Is balancing necessary with TRV

Indeed it does need doing, although a pressurised system with new radiators and a well designed layout dont tend to cause the same issues as an older system with older style rads.
Having said that it is always good practice to do it and sometimes nessacary on larger systems.
Incidentally the TRV's have nothing to do with it whatsoever, they are designed to be adjustable, its the lockshield valves that are adjusted when balancing and hopefully the non rotating cap placed on to prevent fiddling.

 

Dave, you forgot Drayton TRV4 , they are designed to be adjusted to balanced the systems, the lockshield usually stays fully open.

 

Normal TRV's don't adjust the flow rate as such, it's not a gradual process.
They're either on, or off.
That's the principal anyway.

 

That is incorrect.

A TRV is a proportional controller, i.e it adjust the flow through the rad according to the difference between actual and desired room temperatures.
A TRV will act as an on/off switch if the pressure drop across the valve is too high.
Balancing is needed to make sure the mean flow rate through the valve is correct. (300W rad needs 0.4 litres/min, 3kW needs 4 litre/min) A TRV can't handle both - unless it has built in balancing.

 

Yes

 

I may have typed that wrong, but the principal operation of a TRV is not to proportionally meter a certain flow rate through the rad to keep the environment at a pre-determined level.
Too cold, it opens.
Too hot, it closes.
Yes, that process doesn't happen instantly, but that's the function.

If the lockshield was turned off, thus causing an absolute pressure drop, the TRV wouldn't close because of that? Would it?

 

I'm sorry, but you are wrong. A TRV is a proportional controller. See
Balancing of Radiator systems, Page 14.

If the LS valve is closed there will be no flow through the valve, so the differential pressure across the valve would be zero. How far the TRV is open will still depend on the difference between actual and desired temperature

 

"The thermostatic valve theoretically behaves like a proportional controller. In practice, working conditions are not always favourable and the thermostatic valve often works as a temperature limiter. In this case a small proportional band gives better results. Even then, it may sometimes give the impression of behaving proportionally as it moves into intermediate, temporarily stable positions, as a function of its hysteresis"

I think we're both right in our own way.

 

I wondered what you would make of that!

On a correctly balanced system TRVs are less likely to act as an on/off switch. This is mainly due to the mistaken idea that water has to be forced through the pipes and rads, so the pump is, almost always set to maximum. This is not true, the water moves through the rad because the pressure at the rad's return connection is lower than at the flow. 15mm pipe can carry approx 6kW @ 11C, which is a flow rate of 7.83 litres/min. If the pipe only needs to feed a 1kW rad the flow need to be reduced to 1/6th, i.e. 1.3 litres/min The LS valve has to be open a very small amount, 1/4 to 1/2 turn depending on rad size.

Below is a pic of the inside of a TRV4, which has built in balancing, as supplied with balance set to 6. Note the size of the rectangular hole compared to the bore if the pipe connection. At this setting the TRV can handle 3kW without exceeding Drayton's pressure recommendations




(I also tried to upload a pic with the TRV set to 2, but got an error message. The hole is less than half what you see above.)

 

Here's the same TRV set to 2 or 3. The opening for 1 is even smaller, even so it can still carry 600W.

 

If you want your system to run well it should be balanced, so that each radiator gets an appropriate part of the flow of hot water, and each room reaches its set temperature at about the same time.

However, in a system fitted with (nearly) all TRVs, as each room reaches temperature, its TRV closes, leaving more of the flow to go to other radiators. This process will continue (given sufficient heat input) until all rooms have reached their set temperatures, and all TRVs are "off". This in effect is the same as a balanced system, but it takes longer to achieve the desired temperature in some rooms.

 

Correction: The pic show the TRV internal balancer set to 1. This would be suitable for a 600W rad (10C differential).

 

Balancing certainly is necessary to carry out on a heating system to have it working as efficiently as it can, and to do its intended purpose. It's hardly a 'heating system' if it only heats some rads with whopping output and some with barely any or none at all.

TRV's work exactly as their name states. They adjust according to the temperature of the room and are far from an 'on/off' valve. A TRV is not a way of balancing a system.

 

It is, if it has a balancing system built in, e.g Honeywell VT200 and Drayton TRV.

All balancing does is control flow through the rad; it doesn't matter which end that is done, flow or return.

 

What is the proportional band of the TRV'S that you've quoted?