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Author Topic: Controls for heat store/oil-fired boiler  (Read 1433 times)
whyamisocold
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« on: November 16, 2017, 11:30:23 AM »

OK... as I inherited a 2500 Akvaterm heat store when I bought the house, I came across the idea of using it in conjunction with an oil-fired boiler. One nice long burn in condensing mode will make the oil-fired boiler as efficient as it can be as opposed to the on/off cycling typical with these and/or operating above the upper limit for condensing and hence nowhere near the 90%+ that the manufacturers claim.  Heat is then taken out of the store as required.

Before anyone jumps keeping both within the thermal envelope of the house gets around any objections re unnecessary heat loss !

OK...not intending to fit a Laddomat ( no funds) but need some sort of control.  Sensor at the top (A) and sensor at the bottom (B).  While setpoint A hasn't been reached, turn on the boiler for the start of the long burn cycle. Keep the boiler running until setpoint B is reached.  Naturally, while this is happening the temperature at the top of the tank will have exceeded setpoint A.  This temperature will gradually reduce as heat is removed from the tank until setpoint A is reached again whereupon the cycle repeats itself.  I'd like to control this using something simple and/or off-the-shelf in the short term.  Relays ?   I do have the two sensors that together with their black box have sets of volt-free contacts available.

Any ideas ?

TIA
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biff
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« Reply #1 on: November 16, 2017, 12:45:52 PM »

Hello Tia,
        Your Heat store is quite a good one and I have absolutely no doubt that it will tick all the boxs. However,,running  the oil boiler ,just to heat the thermal store would not be a good idea.
  On the other hand, you could set it up with a PV array and use it,s immersions to heat it. You have plenty of coils to incorporate wet solar and pv as well as your oil boiler. It is just a question of sitting down and drawing up your plan.
  I sited our T/S in the hall (25x8ft) next to under the stairs. It takes heat from our W/S and the excess from our wind turbine and PV. It is encased in a full height, floor to ceiling unit, insulated to the hilt but with sliding vents to release the heat. Because it is sited centrally , the heat rises up the stairwell and this area is never cold even when it is freezing outside.So siting it is all important. If you use the oil boiler to heat it, I doubt if you would get a 30% return on the value of the oil. Whoever bough the heat store originally, obviously had plans for multiple heat sources. It is worth investigating.
                                                                                            Biff
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whyamisocold
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« Reply #2 on: November 16, 2017, 01:03:51 PM »

Hi Biff

Thanks for the reply.  Originally it had a ghastly VIGAS gassification boiler that cost an arm and a leg to run, never worked properly and I sold it.

No funds for anything fancy TBH and we'd never see the return on investment.

TIA = thanks in advance  Grin
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Countrypaul
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« Reply #3 on: November 16, 2017, 11:59:32 PM »

WHy not just use two thermostats to switch the boiler on and off?  When the temperature drops below setpoint A have the first themostat cut in and activate a locking relay to fire up the boiler, when the store hits setpoint B the second thermostat simply cuts the power to the relay and therefore boiler, until the temp in the store drops below setpoint A again. This obviously requires a suitable boiler such as a pressure jet one that  can handle power on/off and probably not an old one that relies on a pot of oil.
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whyamisocold
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« Reply #4 on: November 17, 2017, 07:58:45 AM »

Great idea, thanks, CP
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martin W
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« Reply #5 on: November 18, 2017, 08:05:42 PM »

How about a differential controller like the tdc3?. You should be able to set them up with hysteresis so that once store gets to say 65 deg C boiler is shut off. The when below 60 deg C start boiler again...

martin
 
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whyamisocold
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« Reply #6 on: November 19, 2017, 08:43:05 AM »

Many thanks for that suggestion, Martin. I was unaware of these and so another very fruitful line of enquiry.  Amazing what you can come up with when you have the right terms to Google !  And since the techie in me  has always had a hankering to play around with an Arduino, came across several promising projects around the concept of a TDC using these.
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whyamisocold
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« Reply #7 on: January 04, 2018, 09:25:40 PM »

Well sadly that was hugely expensive waste of time. For the secondary side I had three options as my plumber was pretty much insistent that he wanted a correct system with the neutral point in the right place to avoid the chance of pumping over. 

Option 1 - a pressurised system.  Thrown out of court as I'd inherited an existing system of unknown provenance and with many inaccessible pipes.  Given that I'd found one soldered joint that fell apart in my hand, I think the decision not to go for this option was sound.

Option 2 - site the pump down by the heat store and run the two extra needed pipes through the house to that location. Try as I might, because the house has a load of steel beams in it, I could not find any easy viable route especially one that would give a graceful continuous rise to the vent pipe

Option 3 - site the pump up by the HW tank where the existing feed and vent pipes were located.  This necessitated changing the layout  and connections for the main feed pipe. I won't bore you with the details but suffice it to say it meant that there was a very long length of pipe from heat store to the radiators and as a result the heat loss has given me about a 10-11 degree drop in temperature by the time the hot water reaches the rads.  If I ran the heat store at the temperature I intended (to keep the oil boiler in condensing mode and thus max efficiency) then the temperature at the radiators would be pretty puny. 

So I'm now thinking of ditching the connection to the heat store and simply converting back to a bog-standard vented system.  A real pain as I was looking forward to the techie bit controlling the heat store.  But, as I say, all a bit pointless now.

The one crumb I have is that maybe a faster circulating pump might help ?
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martin W
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« Reply #8 on: January 04, 2018, 09:45:21 PM »

I run my radiators at 45-50 deg C.

I did some heatloss calcs and installed suitable rads in the rooms for the lower flow temperature.... you would be supprised that most modern radiators don't actually need to be much bigger than you think.

Before you ditch the idea, do some heat loss calc and see what size rads you need at lower flow temperatures...

Surely the heat loss form the store to the radiators can't be a continuous 10deg C... that could be some serious kW heat loss. Ok there will be a heat loss until the pipe warms up, but it cant be 10 deg c contineuous it might be like 6kW or more depending on pump flow.
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JohnS
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« Reply #9 on: January 04, 2018, 10:30:39 PM »

Could you insulate the pipe with the tubular insulation?
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whyamisocold
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« Reply #10 on: January 04, 2018, 11:52:52 PM »

... but it cant be 10 deg c contineuous it might be like 6kW or more depending on pump flow.

Why not ?  If the pipe is poorly insulated ?  But to put the obverse and respond to JohnS, the pipe has been insulated with pipe insulation wherever I could get to it.
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guydewdney
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« Reply #11 on: June 11, 2018, 07:37:43 PM »

Weather compensation. Saved me a fortune
 Second hand eBay 60 quid. Ymmv
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