23 May 2011
Reaction score
United Kingdom
This what I have understood.

You pre-set the flow rate for the flow section of this type of valve. This is maintained automatically when (if) the pump pressure changes. The device incorporates semi-separately the usual automatic flow-modulating thermostatic valve function.

Normally the flow rate to a radiator is fixed by the manual pre-setting of the lockshield valve carried out at installation to try to achieve good radiator balancing, with all the handwheel valves fully open..

It is widely recommended to have the lockshield valve fully open when a TRV is fitted on the other side of the radiator (normally occupied by the manually adjustable handwheel valve). That is because a TRV modulates the flow through the radiator automatically, responding to ambient temperature. It replaces both the handwheel and the lockshield valve, so a valve is, in theory, not needed at all at the other side of the radiator

In practice one is usually fitted there, even in tandem with a TRV, to avoid (hopefully) complicating a radiator change etc.

If it is intended that the radiator concerned should always be able (subject to the operation of the TRV) to receive the maximum flow rate available, a valve at the other side of the radiator should obviously be tamper-resistant (eg, a lockshield).

Presumably the lockshield valve should also be left fully open if a handwheel valve or "ordinary" TRV is replaced by a TRV4-type valve?


The marketing of this type of device implies that an "ordinary" TRV alone is not sufficient to maintain a constant flow. That is true - if the flow rate varies. But it varies throughout the pipework, more or less equally, not normally at different points. So all radiator performance is altered by the varying flow rate, more or less equally.

If the system pump varies its speed in inverse proportion to the opposing head within the system (eg, the Grundfoss Alpha pump set to automatic rather than 3-speed manual operation), the changes in pump pressure are equal throughout the system, High pump speed occurs when the opposing head in the system is low (all TRVs more or less fully open because the ambient temperature is low, or the boiler and pump would not be responding to a call for heat). As the ambient temperature rises, the TRVs should gradually reduce the flow rate, causing the pump to slow down.

So this type of pump should achieve a fairly constant dynamic pressure and flow rate in the system, irrespective of the varying head.

Ordinary TRVs are intended to, and do, modulate the flow within their radiators, so the dynamic pressure varies within these, to modulate their heats outputs to achieve ambient air temperatures matching (hopefully) what is intended in the areas where they are situated.

If the above is accepted, what additional function or refinement does a TRV4-type valve provide compared with standard TRVs plus fully open lockshield valves, if the pump is the Alpha type?

What if a standard constant-velocity pump is fitted (a single-or three-speed type)? Here there may be an advantage in fitting the TRV4. If the pump is a multi-speed model, but not easy of access, it will probably have been switched to the highest speed, and been left like that.

After the system switches on early in the morning, (usually) the TRVs will be all more or less fully open, so there is a low opposing head for the pump and a high flow rate, but moderate dynamic pressure in the system. TRVs modulate down the flow rates through them as their ambient temperatures rise. The pump has a series-wound motor, so it draws a rising current to try to overcome the increasing opposing head in the system and maintain its intended dynamic pressure performance. An old TRV may not be able to resist the increasing system pressure, which forces its water valve towards the open position, against the compressed spring pressure acting on the TRV's pintle), so allowing more flow of by-now fully heated water than the ambient temperature setting on the valve needs.

This is usually referred to as "overpowering the TRVs". If the pump has a 3-speed switch. If this is accessible, it should be be turned down to whatever is a reasonable normal speed given the ambient temperature in the house. Trial and error needed here.

If that is an excessive chore (it's easily forgotten), possibly TRV4-type valves would be helpful.

But otherwise, especially if the system has a self-speed-varying pump, I don't see what advantages they have over good standard TRVs.

Two asides on that last comment

Most TRV's use a wax-filled capsule to expand and contract and raise/lower, in conjunction with a spring, a pintle which operates the also-spring-loaded water valve in the part of the TRV that is attached to the pipework and radiator. These capsules seem slow and unresponsive, and hard and often "sticky" to adjust. This was not a suitable subject to ventilate at a trade counter, but I tried the "own brand" TRV from my local (as it was) PTS. I had an unusually light action, so I suspected a gas-filled bellows (rather than a wax-filled capsule). I may have been right, because these few (precious!) relics are very much easier to operate than the stiff ones, which I associate with wax-filled capsules.

A year or so later I returned to PTS for some more. The appearance had changed ("New model, mate"), and operation is the common stiff and sticktious.

I don't find much, if any, difference in accuracy of response between the two types, to what feels to be the ambient temperature in the environment of the valve, though the "historic" PTS models respond more quickly to temperature change than their sluggish companions.

Nearly all our TVRs are mounted vertically, because they are older non-bi-directional models, and are on the flow, and at the bottom of their radiators.

Lifting these valves into a part of a room atmosphere where the temperature is more representative of what humans feel, should also, it is said, give a small improvement in radiator efficiency So, over time, I intend to replace very old non-bi-directional TRVs, and fit them horizontally rather than vertically.
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