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Do I keep the thermal store

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Re: Do I keep the thermal store

Postby RogerS » 30 Mar 2017, 08:29

mmmm...I had come round to rethinking this and deciding to keep the thermal store. One of the reasons being that 'best practice' is to locate it within the thermal envelope of the property. It isn't in the house but it would have been inside what was to be my workshop.

But given the decision to PISIAM ...'Pimp it, Sell it And Move'...I think the motto must be 'KISS'. Much as I like the intellectual exercise of working out the extra size required for the radiators in a low-temperature system, the extra cost does not warrant it.

KISS...
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Re: Do I keep the thermal store

Postby RogerS » 01 Apr 2017, 06:17

DD here ...or Dithery Daisy as LOML likes to call me.

Over lunch yesterday, I talked this through with her and what level of heating did she want. Answer....low background heating to keep the damp out of the house but spot heating in a room that she was in. Which appeals to my renovation principle of TWS (Tight Wallet Syndrome) since I don't need to buy larger radiators which saves cost.

And keeping the store does future proof things. And whatever heat leakage will be inside my thermal envelope. And I've already got it. Plus it will be fun working out the controlling system...maybe, at last, an opportunity to do something with those Raspberry thingies.
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Re: Do I keep the thermal store

Postby RogerS » 16 Nov 2017, 12:23

Doesn't time fly.

I went round yesterday to the originator of using the heat store with an oil boiler to look at his controls etc. He uses a Laddomat (which I don't have or intend to get) and that does his controlling. But essentially there is a sensor (A) at the top of the tank and one at the bottom (B).

While the setpoint at A has not been reached, the boiler is fired up. It stays fired up until setpoint B has been reached whereupon the boiler stops. Of course, while this has been going on, the temperature at the top of the tank has gone past its setpoint (A).

As heat is drawn out of the heat store, the temperature at the top will gradually drop until it goes below setpoint A when the cycle repeats.

I asked him why bother with the top setpoint and after some thought he came to the conclusion that i would work just as well with only the bottom setpoint as the controlling mechanism.

I'm not sure. The whole aim of using the heat store is to have very few but long condensing burn cycles from the oil boiler. My gut feel suggests that if there is little hysteresis in the bottom sensor then it will be demanding heat on/off quite frequently. Having the top sensor as part of the controlling mechanism means that there is a very large hysteresis inherent in the system.

What does the team think ?

Assuming that both sensors are needed then any suggestions for a simple interim solution using relays ?
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Re: Do I keep the thermal store

Postby 9fingers » 16 Nov 2017, 12:42

RogerS wrote:Doesn't time fly.

I went round yesterday to the originator of using the heat store with an oil boiler to look at his controls etc. He uses a Laddomat (which I don't have or intend to get) and that does his controlling. But essentially there is a sensor (A) at the top of the tank and one at the bottom (B).

While the setpoint at A has not been reached, the boiler is fired up. It stays fired up until setpoint B has been reached whereupon the boiler stops. Of course, while this has been going on, the temperature at the top of the tank has gone past its setpoint (A).

As heat is drawn out of the heat store, the temperature at the top will gradually drop until it goes below setpoint A when the cycle repeats.

I asked him why bother with the top setpoint and after some thought he came to the conclusion that i would work just as well with only the bottom setpoint as the controlling mechanism.

I'm not sure. The whole aim of using the heat store is to have very few but long condensing burn cycles from the oil boiler. My gut feel suggests that if there is little hysteresis in the bottom sensor then it will be demanding heat on/off quite frequently. Having the top sensor as part of the controlling mechanism means that there is a very large hysteresis inherent in the system.

What does the team think ?

Assuming that both sensors are needed then any suggestions for a simple interim solution using relays ?


I'd consider using the top sensor to start the boiler run on cooling below setting and the bottom one to stop it on rising above setting.
To make best use of the stored heat at the end of the heating day, arrange to stop the boiler a predetermined time before system shut down. Remember that storing hot water is very inefficient over periods when you don't need it.
Our heating switches off about 9pm and we then open the lounge door (thermostat is in the lounge) to let some heat go upstairs to the bedrooms

At start up in the morning, you could arrange for the store to be bypassed until the first occurrence of the demand in the house being met.
It just depends on how far you want to go with economy measures.
You could also scale the store settings back according to inside/outside temperature differential.
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Re: Do I keep the thermal store

Postby RogerS » 16 Nov 2017, 13:16

9fingers wrote:I'd consider using the top sensor to start the boiler run on cooling below setting and the bottom one to stop it on rising above setting.


That's kind of what I had in mind but not sure how to do the 'clever' bit !

9fingers wrote:To make best use of the stored heat at the end of the heating day, arrange to stop the boiler a predetermined time before system shut down. Remember that storing hot water is very inefficient over periods when you don't need it.


Good point and well worth considering. Having said that I do know that the tank is incredibly well insulated.

9fingers wrote:At start up in the morning, you could arrange for the store to be bypassed until the first occurrence of the demand in the house being met.


Plumbing doesn't allow for that.

9fingers wrote:You could also scale the store settings back according to inside/outside temperature differential.


Given the degree of insulation, I'm tempted just to keep it simple TBH. Hell of a lot of thermal inertia in 2500 litres of water.
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Re: Do I keep the thermal store

Postby 9fingers » 16 Nov 2017, 13:27

Not exactly clever but depends on the contacts provided on stats A and B?
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Re: Do I keep the thermal store

Postby RogerS » 16 Nov 2017, 13:38

9fingers wrote:Not exactly clever but depends on the contacts provided on stats A and B?
Bob


I have two Elitech STC-1000 gizmos.

