(re) wiring a CH system with a heat store tank.

[ProDave]

Called to look at a new, almost completed house today. Neither the plumber or the original electrician can get the heating to work properly. I'm not surprised as it's been wired completely wrong. So it looks like I'm going to disconnect it all and start from scratch rewiring it.

I just want to run my proposal past you so I don't end up being the second electrician to fail to get it working:

It is plumbed with a "heat store" tank, heated by an oil fired boiler. If you have not come across these before, think of a conventional hot water cylinder "inside out". So the boiler directly heats the water in the tank, and that water just gets re circulated from the tank to the boiler directly. There is then a heat exchange coil (or two) to remove heat from the tank for hot water and heating.

This is quite a nice system as the tank is just low pressure fed from a built in header tank, so no pressurised vessels to worry about. But the hot water takes a mains pressure feed, into a heat exchange coil, so you get mains pressure hot water out of the other end.

So the plan is the boiler will heat the tank, controlled just by the tank thermostat. That will be a completely stand alone control loop, but I'll feed it from the "hot water" output of the programmer just to give a means of turning it off, though it would normally stay on 24/7. So that will continuously maintain the heat store at working temperature.

The under floor heating will be controlled in the normal way from it's controller and will be fed electrically from the heating output of the programmer. The only difference is when the UFH controller calls for heat, instead of turning on the boiler (as it does wrongly at the moment) it will just turn on the circulating pump to draw hot water from the tank to the UFH manifold.

The hot water needs no controls. Turn on a tap, water flows and removes heat from the heat store.

Does that all sound right?

The basic mistake the original electrician made was to follow the UFH instruction to the letter, so the boiler, and it's pump only turn on when the UFH is calling for heat. So as they have just found out, now the weather has warmed up, no UFH demand means the boiler never turns on, so no hot water as the heat store tank has gone cold.

 

[SPECIAL LOCATION]

Did one similar.. but with a couple of extra bits i.e. a separate circuit for some towel radiators and a wood burner as well as an oil fired boiler to heat the cylinder...

Heres what we had:-

Cylinder thermostat NEAR THE BOTTOM OF THE CYLINDER to keep the closed cylinder loop constantly hot in case of need for hot water.

FCU to boiler via stat to fire boiler & pump. Didn't bother with any hot water control from the programmer. Just use the FCU.

Under floor heating from room thermostats to manifold zones and heating pump, circulates a second closed loop drawing heat from another coil inside the cylinder.

{ So if the heating loop is cold, the cylinder will also be cold and thus fire the boiler from the cylinder thermostat. }

This house also had a wood burner direct into the main cylinder (same water circuit as the oil boiler)...

If the cylinder got too hot due to non-controlled wood burner, a second thermostat NEAR THE TOP OF THE CYLINDER..

turned on another pump to dump heat to the towel radiators fed directly off the cylinder..

We used a 'Hot water On' program option to manually turn on the towel radiator pump if required to control the towel radiators independently of UF heating or wood burner heat dump.

As you say it is not hard system to wire...

But some electricians cannot think outside of "standard instructions" :C

 

[ProDave]

Thanks SL

I plan to feed the boiler control loop from the HW output of the programmer as that allows me to re use the cables that are already pulled in rather than providing a new feed for it.

One thing that has occurred to me is I need to set the programmer for "gravity HW" so if someone programmes it wrong and tries to turn on heating without HW, then the programmer will automatically turn the HW on as well.

There's two cylinder stats, but both at the same level low down on the tank. I'm not sure whether to just use one, or connect both of them in parallel for redundancy?

The way it was wired before, one of the cylinder stats directly switched the pump that sends hot water down to the UFH manifold, but that sounds wrong as the pump would be pumping whenever the tank was hot, even if the UFH was not calling for heat.

There's no other form of heat input so no need for dump circuits.

Apparently it was the owners "mate" who wired this house and couldn't figure the heating. I suspect that may not be the last I hear of this house. It's not quite finished so I doubt it's been signed off by building control yet.

As an aside, I thought the CU looked quite neat. One I've not seen before. Then I saw the make, Proteus. That's CEF stuff isn't it? (goes to wash mouth out with soap) Never thought I would like anything from CEF.

