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Hi I read this description of a stat and would like to know the name of any makes / models out there to help me source one.
"One pipe stat which is on the return pipe to the back boiler on the WBS. The normal position for the pump is OFF when the return temp gets up to 60 deg C the pump starts, the pump stays on until the return temp drops blelow about 45 deg C. This is a characteristic of the switch and is called dead band".
I currently have a wbs, thermal store, loading valve for heating rads and hot water - I plan to add solar when funds allow. This small tweak would be cheap and a interesting experiment for my heating setup.
What Damon said. All mechanically operated control switches have this characteristic - be they thermostats, pressure switches or alike. Its a function of the way the mechanical switches contained within the unit work.
15 deg.C sounds a bit wide, most of the pipe thermostats I have used seem to have between 5 -10 deg C hysteresis. I do have some pressure switches on my water system that have adjustable hysteresis provided by an adjustable bias spring but I have never seen this feature on a pipe thermostat.
If you get a mechanical pipe thermostat and turn the dial you will hear a click as the switch operates, turn it back the other way and you will hear another click. Note the temp. reading on each click, the difference between the two is the hysteresis.
Unless you actually want 15 deg C use a normal pipe thermostat with the typical 5 -10 deg C hysteresis. The usefulness of this characteristic is that it prevents short cycling of motors which can cause them to over heat and will shorten their life.
Thanks for your comments - I have only purchased two stats so far a flue stat and a cylinder stat learning as I go - When other jobs are done making build better prepared for winter I shall seek to improve my controls further, for now a pipe stat with 10 deg hysteresis will do fine :)
Thanks C watters - I have been looking but not found many alternatives - only pictures of circuit boards. The above link is a good start. Peter above has also advised that a regular pipe stat can achieve 10 deg hysteresis I have yet to purchase one. When reading info about different stats some do mention hysteresis others a temperature differential, would the differential be referring to the same thing ?.
It is good to be aware of my options and it makes for an interesting read when the weather is bad.
If you want to control things on and off over a wide temp range then you can easily do it with two stats – one wired to close on temp X and one wired to open on temp Y– depends which way round you want to go. Some stats come with contacts NC and NO so you just get 2 of the same and wire them to effectively reverse what the other has done at the other temp. Sounds confusing like that but work out which is to close to start something at which temp and which is to open to stop something and at which temp.
Although a different application taking for example an oil boiler driven thermal store. The upper stat might be set to close at 40 deg (falling temp) i.e. fire the boiler. When the store is depleted the coldwater has come to the top so upper stat closes and boiler is on. The boiler runs till the delivered hot water reaches the lower stat which is set to open at say 70 deg (rising temp) thus the boiler stops as the hot water has charged the store top to bottom. You need to wire it such that when the store starts to give up heat the cold / hot interface will rise thus the lower stat will re-close, you don’t want the boiler to fire again so it needs to be latched such that it doesn’t close again this time and re-fire. Likewise, when firing the hot water passes the top stat (it would open again) so this is latched in, till the bottom stat breaks the supply. Takes a bit of thought to get the logic, and isn’t easy to describe long hand, but once done its straight forward, if its wrong it runs only on one or the other and is useless.
This will work for anything that needs a wide temp span. The actual values don’t matter too much so long as there is a decent difference between them.
Failing that numerous diff temp controllers exist (similar to a solar controller) for doing all sorts of things heating-cooling etc but they cost a bit so better to build one.
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