I have installed an hot water system which links central heating, solar and wood stove heating through a central 300l hot water cylinder. It is of my own design based on extensive reading and boring friends to extinction . I need advice on using a Resol ES[1] for the control of the central heating.

To my mind the system is simple, and depends heavily on stratification[2]. The cylinder was made by McDonald engineers[3] (NB the choice to use them was arbitary other suppliers were willing to provide this service) it involves 4 exchanges and an immersion heater. The (pressurised) solar circuit is in two layers with two further exchanges (vented) for central heating and the wood stove. The top 100litres is used for domestic hot water (DHW) and the remaining is either pre-heat or store depending on how you look at it (or the time of year).

In summer the cylinder works (well) by the solar doing what it can, the upper layer is set to 60degrees and what sun is left is used to heat the rest. The resol controller also controls an immersion heater to heat the DHW (top 100l) to 47degrees if the tempriture is below 43. All this operation works well and the gravity fed shower is one of the best I have experienced, we have been running this since mid April. I don't worry about legionella because the DHW temprature exceeds 50 often enough and I haven't had a dead leg since I was in high school.

In winter I wish to use the wood stove, this heats all but about 50l. It is based on a simple gravity fed, vented (cylinder <-> burnoff radiator <-> stove) circuit. This avoids the need for a neutraliser. The tricky bit is the pumped central heating (CH) circuit which extracts heat from the cylinder. The CH exchange sits above the wood stove exchange but below the DHW. In normal operation the central heating will be taking out of the cylinder what the stove is putting in.

There is a function of the controller that I am using which will burn off surplus energy for a period of time. I.e. 6am-10pm if the cylinder temprature (at the CH exchange level) exceeds 60degrees pump the central heating; if temp. drops below 55 stop the pump. This means that at 10pm, when we go to bed, we store the unused heat in the cylinder. At 6am when the stove is cold there is some stored heat to knock the chill out of the house. The controller function which is capable of this is currently being used to turn the immersion heater on.

What I would like to do is turn the central heating on as part of a overheat function say at 95degrees reguardless of time of day or year. I would also like to avoid paying for a seperate immersion heater controller. I have contacted the supplier...again..

Cheers,

[1]http://www.resol.de/en/english.shtml
[2]http://www.veryefficientheating.co.uk/solar/article2.htm
[3]http://www.mcdonald-engineers.com/

I think you might be putting too much thought into it...

I assume what you are trying to do is to get the central heat to draw some of the heat off the cylinder automatically if it was ever to get too hot?

Why not just use a normal cylinder stat to switch the central heating circulation pump on?

Cylinder stats usually have both normally open and normally closed contacts, and go up to around 80deg (have seen Hortsmann ones that go upto 90deg)

Just wire it up like you would a frost stat, use a permenant live feed such as the one that feeds the controller, and use the normally open contact on the stat to switch the pump, set the temp to 80deg and it will just switch the pump on when it reaches that temp.