thermo mixing valve on heatbank

been studying a colleague's heatbank setup.
the heatbank has an adjustable thermostatic mixing valve on the boiler return, that appears to mix water from an tapping near the base of the heat bank with water from a tapping at the top of the heat bank....presumably to control the temp of the boiler return to get the boiler into condensing mode all the time.... but why is it needed? why not just take the boiler return from the bottom tapping full stop?
also it has two electrically interlinked thermostats vertically spaced but only by about 100mm (heatbank is abt 2000mm tall) both are set to 60 degC ?
Seems only way for heat to be called is by using the HW channel on the programmer. CH side of programmer causes radiator circulation pump to run but not call for heat (even when hbank temp down to 40deg C on gauge)???
Anyone else have a heatbank and recognise any of this? or has he got a lemon ?
jim
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On Sun, 03 May 2009 07:29:48 -0700, jim wrote:

There are various widely different heatbanks around. What's your colleague's make & model?
--
John Stumbles -- http://yaph.co.uk

Thesaurus: extinct reptile noted for its wide vocabulary.
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jim coughed up some electrons that declared:

The idea is to promote rapid heating of the top of the store to the final temperature.
I'm not using a mixer in that position - instead I'm using a 3 speed pump to the boiler. I'm hoping to find a speed that allows the boiler to dump its full output (24kW) into the slowest moving water, which effectively achieves the same thing, though if successful, my approach ought to feed cold water for as long as possible into the boiler return, promoting max efficiency from a condensing boiler.
We'll see...
Cheers
Tim
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For rapid reheat.

60C? must have lower higher than upper. They are to prevent boiler cycling. Have bottom at say 70 and top 55 to 60C.

About right. The DHW keeps the cyl hot. The CH runs the CH rad pump. You can have a pipe stat set to say 50C on the CH flow from the cyl to prevent cool rads.

No, far from it. Just set the stats properly. Tell him to put in the correct amount of inhibitor. He could also have TRVs on all rads and Wilo Smart pump on the rad circuit.
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ah ok - what should he set the return temp at for optimum performance?

can you talk me through how that works please?

do you mean a pipe stat could call the boiler (and boiler circ pump) if the temp of flow to rads dropped below 50deg?
thanks jim
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jim coughed up some electrons that declared:

Viesmann told me optimum return was 45C or less, with progressive loss of efficiency for higher return temperatures.

Top stat gets cold, boiler fires. Relay interlock requires bottom stat to get hot (a while after top stat has got hot again) before disengaging boiler.

No, the rads and the reheat of the tank are in different control domains.
All the tank has to worry about is staying hot - to this effect it controls the demand for heat to the boiler.
The rad control circuit only has to respect the timer(s) and room stat(s) - this signal normally fires a boiler and optionally switches a control valve to send hot water to the rads. With a heatbank, it merely controls the rad pump, assuming that the tank always contains hot water. It indirectly fires the boiler when needed by depleting the tank of heat.
If you have underfloor heating, duplicate the radiator control system and pump, add a mixer for cooler water (40 ish flow rather than 60-75 ish).
Tap hot water is also independant, the pump being controled by a flow switch.
This isn't to say that you can't have a smart controller that takes account of outside temp, date, general rad demand, expected output from solar, time of day and stuff and overrides all the systems to perhaps:
a) Limit the boiler input when strong solar input is expected later in the morning (ie leave some cold water for the solar to heat)
b) Up the boiler input if it's a cold mouldy day and maybe a weekend when higher demand from radiators is expected;
c) Goes into minimal frost prevention for 2 weeks when you're on holiday
and so on.
HTH
Tim
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The blending valve is usually set at 54C. The idea is to give what the boilers thermal temperature rise is. If it is 30C and store temp of 75C is needed, the return temp should be set should be set at 45C. A little testing of flow and return temps is needed. The boiler is set to max temp.
Having a large boiler, say 45-55kW, a blending valve is not needed in a domestic store. It has enough temp rise to cope, and reheats ASAP, like el Zippo. The store doesn't care what size the boiler is, and it will not cycle. Some large boilers can be bought reasonably cheaply. or use two 24kW boilers, both having their own flow and return connections to the store, both controlled by the cyl stats.

