I thought this was worth a punt on here.

I currently have a oil fired Rayburn 499k which runs radiators and underfloor heating. The issue I have is that ever time the boiler fires the radiator pump comes on. So if the underfoor heating stat says give me heat I get warm radiators. My spark believes that is correct because there is a danger of water overheating in the boiler if it isn't pumped away and he's afraid that when the underfloor valves close there will not be anywhere for the hot water to go. Does that sound correct? Is there anyway around it?

J

Do you have a separate hot water tank with a cistern above it and a vent pipe ? Surely the domestic water can heat up without the heating coming into play so the hot water can go there instead.

I have the older 480K, and the instal guide definately says the pump should run when the heatings on (and also if the temperature is above a certain value, even if its not firing at the time - this is a built in trip). There was a minimum heat loss radiator but I don't recall the size (e.g one that does not stop giving out heat so no thermostat), so that there will always be a way to loose some heat if all the thermostatic ones stop using any heat - I guess because the cast iron construction means it holds a lot of heat, so continues heating for a while when the heat turns off.
Regards
Hugh

Thanks guys.

Minimum heat loss radiator somewhere sounds like the way to go.

There was a similar problem on a job i was on last week, basically the radiators making the room stat turning off the boiler before the underfloor heating had got hot, so the half of the house that only had underfloor stayed cold.

Think you need a plumber who is familiar with how rads and underfloor coexist on the same heat source. There needs to be some way that either circuit can call water without the other getting hot.

We have had a coupe of houses with underfloor heating, and in both cases, the underfloor, the rads and the hot water circuit have always been able to call for heat separately through the use of zone valves.

Each has completely different characteristics. The troube is that some of the plumbers we have had, have had less of a clue than me, and I work in IT...

It used to be the norm with traditional boilers to always have 1 radiator to sink heat, the preferred one being the bathroom towel radiator so this would be locked open and as a room typically away from the centre of the house would not affect the main stat.

[Edited on 7/12/12 by mark chandler]

Spoke to my mate about this today, he says it needs zone valves and proper wiring back to the boiler so that both can operate independantly. Also, apparently its more common nowadays to have a bypass valve rather than a radiator on perminantly.

I guess bypass valves are simpler, but rely on the boilers ability to turn the wick down in the case of the return water being too hot.

In any case an underfloor and rad system definitely requires two zone valves as the properties of the floors and rad are way different. Rads run at 60c ish and floors run at 30c to 50c. They usually have manifolds with a wax stat type blending valve to keep the floor water temp down.

Edited to say, floors usually take a couple of hours to warm a room. Much slower than rads, but then you have a massive warm brick under you keeping your tootsies warm.

[Edited on 8/12/12 by Xtreme Kermit]