What you don’t know about lockshield valves!?

This article is supplementary to the ultimate guide to balancing, please also check that out!

Many lockshield valves have poor design to control the flow rate for modern boilers, they may be better on heat pumps which have much higher flow rates but could still be improved on. Have you ever noticed when balancing on some systems the first few turns on the lockshield do nothing? It’s typically only the last 3/4 turn to closed that has any effect on flow? This is mainly due to their valve authority.

We will go in more depth with KV values and valve authority in another post but to keep it simple, valve authority is the ability for the valve to evenly control the flow of water through the emitter/radiator. The lower the KV value the better the ability for a valve to control the amount of water going through it, and therefore the better its authority.

The main reason for this is that they were originally designed back when we had systems running at a lot faster flow rates (DT11) and haven’t been updated since. Also, as insulation improves, the lower and lower flow rates required at radiators mean their authority is ever decreasing.

Take a look at the graph below from Albion indicating the effect of their lockshield valves on flow rate. If we take for example a 1kW radiator at delta t 20 this would require 43 L/H (or kg/h). You’ll see that 1/2 a turn open barely even begins to impact on the pressure difference. You’ll also notice that you have to move the valve further and further only to get less impact.

Flow and pressure drop across a half inch valve
balancing valves

Let’s look at what exactly valve authority is. Valve authority is how much control a valve (not necessarily a lockshield valve) has over the flow rate through the specific circuit it’s installed on. The same valve can have a different authority if it’s installed on a different circuit.

Authority is measured as a relationship between the complete pressure loss of the whole circuit and the pressure loss across that valve when fully open at your design flow rate.

Valve authority can be expressed as a ratio. An authority of 0.2 essentially means the valve represents 20% of the total resistance around that circuit. An authority of 0.2 and above would be a good authority over the circuit. An authority of 0.5 or above is excellent. The only downside to having higher authority is additional pumping costs, so there is a balance to be struck.

Interestingly, higher authority valves mean the system is much more likely to be in balance even before we begin to balance radiators manually. If the majority of resistance is at the valve, each circuit is likely to have similar resistances and so the flow will be spread out more equally as described in pressure and flow.

It also means the flow will continue to be more evenly spread through each circuit when the flow dynamics change. For example; when a zone valve closes, this can throw some of your balancing out (perceivable or not) if your valve authorities are low.

TRV’s typically have a very good authority, the reason for this is clear when you look in the bottom of one you will see the actual flow path is vastly smaller then 15mm. This gives the valve more control over the circuit and prevents the TRVs from ‘hunting’ which is where is continually over and under supplies flow.

Smaller bore gives higher valve authority
Smaller bore gives higher valve authority
thermostatic radiator valve authority

That’s not all. Higher authority valves also enable us to achieve a greater degree of accuracy when balancing. Because the valve has more control over the circuit, our ‘valve stroke’ has a more even control over the flow.

Look at the graph from Calleffi below, this shows you how much authority your valve may have over the circuit, but more specifically, how much control the valve has over the flowrate in relation to the ‘valve stroke’ (how open the valve is).

A valve with good authority will ideally have a ‘concave up’ curve as opposed to the concave down in the image. As lockshield valves typically only have control of flow over the last quarter turn their graph would be dragged right up into the top left-hand corner.

Flat disc type valve authority

The concave down shape of the curve above is mainly determined by the valve authority. You can look for valves with a lower KV value (read as higher resistance), but typically the only way to increase the valve authority is to purchase smaller valves, or find valves with smaller internal bores.

However, looking for slightly different types of valves can also improve control of the circuit.

Because some lockshield valves are ‘quick opening’. That is to say, they immediately have a very little restriction to flow, and therefore allow a very high flow rate, the moment the valve is even slightly opened.

Looking for valves with a more ‘conical disk’ that inserts deeper into the seating will give the valve a ‘slow opening characteristic’ by opening the water pathway only a small amount, for the same amount of turns.

This interrupts the flow path and puts more resistance to the flow of water when the valve is only slightly open. The result is the curve in our graph is brought much further over toward the center, and improves the accuracy that can be achieved from balancing. This translates to much less going back and forth over and over making valve adjustments.

So what do we mean by a more conical shape and valve seating?

