Wiring a thermal store

[Shell820810]

Can someone explain the logistics of wiring a thermal store? I think ours is set up wrong and we are short cycling the boiler and went through a horrendous amount of oil in the spring.

 

Oil boiler, multifuel stove, solar PV

 

[PeterW]

Does it have one or two thermostats in it...?? And how far up the tank are they ..??

[Shell820810]

11 minutes ago, PeterW said:

Does it have one or two thermostats in it...?? And how far up the tank are they ..??

Two thermostats, one at top and one at bottom. Space for a third in the middle. He had the top one set to trigger an overflow tank above if multi fuel stove overheated the thermal store, but I think we need top and bottom to control the oil boiler?

[PeterW]

Ok sounds like the top stat maybe incorrectly wired - if it only has a single set of contacts it will need switching out. 

 

Top stat is set to around 50c so the stat will not open until the tank gets this cool. This is wired through the bottom stat set to around 70c, which is wired as a normal stat - it closes when the water reaches 70c which means by definition the whole tank top to bottom has a minimum of 70c .

 

The other way is a latching relay that triggers the boiler. 

 

[Shell820810]

Thanks, hubby is the spark. He hasnt wired a setup like this and is trying to understand what should be happening to control the boiler correctly.  On an aside, We put on the oil last week, its been off since, and PV diverter seems to be keeping tank warm. Pipes at top of tank are warm, pipes at bottom aren't. Does this sound right?

 

This is something I found on another forum when I googled:

 

"

Hopefully I can describe the basic operational principal when supplied by a boiler, before considering the wood burner input you need to get the upper / lower stat operation and control of the boiler straight.

As mentioned above the store stats control the boiler including a time clock, and the room stats (inc time clocks) control the pumps that take heat from the store. There is no connection between them.

If the store has 2 stats an upper and lower then volume in between is the buffer volume. If there is a DHW take off to a plateX or internal coil this will be above the upper stat.

The usual twin stat setup using a controlled on/off boiler such as oil or gas is as follows.

The store charges from the top downwards so when charging the interface between hot incoming water and the cooler water fed to the boiler moves downwards.

This is easiest to visualise in a batch charging scenario where the store is fully charged then depleted, then recharged and so forth. Alternating between charging up and feeding heat loads. Obviously they can be simultaneous, but for simplicity visualise as follows. Starting from a cool store.

Store is cool throughout therefore it demands heat from the boiler (store controls boiler & room stats control store). The upper stat is sitting in cool water and is thus closed, the boiler gets the fire signal - providing time clock allows.

Boiler supplies hot water to top of store and draws cooler water from the bottom, therefore the hot / cool interface moves downwards.

The bottom stat will eventually open at say 75 deg when the hot water arrives at the lower part of the store. Opens on reaching set temp. This will switch off the boiler, the store is fully charged. It can sit like this with some parasitic heat loss until there is a demand for heat.

If or when there is a demand for heat the system (what ever it is) pulls hot water from a highish point on the store. If there is a DHW coil then the heating will pull from a tapping point lower than the DHW coil, if it’s a take off for a plateX then there will be another tapping higher up for that. The returning (now cooler) water from the heating system returns to a lower return tapping. Assuming the boiler is off – the hot / cool interface is now moving upwards. 

So boiler draws from the lower part and supplies to the top, heating load draws from the upper part and returns to the lower part.

When the cool water rises past the upper stat as the heating load uses the water (i.e. the whole store buffer volume is cool) then the upper stat is again sitting in cool water and there is a call for the boiler to fire and recharge the store. For simplicity assume at this point the heating load also stops. The store is again full of cooler water and the cycle repeats with the boiler charging the boiler from the top down.

So using 2 stats the volume of hot and cool water is moving up and down. In reality this is much harder as mixing stirs things up, so keeping flows steady and slow is good.

The tricky bit is to get the stats wired correctly – the values are fairly arbitrary so long as the lower is greater than the upper by say 15 deg and importantly the activation from closed to open or open to closed is round the right way.

The upper stat closes when cool – to switch on boiler
The lower stat opens when hot – to switch off boiler

However, once triggered the upper stat must remain closed until the lower stat is activated, otherwise it would close because of the incoming hot water. Likewise the lower stat that opens when the hot water arrives needs to remain open whilst the store is depleting – other wise the boiler would fire again. This means the store stats must be latched to remain closed (or open) whichever is the required activation until the other stat is activated (or deactivated dependant on which way round), so its not simply 2 sets of contacts in series.

You need to work out a logic diagram based on whether the existing stats are normally Open (NO) or Normally Closed (NC) and whether they activate on falling or rising temp. You need an electrician who understands the logic of operation – or work it out yourself and get them to wire it. Beware asking someone who doesn’t understand the logic. You can get a ready made box to do this, or use regular cylinder stats and use some relays to provide the logic."

[ProDave]

You are trying to over complicate it.

 

The "hot water" output of the time clock is what I normally use to control the boiler heating up the thermal store. So just that switched output and the tank thermostat on the thermal store, probably the middle one.  A tank stat has quite a lot of hysteresis (unlike a room stat)  so once the TS has heated up and the boiler shut off it will need to drop several degrees before the stat turns on again and calls for heat from the boiler.

 

Yes you could have two stat's and a set / reset arrangement to give a wider hysteresis but you won't do that with all off the shelf parts.

 

The second part is then use the heating output of the programmer, in conjunction with room thermostat(s) and zone valve(s) to turn on the pump to circulate water from the TS to the radiators or UFH.

 

And the third part if you have a stove is an over temperature stat on the tank that turns ON when the tank gets too hot. This should override the heating programmer and rooms stats and force it to dump heat to the radiators or UFH.

 

 

[Nickfromwales]

3 hours ago, ProDave said:

This should override the heating programmer and rooms stats and force it to dump heat to the radiators or UFH.

Agree that the quoted text just massively over complicated the setup. 

The only thing from your comment that I'd change, Dave, is where excess heat gets 'dumped' to. I really don't like dumping such high grade heat to anywhere where it can cause injury, such as a rad in the house, as it can easily get to temps of 80oC or above and a child / elderly person accidentally coming into contact with that would be a burns victim. 

I always look at fitting a dedicated heat loss rad, something like a 1800x700 double convector rad positioned in a cold attic space or similar void, where it can dissipate heat with no detriment to the house or danger to its occupants. 

Most modern TS's will have a T&PRV 'blow off' which will open at ~90oC, ( unless it's been purchased specifically without one ), but for a gravity install these  don't operate for over pressure, but are good as a secondary / fail safe for overheat. They DO NOT negate a heat dump arrangement, but are better to have than not to have. They're not critical for an open pipe install as the expansion is designed to happen at the F&E header tank, and that also negates having to get a discharge pipe from the TS to drain. 

One stat at the midway point for boiler control, one at the top for excess temp heat dump control and Bobs your Uncle.