Fast reacting pipe-stat

[Fredxx]

I'm currently using a conventional pipe-stat so the water flowing from the boiler is above a certain temperature. Unfortunately the lag is probably measured in minutes, and so the boiler shuts off before the pipe-stat operates. It has a bypass for when valves are close.

Of course the pipes-stat then switches on but there is a 5 minute wait for the boiler to come back on and in the mean time the pipe-stat switch opens from now circulating cold water and the cycle repeats.

Is there a fast reacting pipe-stat? I don't really want to build my own.

[Fredxx]

Just to clarify, the pipe-stat is used to operate a motorised valve, when the water is above a certain temperature.

[Brian]

We recently had an issue with our CH. As we have a service contract, we just call the company. I usually take an interest in the repair- not because I don?t trust the company, we are very pleased with them. The recent fault led to a very interesting briefing on how different boilers sense water flow, temp, etc. ( Our problem was a flow sensor.)

One thing which seemed obvious, not only from what the technician told me but from the symptoms when it failed, the water in the boiler gets very hot

- hot enough to trip overheat sensors- very quickly if there is a flow problem.

From the briefing, all boilers seem to have some system to check there is water flowing in the heat exchanger / ch circuit and also have an over temp sensor.

My guess would be, the over temp sensor in the boiler is just too sensitive / quick for your design to work - it will always trip if, as I think you are trying to do, you attempt to limit the flow until it reaches some temp you are measuring elsewhere in the CH loop.

[Fredxx]

There is a pump and bypass valve that always ensures a minimum flow in the boiler, even when the motorised valves are shut. The return gets very hot, such the boiler cannot modulate low enough.

I don't believe this is down to boiler sensitivity.

[SH]

it sounds like that the temperature sensing part within the pipestat is outside the pipe so you have thermal mass and thermal resistance to consider.

You might be better off in putting in a T piece and then fitting a type K termocouple designed for direct liquid immersion to overcome the temperature lag/delay issue and then an interface circuit to allow teh Type K thermocouple to interface with the motorised valve.

[Davidm]

Are you sure that the bypass valve is not stuck open. I've had 2 of the Honeywell auto bypass valves fail in this way in the past few years, so that the return gets very hot.