TRV flow characteristics chart and graph - how do you use these?

[mactatty]

Can anyone help me to read the graph of pressure drop/ flow rate, and the chart of Kv values? I shoulc think they are really for commercial installations, but I wonder if they are any value for domestic properties?

 

[SimonJohns]

In my experience these will only come in to play of you exceed 12 rads. But tbh I would just fit a bigger pump

This is primarily for larger installations where you would utilise a llh and have to rate the pump suitable for the system

 

[doitmyself]

Can anyone help me to read the graph of pressure drop/ flow rate, and the chart of Kv values? I should think they are really for commercial installations, but I wonder if they are any value for domestic properties?

They are mainly used in large complicated commercial installations - office blocks etc. The designers will have calculated the required flow in every branch and radiator and installed control valves to regulate the pressure drop in each circuit. They will therefore know what setting the TRV has to be on.

The Drayton TRV4 and RT212 are adjustable (you need a special box spanner) and Danfoss also make them, but they are sold as "commercial" TRVs.

What is interesting is to see how opening in the valve varies with setting. Here is a pic of a TRV4 with the valve set to 1, the lowest. Note the very small rectangular opening compared to that of the 15mm opening. On setting 6 (max), which how they are supplied, the opening is only the size of the small round hole.

 

[mactatty]

Thanks very much. Quite surprising to see how small the aperture is even at max setting - I've looked at one of my TRV4s to check.
I think you are essentially talking about system balancing with the Kv values. Is it not quicker to use these for a quick setting, rather than go round the system checking the flow and return temps with a pair of thermometers? (...not that it is necessary to go this far with a 3 bed semi for instance. A pair of hands is good enough!).

 

[doitmyself]

Thanks very much. Quite surprising to see how small the aperture is even at max setting - I've looked at one of my TRV4s to check.

It's not surprising if you understand that water flows through a rad because there is a pressure difference across the rad; it's not "forced" through the rad by the pump, as so many think.

I think you are essentially talking about system balancing with the Kv values. Is it not quicker to use these for a quick setting, rather than go round the system checking the flow and return temps with a pair of thermometers?

Yes, it is system balancing. If you use the TRV to balance, the lockshield valve is left fully open. A good starting point is to set the TRV to 4 (that refers to the numbers on the black ring round the pin) and adjust up/down if a rad is too cold/hot. Drayton will supply you with the spanner free gratis.

 

[mactatty]

I'm actually intrigued by the concept of pressure difference, and I'd appreciate your interpretation of this. I can't really envisage why the pressure drops round a circuit from a circulator and back to it. I imagine a circuit like a row of dominoes - if you push one in a row, the last one is pushed too. The push force is the same at the end of the row as at the start. Friction will play a part, but not enough to make that much difference.

 

[doitmyself]

I'm actually intrigued by the concept of pressure difference, and I'd appreciate your interpretation of this. I can't really envisage why the pressure drops round a circuit from a circulator and back to it. I imagine a circuit like a row of dominoes - if you push one in a row, the last one is pushed too. The push force is the same at the end of the row as at the start. Friction will play a part, but not enough to make that much difference.

But radiators are not in a row! They are more like the rungs of a ladder.

Read Copper Tubes in Domestic Heating Systems and Small Bore heating Systems, then come back if you want more information.

 

[mactatty]

I was looking through my Copper Board Installation Tips early this week, and the Copper Tubes one isn't in it, or listed on their website either. I've contacted them about this.
I've done pipe sizing a long time ago, and never needed the info since for my work.

By a row of dominoes, I really had in mind the incompressibility of water, so water going in one end of a system must push out the same amount of water out the other end. But in a central heating system, the loss of pressure is all about frictional resistance. If it is great enough, the circulator can push as hard as it likes, but there will be no flow at all, and at some point along the pipework, no pressure at all.
Across a rad the pressure difference is the difference between the flow side and the return side at that rad. Maybe the water is in effect sucked through the aperture of a TRV, or at least there is a combination of pull as well as push from the circulator.
Would you agree with this?

 

[doitmyself]

I was looking through my Copper Board Installation Tips early this week, and the Copper Tubes one isn't in it, or listed on their website either. I've contacted them about this.

It is there - click the link I gave earlier.

In a central heating system, the loss of pressure is all about frictional resistance. If it is great enough, the circulator can push as hard as it likes, but there will be no flow at all, and at some point along the pipework, no pressure at all.

On systems with a F&E tank the neutral point is where the feed pipe connects to the flow, usually just before the pump. The pressure loss round the circuit, aka the head, determines the pump size.

Across a rad the pressure difference is the difference between the flow side and the return side at that rad. Maybe the water is in effect sucked through the aperture of a TRV, or at least there is a combination of pull as well as push from the circulator.

I agree that the difference is due to different pressures either side of the rad. I'm not so sure about the pump pushing as well. If you have a 15mm pipe containing a restriction about 3mm square teed into a 22mm pipe the water will naturally want to follow the easier path, i.e through the 22mm pipe. TRV manufacturers give a maximum recommended pressure drop across the valve, usually about 0.2 bar or 2m H₂O.

This all relates to balancing the system, so you get the correct flow through the radiators. You might like to read [DLMURL="http://www.draytoncontrols.co.uk/WorkArea/DownloadAsset.aspx?id=24266"]Drayton's advice[/DLMURL] on this.