Cold start: store heats up top down to 75C. 75C hits the first stat down set to 60C - stat is ignored. 75C hit the bottom stata and boioper switched out.
Store cools bottom up: bottom stat is ignored. top stat gets below 60C and brings in boiler.
No boiler cycling as boiler heats in one long efficient burn.
Having both to 60C will still give a lag, well with 1C between. Work through the operation.

The pipe stat holds off the CH pump pumping to the rads. Above 50C the pump comnes in. Fitr pipe stat near the boioer sop herat floats out of cylinder into pipe. A cylidner stat at the point of the rad flow tapping is better.
Most do not find the rad pump pump a problem. The boiler will come in and re-heat ASAP.
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interesting - additionally there's a further "overheat" stat near the top of the cylinder set at 85 degC - would it be actually possible to satisfy the two stats (only 4" vertically" apart, half way up the cylinder) without triggering the overheat stat and (as i understand it) dumping the "excess" heat into the rad circuit?
if that happened the boiler would be firing most of the time (to try to satisfy the two anti-cycling stats) and the heatbank would be dumping heat into the rads (even if the CH programmer channel + room stats were "off") to stop the overheat stat from being triggered.....??

so presumably *only* the "HW" channel on the programmer plus the thermostats, control when/if the boiler fires? the heatbank thermostats *should* limit cycling of the boiler
so real question is - with the location, spacing and settings of the two "anti-cycling" stats, can the heatbank work as it should and limit boiler cycling without overheating and dumping heat to the rads??
any thoughts on how we could go about testing the theories please?
thanks jim
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wrote:

The boiler can only give out 82C. The top stat is a high limit only.

Not sure what you mean. The CH pump only comes on when the programmers say it can..,.and if a room stat is fitted and says it can.
The DHW must be on to get hot water for CH.
The boiler is controlled only by the two anti-cycle stats. The boiler does fire most of the time at all. The bonier is off most of the time. It heat the cylinder and only comes in when the heat is exhausted to an useable level and re-heats in one burn.

Yep. The stats prevent cycling. The prime purpose.

Yes. set to 70C at the bottom and 55C at the top and see what happens. In summer you get away with 65C if using a plate heat exchanger.

Just set as I say and see how it goes.
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will give it a try thanks NB the overheat dump on his setup is the CH circuit so I expect that if ever triggered the overheat stat must activate the CH pump ...
jim
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wrote:

sounds that way. It may activate the DHW as well to take heat from the top to the bottom. Could be two-stage: 1. DHW pump, 2. CH pump. Having two stage means that heat stays inside the cylinder, unless there is a lot of heat input then it gets rid to the rads.
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The blending valve is usually set at 54C. The idea is to give what the boilers thermal temperature rise is. If it is 30C and store temp of 75C is needed, the return temp should be set should be set at 45C. A little testing of flow and return temps is needed. The boiler is set to max temp.
Having a large boiler, say 45-55kW, a blending valve is not needed in a domestic store. It has enough temp rise to cope, and reheats ASAP, like el Zippo. The store doesn't care what size the boiler is, and it will not cycle. Some large boilers can be bought reasonably cheaply. or use two 24kW boilers, both having their own flow and return connections to the store, both controlled by the cyl stats.

Cold start: store heats up top down to 75C. 75C hits the first stat down set to 60C - stat is ignored. 75C hit the bottom stata and boioper switched out.
Store cools bottom up: bottom stat is ignored. top stat gets below 60C and brings in boiler.
No boiler cycling as boiler heats in one long efficient burn.
Having both to 60C will still give a lag, well with 1C between. Work through the operation.

The boiler heating the cylinder is independent of DHW or CH demand. The pipe stat holds off the CH pump pumping to the rads. Above 50C the pump comes in. Fit pipe stat near the boiler sop heat floats out of cylinder into pipe. A cylinder stat at the point of the rad flow tapping is better.
Most do not find the rad pump pumping a problem. The boiler will come in and re-heat ASAP.
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