Adding cyl thermostat to Gravity HW system

[pete]

Our central heating is pumped heating/ gravity HW

I would like to add a tank therm to give some control over heating water.

How can I acheive this without the complication of going to fully pumped?

[Andy Hall]

You need a motorised zone valve and a thermostat and possibly an additional/new programmer and then follow the C Plan instructions from Honeywell..

.andy

To email, substitute .nospam with .gl

[Chris J Dixon]

I added a tank stat to my similar system much more simply. I know it doesn't give full control, and can't remember exactly how I wired it (probably followed the instructions in the pack) but it serves to ensure that, if there is no heating demand, instead of the boiler cycling on its internal stat, the cylinder stat takes control.

Chris

[Richard Sterry]

was circulating all the time the boiler was on. I had previously fitted a programmer with separate hot water and central heating controls, so just got 'the man' to add the zone valve and cylinder thermostat for me.

Rick

[IMM]

fit a zone valve you may as well make it a fully pumped system and improve efficiencies all around.

You do not have to fit a zone valve on the gravity primary pipes to the cylinder. A cylinder stat will prevent the cylinder overheating in summer when the heating is off. In winter when the heating is on, the cylinder temp will determined by the boiler stat.

In summer to prevent boiler cycling, two stats, one top and one bottom on the cylinder can be fitted.

[Dave Plowman]

You used to be able to get a device similar to a TRV designed for this - it had a remote sensor which clamped onto the tank. I've never fitted one or seen them for sale recently, though, and don't know how well they work.

[Terry D]

Surely a valve would be required in the primary from the hot water tank. Otherwise, what would the cylinder stat be controlling? I have a similar system and would like to do the same. I'm considering inserting a motorised valve in the primary return between the hot water tank and the boiler and controlling it from a tank thermostat. It's important to maintain venting in the primary, hence the valve positioning in the return, not the flow. I've also considered fitting a small bore by-pass around the valve, just in case.

Terry D.

[IMM]

They are in the return. They are wax thermostats with some having remote sensors that clamp to the cylinder. they were popular about 25 years ago when many existing, and still new, systems had gravity primaries. Depending on the make, they were a bit dickie. I think the likes of danfoss still make them, but they are now very expensive. Also "plumbers" would use the a lot as they were mechanical, with their limited ability could understand them. Heating engineers would use more efficient and controllable electric zone valves.

As an add on they are not worth it, as if you are breaking a primary pipe you may as well insert a zone valve and make the whole system fully pumped.

You also, cannot put the valve blocking the cold feed to the boiler either.

This by-pass would have to run in an up and down manner to prevent gravity circulation. Making a system full pumped can mean re-directing pipes, etc. If you want minimum fuss, and are going to drain and break into a pipe, then insert a new pump on the gravity primaries with a non-return valve and a cylinder stat controlling the pump. You may need to insert a non-return valve before or after the existing CH pump too. You may as well do this as the system is drained down. Then you will have a cheapish fully pumped system. That is how I would approach it.

[Stuart]

Like this from

under Other Domestic Controls/Hot water

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RAVI Thermostatic Cylinder Controls

Item Number(s): RAVI thermostatic cylinder controls The RAVI offers a simple non-electric alternative to the more traditional motorised valve and electric cylinder thermostat. The RAVI is ideal for those system upgrade situations where the consumer wants the benefits of temperature controlled hot water, but without the inconvenience or upheaval of lifting floor boards and disturbing decoration to run electric cables.

The RAVI is available in 2 port versions for Pumped and Gravity systems, and in a 3 port version for Pumped systems. The table opposite details the sizes available in the various types.

To ease installation, the sensor is designed for "clamp-on" mounting, using a strap and adhesive pad provided. For situations which demand immersion sensing, an immersion pocket. or capillary gland seal is also available.

A low temperature version, type RAVK is also available and is particularly suited to under floor heating applications. For suitable valve bodies, refer to small seated valves section.

Simple, self acting control, requires no wiring Versions available for Pumped and Gravity systems Two port Pumped versions in 15 and 22mm sizes Two port Gravity versions available in 22 and 28mm sizes Three port Pumped versions available in 15mm or 1/2" BSP sizes Prevents scalding hot water

[IMM]

Not cheap though. Se my post of this.