Digital cylinder 'stat?

Just a musing really. With the large number of digital (programmable - but that's not really the issue) *room* thermostats now on the market at reasonable prices, have I missed something, or is there a reason why no-one seems to be producing a digital *cylinder* thermostat?
Surely with one of these you could do all sorts of clever things like... ummm... pre-firing the boiler as hot water is drawn off but before the tank reaches the "low" setpoint... have two sensors to allow day-period variances (e.g. keep the top 1/3rd hot during the day, but fill the tank in the evening)... allow user choice over "hysterisis" (or however you spell it) so that boiler cycling is minimised... and probably loads more that I haven't thought of.
This is the point where someone either tells me it's been done and is available or tells me that there is no need to ditch the bimetal thermostat because...
:-)
Hwyl!
M.
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Martin Angove wrote:

Get the water piping hot for when you come home from work at the pit but set the temp lower for night and daytime when you only want to wash the dishes, or somesuch. Looks like an idea with scope.

I think I saw one once but couldn't find it when I trawled the usual websites.
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But why ? Do you need to check the exact temperature of the water ?

You can have thermocouples installed around the system that feed back to a selectable LCD display. This lets you check the temperatures at various points around the whole system. I've only ever seen them used in commercial systems though.

No. Can't say I've seen anything other than the bimetal strip being used again the outside of a cylinder wall. As I said above, I've seen systems with thermocouples placed in tanks and on pipes, that feed signals back to a display board. But never in a domestic system.
There is a product called a boiler manager:
http://www.deeter.co.uk/boiler.htm
which we tested for Deeter when in prototype, and have since installed in a few mainly local authority buildings. It checks the flow temperature from the boiler against the return temperature, and only allows the boiler to fire when the difference is just 3 degrees C between them. It is fitted across the main boiler stat', so doesn't really take anything away from the control of the boiler. You set it manually to the top temp' and it automatically controls the lower difference itself.
I've been told it has saved two childrens homes we visit regularly, loads of money on their bills over just the first years running period. I have one fitted here in the house, and between the the first two years of having the heating system and the second two years after fitting the manager, we noticed a saving of at least half on the running costs. And that's no joking. We haven't noticed any difference in the atmosphere or the hot water supply, so it is basically only controlling the boilers firing cycles.
It is an expensive piece of kit to buy though, we got ours because of the testing, but you should be able to get the outlay back withing two years of running the system with it attached.
Boy can I go on a bit. :-))
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This is just a load compensation controller. You say it fires when the flow/return delta T (differential) is below 3C. This would just cycle. Having a brief read of the front page, I'm sure it does not do this.
Load compensation controls are now an integral aspect of many condensing boilers. They do the same thing by sensing the flow and return. If for e.g., the boilers delta T should not be more than 22c, then it will not allow the boiler to fire until it can give a flow of 62C, if say the return is at 40C. Also if a system is fitted to a radiator delta T of 11C then it will not allow the flow temp to be less than 11C higher than the return. This way the boilers heat exchanger is protected and the flow and return delta T is never less than 11C.
That is the basics, but many makers do it differently.
Thermal stores/heat banks reduce boiler cycling when two stats are fitted to the cylinder - preferably electronic stats. Only when approx 80-90% of the cylinders heat is exhausted will the boiler fire, then one long, continuous, efficient burn until the store is fully heated. Two stats can do this on a cylinder, and with a quick recovery coil in any type of cylinder using two stats can also promote efficiency.
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Yeah. Something like that.
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This has been around for about 10 years. I don't know the prices: http://savastat.co.uk
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I do now. 170 + VAT
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"at least half". Have you took measurements? data over periods when fitted and not fitted? What type of boiler do you have. On a condensing boiler I can see big gains. On a regular boiler, gains in that only boiler dry cycling is reduced. In a regular boiler you don't want the return temp to drop low as condensation may occur. Anti-cycling can be achieved via a simple electrical timer. Just delay re-firing for may 5,6,7 mins or so.

A Savastat is 175 + VAT which I think is steep for domestic. The Savastat only has a return pipe sensor, relying on sensing "rate of temp change" to judge the load of the system. The Savastat will work well with basic commercial systems, which is its main target market.
The Deeter has a sensor on the flow and return which is more encouraging. But it depends on how they have programmed it. How much is this unit?
I can see both of the above giving gains with a modulated burner boiler that modulates to maintain a set flow temp, as do combi's. Just keep it on full temp and let the load compensator stat do the rest. That also includes combi's. But there must a way of cutting out the load compensator to allow full heat for DHW. Fitting one on a boiler that already had load compensation built-in is needless.
With a simple cheap condensing boiler, I would go for an outside weather compensator. The Danfos model is 166 +VAT from discountedheating. This is a good unit and takes into account the cylinder stat and that the boiler needs to be on full heat when re-heating water. It senses the return pipe temp to keep the boilers return in key with the outside weather.
Better units than the Randall, although more expensive, have inside temperature influence, in that if the room temp is being approached it drops the return temp even further and if over room setpoint by a degree or so cuts out the burner. Anti-cycle control is also built-in. Very effective. I know I have one like this.
http://www.danfoss-randall.co.uk/prod_bem5000.htm

The BEM 5000 is a sophisticated electronic domestic boiler energy manager which when added to almost any central heating system. new or existing, will reduce unnecessary boiler cycling. improve boiler seasonal efficiency and reduce running costs.
The BEM 5000 integrates the operation of the heating system and domestic hot water systems and includes a pump overrun.
The unit is a weather compensator which during periods of heating demand varies the temperature of water flowing to the radiators as a function of the outside temperature The technique which is well established in the commercial market. adds significantly to system efficiency and reduces running costs in the process. During periods of hot water demand. the weather compensation function is cancelled to enable the cylinder to be reheated as quickly and efficiently as possible
The BEM 5000 is an all electric solution which can be installed without the need to drain down the system, making it an ideal system upgrade option
Improves system efficiency Provides electronic boiler interlock Low cost weather compensator Provides HWS priority Built-in pump run-on control User adjustable <<<<
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I use an RF one. Saved me from running a cable. Danfoss WP75RF. Rgds

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Can't really see any benefit. With a well lagged tank I don't think you'd separate the top third from the full tank for the time scale involved. And my boiler cuts in before the hot water is exhausted anyway.
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