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The Real Seal Deal

When and how to replace the slide valve seal.

Customers sometimes ask us for a “cartridge” for one of our TMVs. This is probably because they’ve heard that in order to repair a TMV one merely has to change the cartridge. This is occasionally true, but it can be hard to source the right one (as models change frequently) and can sometimes result in a bill that is almost as large as the cost of a new TMV. In any case, a new cartridge for one of the cheaper valves on the market will not likely fix the problem as scale and debris build-up can very quickly increase friction inside the valve to the point where the valve cannot work properly, perhaps resulting in a scalding incident. The Horne point-of-use TMVs have a design which minimises internal friction, resulting in a valve that gives great performance and maximises resistance to limescale. The Horne TMV, however, does not have a “cartridge”.

Therefore, it is helpful to have a working understanding of the TMV. With just a little understanding of what the individual components are and what they do, any competent handy-person can become acomplished in servicing a Horne TMV and bring it back to “as new” condition. This can be a very rewarding process, especially if you are willing to spend a little time mastering the basics. It also explains why many of our mixing valves are still in service after decades of use. The new Horne website has a maintenance training section with narrated videos showing the main parts of our point-of-use TMVs and information on the more detailed points of servicing them.

The Cold-water-isolation test

The cold-water-isolation test should be done routinely, according to local risk-assessment. If done along with its partner, the hot-water-isolation test, it becomes a kind of “MOT” (for non-UK readers: car roadworthiness) test on the valve, because then every component in the mixing valve is tested, and can be certain to provide protection from scalding. This test, however, must be done regularly; because, like an MOT it only guarantees safety at the time of the test.

If the valve fails its Cold-water-isolation test AND the Hot-water-isolation test

If the valve fails both the cold-water-isolation test and the hot-water-isolation test, then a new o-ring seal is required in the valve body (around where the slide-valve goes). Watch the video below for a demonstration of how to do this. The o-ring removal rods shown in the video are Part No.4411, and are available from the Horne spares department. Order them when you order your spares kit. Please have speakers enabled to hear the video narration.

If the valve fails its Cold-water-isolation test but passes the Hot-water-isolation test

If the valve is okay on one of the isolation tests, but not the other, then it is more likely that the problem is caused by dirt in the valve or corrosion/scale on the hot valve face. See the video from our maintenance site entitled “Lapping”. You will need to obtain a password to access this part of our website.