 

[Steve3948]

There's two cylinder stats, but both at the same level low down on the tank. I'm not sure whether to just use one, or connect both of them in parallel for redundancy?

I think one is a High Limit Stat in case the thermostat fails, wired in series for safety.

 

[SPECIAL LOCATION]

Thanks SLI plan to feed the boiler control loop from the HW output of the programmer as that allows me to re use the cables that are already pulled in rather than providing a new feed for it.

One thing that has occurred to me is I need to set the programmer for "gravity HW" so if someone programmes it wrong and tries to turn on heating without HW, then the programmer will automatically turn the HW on as well.

There's two cylinder stats, but both at the same level low down on the tank. I'm not sure whether to just use one, or connect both of them in parallel for redundancy?

The way it was wired before, one of the cylinder stats directly switched the pump that sends hot water down to the UFH manifold, but that sounds wrong as the pump would be pumping whenever the tank was hot, even if the UFH was not calling for heat.

There's no other form of heat input so no need for dump circuits.

Apparently it was the owners "mate" who wired this house and couldn't figure the heating. I suspect that may not be the last I hear of this house. It's not quite finished so I doubt it's been signed off by building control yet.

As an aside, I thought the CU looked quite neat. One I've not seen before. Then I saw the make, Proteus. That's CEF stuff isn't it? (goes to wash mouth out with soap) Never thought I would like anything from CEF.

Is the heating purely underfloor? or are there any traditional radiators as well?

If purely underfloor how many zones are there?

Typically you have a manifold to the zones and each zone has its own room thermostat..



Each room thermostat just goes back to a control box that operates the zone valve on the manifold and turns the circulating pump on..



There is no need for any connection back to the main heat cylinder.

Also no global heating ON trigger either as each room/zone has its own individual programmer.

I really cant see much need for a traditional water and heating two channel programmer with this sort of heating..

But if you have a mixture of under floor and traditional radiators then that may be different...

You will also need to double check that the plumber has got his pipe circuits and pumps in the right places!!

As it sounds like neither the plumber nor the electrician understand it much! :|

 

[ProDave]

It's all UFH and the UFH has it's own control box a bit like the one you pictured.

The way all (at least all that I have seen) UFH systems work is the UFH controller reads the room stats and controls the manifold valves and the manifold pump. BUT it sends a "call for heat" which normally fires up the boiler, if it's fed directly from a boiler.

In this case, it "calls for heat" from the heat store, so instead of firing up the boiler, will fire up the pump that pumps hot water from the heat store to the UFH manifold.

A lot of people don't have UFH on a timer but have it on 24/7, usually because of the long heat up times of UFH, but in my own house we have it on a conventional programmer and it works well.

I've spoken to the plumber and am confident he knows what he's doing and has plumbed it correctly. It was the first electrician that didn't understand it, and instead wired the UFH "call for heat" directly to the boiler, as it says to do in the UFH sytem instructions. He lacked the knowledge to realise a heat store system operates differently.

It's funny how I seem to be doing a lot of heating systems this week in all this hot weather. On Wednesday I was finalising the finer points of another, different heat store system powered by a wood burning stove. We had the stove lit and all the doors and windows open to let the heat out.

 

[SPECIAL LOCATION]

Perhaps they know something you dont....

Cold snap is on its way...

You will be under 3 foot of snow this time next week..

once summer has finished again!!!! :^O :^O

 

[kme]

Couple of thoughts:

The thermal store is designed to stay within certain temperature levels permanently - I`d be giving the primary from the boiler a perm. feed, with the thermostat as SL says.

The next bit isn`t so much of an issue unless you have a solid fuel appliance - but the tank has a max temp. that musn`t be exceeded, hence the heat dump.

The first one of these I did, I also wired a pump override to the upstairs rads, in order to dump excessive heat when the wood burner was on........

Then had to go back, and fit a large radiator on the outside wall of the house, to use as a dump - the system was SO efficient that, if you used internal rads, the customer thought you were trying to cook `em alive.

Mind, that one also had solar thermal feeding the store, too - no wonder the plumbit took one look, and `phoned for assistance!!

I wouldn`t use a standard timer circuit for the store though - to come back to your query - the whole premise behind a thermal store is that it is permanently at operating temp.

