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Small Radiator Connected to DHW and Hot Water Tank

Postby giomach » Mon Jan 23, 2017 10:33 pm

We have an oil-fired boiler with separate programming for ground-floor radiators, first-floor radiators, and domestic hot water.

We recently had a very small radiator fitted in our very small downstairs toilet, and (presumably because it was most convenient for him) the plumber connected it to the DHW. That is, the radiator is connected to the hot water tank by two (short) 15mm pipes.

The problem is that this radiator does not heat up, except when the tank thermostat has triggered the DHW motorised valve, so that the pump and boiler are on. But as soon as the tank thermostat opens again, the radiator rapidly cools down. The result is that it is warm only for a short period in the middle of the night! The rest of the time, we have a tank of hot water and a cold radiator, without any movement of water between the two (they are at the same height).

People tell me that the radiator should be removed from the DHW circuit and reconnected to one of the central heating circuits. I'm looking for a simpler alternative, though.

To move water between the hot tank and the radiator, why not a separate pump? Electrically it could be driven by the live wire coming from the hot water programmer, via a wall thermostat to be installed in the small toilet. But what sort of pump? It should connect to 15mm piping, would be active for short periods only, would need to pump only the capacity of the radiator each time, through a short length of piping, with no gravity to overcome. Or is this a crazy idea (if so, why?)

An alternative possibility is to wire the new wall thermostat in parallel with the tank thermostat, to drive the DHW motorised valve. No new pump needed. The obvious danger is that the water tank could overheat while the radiator is being heated, but the speed at which the existing pump would heat the radiator with hot water from the tank could avoid this happening.

Thanks for your thoughts, on either of the above suggestions.
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