill flowrate?
No.

Does this approach work where the average temp drop has been engineered to keep the boiler in cond mode?
You get my points?

 

[Madrab]

If all the radiators in a house heat up at the same rate then of course the whole space will reach an equilibrium as quickly as is possible, why would it not? The problem starts when the radiators don't heat up at the same time and the whole space then has cold/cooler spots. That is all before any of the dynamic controls even come into play. That's when a system that can heat up evenly and consistently is desirable as it can minimises run time at higher rates and satisfies the core demand as quickly as possible. It's not about killing all the other rads, sometime a rad stuck out in the 'wild' cannot be heated up without slowing the others down, that doesn't mean the others are being killed rather it's ensuring everything runs equally and speeds that slow one up, as it then gets it's required share of the flow.

By balancing the rads they are not necessarily being throttled back. Each rad requires a max flow to allow it to warm up properly, consistently and allow it the correct flow rate to release it's heat efficiently. If that isn't optimised then the rad cannot do it's job properly. A system can have lots of rads of all difference sizes on the end of differing lengths/resistances of pipes and at different heights, therefore by it's very nature if everything was fully open then the system water will naturally take the path of least resistance - unless the system was relatively small with similar size rads and valves all with similar supply pipework. That then means that the rads closer to the pump/boiler can actually receive significantly more flow that they need. That excess return heat then finds it's way back at the boiler and the boiler will then modulate down before the others are even warmed up properly and that cycle will continue. Modifying and optimising the flow to each radiator to find a happy medium then ensures that is kept to a minimum and each rad still receives what it needs to be efficient.

 

[Madrab]

We all seem to be dancing on the head of a pin here and and probably wont achieve consensus.

I'm not sure whether any of this is of value to the OP and whether it helps him with his original request of not @berty3000, apologies if your post has rambled and been stomped on somewhat