KME

 

[ProDave]

Well these are like buses. the new build I am working an at the moment also has a heat store tank, thought this time a pressurised one (which seems a bit pointless)

But this one has a wood burner and an oil fired boiler to heat up the heat store.

The plumber has made no provision for a heat dump in the event of over heat, so on this one the only option I can see is for the high temperature cylinder stat to override any timers etc and bring on the whole central heating system as a heat dump.

 

[dave2]

Pro Dave

Try looking at this site.

There are loads of information and wiring diagrams for heat banks etc.

 

[ProDave]

well I've been and rewired the first one of these today and it's all working well now.

Problem. It's only an oil fired boiler now, but pipework is in place to add a wood burning stove later on. So with a woodburner, it needs some form of over heat dump.

The only heating in this house is under floor heating. You can't rely on turning that on to dump excess heat, because if the individual room stats are all up to temp, the heat won't go anywhere.

The only thing I can think is to tell the plumber when he adds the woodburning stove to the system, to also add a radiator somewhere as an over heat dump load.

 

[wozz]

I like heating controls.

Any of these system have set back stats?

 

[Steve3948]

I've found set-back stats are a total set-back :innocent

Confuses the hell out of the customer.

 

[ProDave]

A set back stat is just an admission from under floor heating suppliers, that under floor heating takes a long time to heat up. Sometimes too long in some installations, so a set back stat is just a bodge to keep the heating going, but at a lower setpoint, so it won't take so long to warm up.

My own system doesn't have it. Yo just learn what the warm up times are for your system, and make allowances when setting your programmer.

The system I was working on today goes one dafter. Not only does it have a set back stat, it also has an outside temperature probe so it can compensate for weather changes. WTF would it need to do that? it has a room stat for each zone so what more does it need?

The owner wants me to remove the set back stat and outside temp. probe but I'm waiting on him getting another ordinary thermostat and to find the user manual (because you have to program the system to tell it what stat controls what etc)

 

[wozz]

Are you sure the outside temp sensor isnt for a compensator?

 

[ProDave]

Are you sure the outside temp sensor isnt for a compensator?

I can't figure out what it's for. The only instructions I have seen say it should be there. Not WHY it should be there.

 

[kme]

One of the reasons I like dry (electric) underfloor - my thermostat "learns" how long it takes the room to reach the requested temp, dependant on heated area, & Tamb. Within a week, it has worked out what time to switch on, so that it can provide the requested room temp by the requested time

That is the "standard" timer stat - the "all singing, all dancing" in the kitchen is touch screen, allows you to set a target temp. that is different for any time of day, with a controlling air and / or floor temp - expensive stat, but is saving a huge amount in electric, and shows usage as a graph for today, weekly, monthly, quarterly or annually. Clever piece of kit!

 

[Steve3948]

I can't figure out what it's for. The only instructions I have seen say it should be there. Not WHY it should be there.

You don't have to fit it, however the system will be more efficient if fitted.

With it, the controller monitors the outdoor air temperature against the inside air temperature and as already stated compensates for the difference, if its in setback it comes on full, earlier or later to achieve the set temperatures on the thermostats.

If you have the UFH through a time clock then it's no use anyway as it's designed to work 24/7.

 

[ProDave]

You don't have to fit it, however the system will be more efficient if fitted.With it, the controller monitors the outdoor air temperature against the inside air temperature and as already stated compensates for the difference, if its in setback it comes on full, earlier or later to achieve the set temperatures on the thermostats.

If you have the UFH through a time clock then it's no use anyway as it's designed to work 24/7.

Yes you are right.

My "problem" is this level of control sophistication is beyond the comprehension of the average home owner.

I was first called to this property, because they got no hot water when they turned the heating off. that was because the system was not wired to take into account it being a heat store system.

you would think that is the end of it?

No.

The customers complaint now, and hence him wanting it on a time clock, and this "clever" thermostat removed, is he is complaining the house is "stifling"

Well if it is stifling, that must be because something is set wrong, as every room has it's own thermostat, plus this one clever one, or it's just that he got used to living in a cold damp draughty static caravan while the house was built (been there, done that) that he's forgotton what a warm, dry, draught free house feels like.

He's away for a few weeks (works off shore) so when he's back I'll try and get to the bottom of exactly what he thinks is wrong with it and fix it properly rather than just bodge it to remove his issues.