A flat face valve is much like you have in a traditional tap, the flat face of the gland presses against the seat to stop flow. Great for turning on and off, but the moment the gland is lifted from the seat theres not much pressing back against the flow of water.

Flat face radiator valve side view

A more conical gland has more of a cone shape. The gland actually enters inside the seat before the flow of water is even stopped. This interrupts the flow of water and ads resistance BEFORE the valve is closed. The more cone-shaped the gland or the more it protrudes into the seat, the better control over flow you will have and the less time spent balancing!

Take a look at this excellent valve. The Regutec by IMI has a gland that drops right into the seat giving you more interruption of the flow path, and less shifting around as the rest of the system is balanced.

IMI have taken this to another level with the Regulux. The Regulux Encorperatures a max setting limiter, this effectively lets you set your KV value (or valve authority) to suit the system however does still limit your valve travel.

The Regulux does this by setting a maximum opening of the valve which also means anyone can turn the radiator on or off via the lockshield for maintenance, and when they fullly open the valve again it will be set at the correct value.

What’s more this valve also has a built-in drain off and filling connector, pretty incredible for a lockshield valve!.. 🤣

If installing the Regulux we would suggest ignoring the instructions as regards to the KV/max flow setting. The instructions are for a system with constant differential pressure (DP) and as most of us should be fitting modulating boilers will not suit (unless you have a separate system pump and not using DT control).

Instead of the method included in the instructions here, I would instead set the lockshield via the normal method with an alan key, count the turns untill closed, then adjust the presetter untill the valve stops opening at your counted number of turns.

Back to our authority curve. As mentioned, the more the gland extends in to the seating the more control you have over the flow. At the extreme end of this we end up with an equal percentage type valve.

This is where the glands plug travels right into the seat and doesn’t actually allow full flow until the valve is opened all the way.

Radiator valve plug types
Valve plug shapes

Somewhere in between a flat face type valve and and equal percentage valve we have a linear type valve which is what the Regutech and Regulux seem to be.

Radiator valve opening characteristics table

Most lockshields do have a slightly conical shape, however only drop into the seating very slightly. As mentioned earlier, we don’t find many problems with the traditional quick opening valves even on larger domestic really depending on what valves you use. But If you are going to swap the valves anyway you may as well select valves that will save time later on.

How much any of this matters will vary depending on your system size and flow rate. For example Heat pumps typically work at a dt of 5-7°. Because the flow rate is 3 to 4 times faster than with gas boiler systems your valve authority is automatically increased making balancing potentially easier.

Now that’s the theoretical.. what about reality, how much difference will this really make? Here’s a quick video on this very subject that might just help.

Well that was worth doing! So from the findings above it definately seems important to find the dud ones, that makes the most difference.

Here’s the very rough results from our ‘experiment’.

15mm compDanfoss RasIMI Regutech3/4 Union
1/2 turn2.5211
1 turn1141.52
2Out of range646.5

And here’s what the opening characteristics look like.

Lockshield valve test results table
Valve opening characteristics

To be perfectly honest the main lesson here is that our experiment is flawed by our flow gauge and high flow requirement. If we tested these valves at lower flow rates you may see a much more ‘concave down’ line (like the blue line).

Still, they are much more similar (excluding that compression one) than we thought, but bare in mind this represents the % of the maximum flow rate which is unrealistically high in this experiment. Although the Danfoss and 3/4 valves look relatively steady, it’s steady over a very high flow rate which is of no help, the IMI has much greater control over lower flow rates which is much more helpful. This will be repeated if anyone can suggest a sensible, high accuracy flow gauge!?

Remember, this isn’t just for lockshield valves, this is for any valve type and will become more important the bigger the systems you work on.

You may want to use equal percentage or linear valves on larger systems where you are balancing whole circuits with 1 valve, not just 1 emitter. This will give you a very easy and quick degree of control and will save time when then balancing the emitters within each circuit later.

Also worth mentioning now that all these must be installed the right way round with the flow of water pushing into the plug/stem or they will behave erratically when the system dynamics change.

Something that’s regularly overlooked when balancing is what logic your pump is using. You could have burner linked, DT controlled, outside sensor control, constant pressure, constant speed, proportional pressure and more (an article to follow on these).

Which one of these you are using will dictate which valves best suit your installation. This is covered in the second half of our ultimate guide to balancing article.

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