Each has one set of normally open contacts (demand for cooling) - energized when

measured temp ≥ set value + difference value (which is setable from 0.3 to 10 C)

and a second set of normally open contacts (demand for heating) - energized when

measured temp ≦ set value - difference value (which is setable from 0.3 to 10 C)


contacts rated 10A 220Vac
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Re: Do I keep the thermal store

Postby 9fingers » 16 Nov 2017, 13:50

Will draw something later.

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Re: Do I keep the thermal store

Postby RogerS » 16 Nov 2017, 15:07

9fingers wrote:Will draw something later.

Bob



Thanks Bob
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Re: Do I keep the thermal store

Postby 9fingers » 16 Nov 2017, 15:12

Try this:-

Rogers Diagram.jpg
(76.43 KiB)


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Re: Do I keep the thermal store

Postby RogerS » 16 Nov 2017, 15:29

Thanks Bob.

I'm not 100% convinced but probably my misunderstanding.

Assume tank totally cold. Both top and bottom close, DPST relay energised and one contact provides a permanent feed to keep the relay energised once the Top Close has reached temperature and released.

Finally when Bottom Close is up to temperature, it breaks the circuit and the DPST relay drops out.

Heat gradually taken out of the tank. Top Close gets energised but Bottom close stays open (IF it's temperature is still fine) and so the DPST relay doesn't get energised until Bottom Close temperature falls.

Have I got that right ?

Having said that, his two set points were both the same (55 degrees) but the actual temperatures were 61 degrees top and 47 degrees IIRC at the bottom. Which means my concern re Bottom Close not energising as it's hot enough...is not valid.
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Re: Do I keep the thermal store

Postby 9fingers » 16 Nov 2017, 15:57

I think it should do what you want ie only refire the boiler once the whole store has dropped below the temp set at the bottom thus stopping the cycling.
Possibly best to start with the two settings the same and yes the actual top temp will likely be higher than the bottom.
Once the bottom has got up to temperature and the boiler is off, the bottom will start to cool and the interface between the cooler and warmer water will rise to the top and eventually trigger the top sensor and the boiler comes on.
The precise performance will depend on the internal design of the store and how well it stratisfies. There should be baffles various inside to stop the turbulence of the water flow destroying the stratification. best aspect ratio is a tall narrow tank but that is not always possible.

I don't have to bother with that on my store as there is no water flow in and out and the only currents internally are from convection caused by heat flow in and out of the heat exchangers. I presume there are no heat exchangers on your store it is simply a large insulated vessel in series with the primary circuit from the boiler.

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Re: Do I keep the thermal store

Postby RogerS » 16 Nov 2017, 16:08

9fingers wrote:I think it should do what you want ie only refire the boiler once the whole store has dropped below the temp set at the bottom thus stopping the cycling.
Possibly best to start with the two settings the same and yes the actual top temp will likely be higher than the bottom.
Once the bottom has got up to temperature and the boiler is off, the bottom will start to cool and the interface between the cooler and warmer water will rise to the top and eventually trigger the top sensor and the boiler comes on.
The precise performance will depend on the internal design of the store and how well it stratisfies. There should be baffles various inside to stop the turbulence of the water flow destroying the stratification. best aspect ratio is a tall narrow tank but that is not always possible.


Many thanks Bob...really good starting (and probably ending!) point for me. It was that second set of contacts on the DPST that gut instinct suggested I needed but couldn't work out where or how.

9fingers wrote:....I presume there are no heat exchangers on your store it is simply a large insulated vessel in series with the primary circuit from the boiler.

Bob


I had to go and check (!) but you are correct. And 2500 litres in a steel tank needs an awful lot of inhibitor. £300 or so. :cry:
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Re: Do I keep the thermal store

Postby 9fingers » 16 Nov 2017, 16:59

The old way of making stores when people were experimenting was to get hold of certain types of redundant brewery vessels which apparently were glass lined and of course no corrosion problems. People used to bury them underground and back fill with insulation to hide them away.

Mine is all copper and brass fittings and I used about 4 x 500ml containers of the concentrated stuff. I put a magnaclean type filter in my primary circuit and am impressed how much rubbish it filters out.
However I expect your boiler has larege bore heat exchangers and so less prone to a modern gas boiler with near zero water content and fine bore HE.

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Re: Do I keep the thermal store

Postby RogerS » 10 Sep 2018, 08:39

So nearly a year on and found a flaw in the basic idea of using the heatstore with the oil boiler namely the need for the odd days with a bit of heat just to take the chill off. If the CH has been switched off then before you can get any heat into the house, you have to heat up that massive tank first.

So I asked my plumber when he was round if he could put some manually controlled diverter arrangement in so that on those days I could feed the boiler directly into the house rather than the thermal store. 'No problem' he said. Until.

Until he came to do it and apologised profusely for leading me up the garden path since the thermal store is vented and the house CH now unvented. The thermal store does not use a heating coil inside it that is plumbed into the CH boiler but rather an open tank effectively. We'd have to convert the thermal store to unvented which in turn would need rather a large expansion vessel given the 2500 litres of water. (I just calculated that 2500 litres of water will expand by nearly 40 litres at 80 degrees C). So we decided to simply cut-off the thermal store and feed the CH boiler directly into the house.

TBH I was never going to get the burn improvement that the originator of the scheme achieved simply because he used underfloor heating and so was able to run the CH boiler within it's maximum efficiency range ie in condensing mode.

The thermal store is still there and can be piped back into the system as per the original plan very easily should the need arise.
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