Boiler controls

OK so you can prioritise hot water with a simple diverter valve but don't jump in with a Heatmiser UH4 set up!
The R. Cott. heating system now works but wasted 2 days of electrician time.
Detail available if anyone interested.
Further question arising from a perceived risk to underfloor pipe work if the mechanical thermal regulator fails.....
The suggestion is to fit an *over temperature* cut out to the manifold pump. Not good for me because the diverter valve circulates water at 70 deg. plus during overrun when the DHW stat opens. My Viessmann boiler offers weather compensation and I am wondering if this can be adapted to give a *set back* output temperature for the underfloor system. Easy with a combi but system boiler...?
--
Tim Lamb

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On Sun, 20 May 2018 21:45:02 +0100, Tim Lamb wrote:

Presumably a thermostatic mixer valve fo some sort. Pretty damn reliable IMHO.

I'm assuming the "manifold pump" is a seperate pump just for the underfloor heating system and there is a another pump doing the overrun. If so a pipe stat set to 50 C (?) mounted on the output side of manifold pump (or input to the manifold) with its normally closed contacts in series with manifold pump motor? Crude but effective. Thinks a bit more, better to put the contacts in the underfloor heating stat circuit so if the water to the floor gets to hot it turns off the demand thus stopping the pump and boiler. This would obviously cycle as the pipe cooled and the pipe stat contacts re-closed but ought to limit the max temp of the water in the underfloor system. You could make it latch to off by adding a relay but then you'd lose heating until reset. A latching indicator that the pipe stat has operated might be useful otherwise you may not notice that it has/is.
--
Cheers
Dave.
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Dave Liquorice used his keyboard to write :

That idea will do that, but not a good idea so far as the boiler is concerned, to frequently cycle it like that.
My boiler has only one boiler output temperature setting on it (New heat only type). I wanted separate settings for output temperature for CH v HW, so decided set the boiler to max, then have a separate stat on the pipe for when the demand was for CH only. It caused rapid boiler on / off cycling and defeated the boiler's output modulation system.
The pump run on, quickly pumped the cooled water back round, so the boiler fired up again, the pipe stat opened, stopped the boiler and so on as it repeated until the room stat was satisfied. I quickly removed the pipe stat.
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On Mon, 21 May 2018 10:36:01 +0100, Harry Bloomfield wrote:

Agreed, I'm having similar battle but the otherway round with the boiler and thermal store, the max temp from the boiler is lower than what I'd like the thermal store to be for the CH circulation.

Setting to max was probably the mistake, as it runs and slams into the overheat stat rather than detecting it's getting close to the set point and modulating back.

Thinking a bit more the "don't let the floor get too hot" pipe stat ought to be post mixer valve pre manifold so it monitors the actual temp of the water circulating around the floor and never actually gets hot enough to open. Rather than the flow into the underfloor system from the boiler, pre mixer valve.
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Dave.
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Dave Liquorice expressed precisely :

No, it modulate perfectly set at its 75C max on both CH and HW, it was just that 75C was unnecessary for the CH and less efficient/ above condensing temperature.
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On Mon, 21 May 2018 14:28:35 +0100, Harry Bloomfield

75C will damage small fingers.
http://www.bsee.co.uk/safe-heating-for-vulnerable-people/
--

Martin in Zuid Holland




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After serious thinking Martin wrote :

75C is the boiler output temperature, the HW in the cylinder will be less than that and the small fingers only use a thermostatic mixer tap set to max out at 50C.
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On Mon, 21 May 2018 17:02:37 +0100, Harry Bloomfield

CH not water coming out of the tap.
My grandson burnt his fingers on a radiator with a thermostat on it. The radiator reached the water temperature before the thermostat shut off the flow.
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Martin in Zuid Holland




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A TRV is a *room* thermostat, not a radiator thermostat.
I did some experiments when I first installed my central heating, and they are incredibly accurate - the TRV4's were achieving a very stable temperature with only around 0.3C drift, which is well better than most wall stats manage. I had been rather sceptical of them until I actually measured their performance.
BTW, nice thing about my heating system was a) I designed it for efficient condensing operation, i.e. large radiators which therefore don't need to be very hot, and b) The beta version of the Myson heat loss calculator I used over estimated all the building material heat loss values, probably deliberately to make you buy bigger Myson radiators than you really needed.
This actually meant the system is even more efficient than intended. The net effect was the heating system will keep the house warm at -3C outside running at 45C flow temperature, and you can't really burn yourself on that. (This is ideal where young children or elderly are around.) However, I can bang it up to max to heat the house up from cold extremely quickly, fast enough that it's not really necessary to turn the heating on before ariving home (although I can do that too). The boiler's max flow is 83C, although I can't get it quite that high with all the radiators on because they would give off more than the boiler's max power output at 83C. IIRC, I can get them up to around 77C at max boiler output.
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Andrew Gabriel
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New to me:-) With two manifolds, I have two mixers.

That is the right advice. However, thermosyphoning (if that is the word) leads to the upstairs manifold slowly reaching flow/return pipe temperature. The mixer valve is directly coupled to the pump and close to the only practical place to fit an overtemp stat.
I am hoping a *set back system* on boiler output might be easier.

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Tim Lamb

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On Mon, 21 May 2018 10:47:38 +0100, Tim Lamb wrote:

side

closed

Ah? Presumably there is a vertical run somewhere to feed the upstairs mainfold? Bunga gravity valve in the flow somewhere.
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Dave.
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