The idea of balancing a system is to try and get the pressure drop of all circuits equal to that of the index circuit.

[Ric2013]

Hi. Just trying to get my head around how this relates to the temperature drop across the circuit. Not disputing that the two are related (obviously we measure temperature as that is easier) but I don't understand the process. This came up on another thread and I didn't want to sidetrack the discussion to the detriment of the OP.

 

[SJB060685]

Hi mate.
The idea of balancing is as I said in the other post and as you have just said is to try and equalise the pressure drop across the various zones.
In order for flow to occur there must be a pressure difference, the greater the pressure difference the greater the flow. By equalising you are affectively keeping the flow the same throughout each sub-circuit and therefore have a balanced system. This will in turn keep the DT across each emitter close, if not identical.
Remember the greater the flow rate through an emitter the lower the DT is and the higher the mean temperature of the emitter is, likewise you widen the DT by decreasing the flow rate the lower the mean emitter temperature is.
As I said, an unbalanced system can but not always affect the return temperature to the boiler, which can in turn affect its output.
Calculating mixing temperatures and flow rates is a lengthy task, especially when you have multiple and would take me a little while to calculate with certain information at hand.
Google heatgeek guide to balancing and have a read, it really will explain it all to you, probably better than I have.

 

[Ric2013]

Thanks mate. Will do!

Okay, I've had a look. The caveat is that they mean pressure drop across each radiator (they say circuit but go on to contradict) and this assumes that the radiators are identical, not different types and sizes. It's a very interesting site though with plenty of interesting articles and it's now on my rainy day reading list.

 

[SJB060685]

Thanks mate. Will do!

Trust me, the guy Is nothing short of a genius! Myself and a couple on here are friends with him elsewhere. Please do listen and take on what you read or see. It boggles my mind how someone can be so knowledgeable

 

[John.g]

Be interesting to see these installed even if one never used the TRV side of them.

 

[SJB060685]

Be interesting to see these installed even if one never used the TRV side of them.

There's videos on YouTube about them mate. Can't remember which video I watched but they look impressive and would certainly making nuisance systems balancing easier.

 

[Ric2013]

Be interesting to see these installed even if one never used the TRV side of them.

You still need to set them, mind. I'd be interested to have a go. Price is what?

 

[John.g]

Don't know cost, there is a app to set them up , I find the normal TRVs work quite well even though I have never used the L/shields to do any balancing but I run my heating > 15 hrs/day so the TRVs even though slow acting will have taken up their relative positions so I wouldn't see any great advantages for me.

 

[Chuck]

Hi. Just trying to get my head around how this relates to the temperature drop across the circuit. Not disputing that the two are related (obviously we measure temperature as that is easier) but I don't understand the process.

The two key physical variables are flow rate (litre/min) and temperature drop (degC), which together determine the heat transfer rate (joule/sec). The former is set by the displacement rate (litre/min) of the pump and the boiler power (joule/sec) then sets the latter. The process of balancing adjusts local impedances (mbar.sec/litre) to tweak the flow through each emitter so it is comensurate with the heat requirements in its vicinity.

If the pump has a constant displacement rate, adjusting the lockshield to reduce the flow through one radiator results in an increase in flow through other radiators to keep the total constant. In such systems you need go round making adjustments a couple of times get everything sweet.

If the pump has a controller that modulates the displacement rate to keep the pressure drop constant life is a lot easier. In this case reducing the flow through one radiator with its lockshield causes the pump to reduce the displacement rate but to keep the pressure difference between its ports constant. As long as the pipework impedance is neglible, which should be a reasonable approximation but isn't always, the flow through the other emitters remains constatn. In such systems balancing is a single pass proces.

When a TRV closes on a 'constant displacement' system the reduction in flow has to be balanced by an equivalent increase shared amongst the remaining radiators, which will probably all get a bit hotter as a result. On the other hand, when a TRV closes on a 'constant pressure' system the flow through the pump reduces and the other radiators remain unaffected.

A pedant will point out that real systems are never ideally constant flow or ideally constant pressure due to the finite impedance of the pipework. In practice, however, the behaviour is largely determined by the pump type and constant pressure systems are, IME, easier to set up.

 

[Ric2013]

Agreed Chuck, especially your point about constant pressure pumps. I remember the first time I balanced a system on a PP set UPS2. I set the valves, checked the others, tweaked a few and then found myself surprised that I'd already finished.

 

[John.g]

The two key physical variables are flow rate (litre/min) and temperature drop (degC), which together determine the heat transfer rate (joule/sec). The former is set by the displacement rate (litre/min) of the pump and the boiler power (joule/sec) then sets the latter. The process of balancing adjusts local impedances (mbar.sec/litre) to tweak the flow through each emitter so it is comensurate with the heat requirements in its vicinity.

If the pump has a constant displacement rate, adjusting the lockshield to reduce the flow through one radiator results in an increase in flow through other radiators to keep the total constant. In such systems you need go round making adjustments a couple of times get everything sweet.

If the pump has a controller that modulates the displacement rate to keep the pressure drop constant life is a lot easier. In this case reducing the flow through one radiator with its lockshield causes the pump to reduce the displacement rate but to keep the pressure difference between its ports constant. As long as the pipework impedance is neglible, which should be a reasonable approximation but isn't always, the flow through the other emitters remains constatn. In such systems balancing is a single pass proces.

When a TRV closes on a 'constant displacement' system the reduction in flow has to be balanced by an equivalent increase shared amongst the remaining radiators, which will probably all get a bit hotter as a result. On the other hand, when a TRV closes on a 'constant pressure' system the flow through the pump reduces and the other radiators remain unaffected.

A pedant will point out that real systems are never ideally constant flow or ideally constant pressure due to the finite impedance of the pipework. In practice, however, the behaviour is largely determined by the pump type and constant pressure systems are, IME, easier to set up.

Yes there will be some small effect on the other TRVs when one shuts in/opens up but these will automatically correct for this. Also, if, like mine, you run the system in PP pump mode then the pump will ramp down/up to compensate for the change in head/flow so normal TRV control should be excellent.
The pump manufacturers generally reckon that half the head losses are in the piping and half in the rads and this is why the PP uses max flow at H and zero flow at H/2. Grundfos use slightly different settings than H/2.