wiring up a Gledhill thermal store

[oscar21]

I've always thought I was quite good a wiring heating systems, I can do Y plan, S plan, UFH set ups, "normal" thermal stores etc, you name it and I can do it and have done for years.

However this system opened my eyes today, the thermal stores I have done in the past have had a coil that carries the hot pressurised water from the mains and its heated by the mass of water round it. This one doesn't have any coil, all water ports in the cylinder are open to the whole tank, so the boiler just pumps the water from the tank round the cylinder and boiler and the radiators just draw off that same water from various bits of the cylinder. The hot water also comes from the same water but this is pumped round an external heat exchanger bolted to the side of the tank. This is just a 3 bed domestic house but the system has 4 pumps on it and a big storage tank in the loft, the CH isn't even pressurised, its all open vented.

I think I know how to wire it, its essentially 3 separate systems, the CH/UFH is one system and I think instead of the zone valve firing the boiler I think it needs to just start the circulating pump as the water will already be hot in the tank. The HW is also a similar separate system, it has a flow switch so when a tap is opened the HW pump starts up and pumps the cylinder water round the heat exchanger. none of these need to fire the boiler as far as I'm aware.

Now this is the tricky bit, if you're still with me the third system is the boiler itself, its just the boiler, a pump and the cylinder. On the cylinder there is a thermostat like the old fashioned strap on stats, it doesn't have a high limit cut off on it so it will control the water temp in the cylinder but doesn't appear to have a safety aspect to it, I know it won't blow up as its open vented but the instructions talk about multiple thermostats and 2 pole relays etc, the customer bought it as a complete kit and the single thermostat is all it came with.

This is the instruction manual for it, it also shows normally closed and normally open zone valves in the wiring diagram but neglects to say where they are supposed to go. And if things couldn't possibly get any more complicated he also has a log burner piped into it as well and the plumber is taking about pipe stats and yet more zone valves to divert the hot water to and auxiliary radiator in case the log burner gets too hot.

https://www.gledhill.net/uploads/Torrent-Stainless-Iss-4-1.pdf

This is all in a 3 bed semi-detached house, the plumber listened to the client when he was describing what he wanted and said that all sounds great but I'd go for a combi.

 

[Simon47]

However this system opened my eyes today, the thermal stores I have done in the past have had a coil that carries the hot pressurised water from the mains and its heated by the mass of water round it. ... The hot water also comes from the same water but this is pumped round an external heat exchanger bolted to the side of the tank.

There are pros and cons both ways.

This is just a 3 bed domestic house but the system has 4 pumps on it

4 ? Needs 3 generally. A 4th could be for DHW circulation, or for the wood burner mentioned below.

and a big storage tank in the loft

Shouldn't be a "big" tank, just a F&E tank sized to the system - but see note below about regulations.

the CH isn't even pressurised, its all open vented.

If you refer to P 15 of the manual, you can run the heating separately via S or Y plan. But IMO that negates many of the benefit of a thermal store.

I think I know how to wire it, its essentially 3 separate systems

Correct

the CH/UFH is one system and I think instead of the zone valve firing the boiler I think it needs to just start the circulating pump as the water will already be hot in the tank

Yes

The HW is also a similar separate system, it has a flow switch so when a tap is opened the HW pump starts up and pumps the cylinder water round the heat exchanger. none of these need to fire the boiler as far as I'm aware.

Yes

Now this is the tricky bit, if you're still with me the third system is the boiler itself, its just the boiler, a pump and the cylinder. On the cylinder there is a thermostat like the old fashioned strap on stats, it doesn't have a high limit cut off on it so it will control the water temp in the cylinder but doesn't appear to have a safety aspect to it

The boiler isn't an uncontrolled heat source, so no safety function needed. The boiler will have internal over-temp shut down so that will limit the store temp to (probably) 88˚C. Safety functions only come into it when you have uncontrolled heat sources such as wood burners with a back boiler.
You are possibly thinking of unvented cylinders - where a number of safety functions are mandated to eliminate the risks from having a large pressurised store of potentially superheated water just waiting to cause a BLEVE "explosion". Having seen one first hand*, they are "interesting".

the instructions talk about multiple thermostats and 2 pole relays etc, the customer bought it as a complete kit and the single thermostat is all it came with.

OK, page 20 in the manual.
The single stat is all that's needed, it will fire the boiler when the store cools, and turn it off when it's heated up again. What is described in the manual is a technique for making the boiler fire for fewer, but longer, burns.
A single stat, if it's any good, will tend to turn the boiler on and off quite frequently for short bursts. This is generally not the most efficient way of doing it, and for things like oil boilers will tend to make more ash deposits.
By using two stats as shown, the boiler will turn on when the top stat calls for heat, but the relay will hold the boiler on until the bottom stat is satisfied. So the boiler runs for longer, but less frequently.

This is the instruction manual for it, it also shows normally closed and normally open zone valves in the wiring diagram but neglects to say where they are supposed to go. And if things couldn't possibly get any more complicated he also has a log burner piped into it as well and the plumber is taking about pipe stats and yet more zone valves to divert the hot water to and auxiliary radiator in case the log burner gets too hot.

Refer to P 19 where it shows the location of the pipe stat and zone valves.
In normal operation, the zone valves are both powered, hence the NC one is open, and NO one is closed. This connects the wood burner to the store and heat goes into heating the water.
If the pipe stat switches over, the valves swap positions, and heat is transferred to the heat dump radiator. The diagram on P 21 is applicable here.
In normal operation, the pipe stat is "off", and so power goes via the pipe stat, terminal 2 (lower block), the overheat stat, control stat 2 (which needs to be positioned appropriately in the store - I'm not actually sure where that is), terminal 3 (lower block), and powers the motor in the NC valve to open it. When the valve is open, it's switch them powers (via terminal 3, lower block) the NO valve to close it. The NO valve then blocks flow to the heat dump rad.
If any of those 3 stats switches over because things have got too hot, then the NC valve will shut, and the NO valve will open - dumping heat into the heat dump rad.

EDIT: I personally have some reservations about this arrangement. The main one is that if you leave a typical zone valve powered for months on end, there's a definite question as to whether it will spring return when power is removed. There are alternative arrangements, but with the one shown, it would need regular testing to check that the valves aren't stuck.

The wood burner could be where the 4th pump belongs. Ideally you arrange for gravity circulation which is near enough foolproof - and in particular doesn't need power for a pump. There may well be a thermostatic valve unit of some sort as wood burners don't like running too cold so there may be a need to restrict the flow, localise the circulation until the water is hot enough.
Where gravity flow isn't possible, the system can be pumped, but you then get into interesting safety issues should the supply fail.
This is an area to leave to the "experts", or at least, those who know what they are doing. You just need to wire it all up to their specification - and probably the diagram on P 19.

As an aside, there are some strict rules on the F&E tank when there's an uncontrolled heat source such as a wood burner. What you aren't allowed is a plastic tank, unbunded, and located where it will dump scalding hot water over someone's bed if the system fails and the tank fails through excess heat. As is common, these rules came in after an incident - a plastic tank did that, and dumped boiled water over a young girls head.
I believe (not an expert in this area) that a copper F&E tank, properly supported, is normally specced for such systems.

This is all in a 3 bed semi-detached house, the plumber listened to the client when he was describing what he wanted and said that all sounds great but I'd go for a combi.

I have a thermal store in what was a 2 bed end of terrace ex-council house. It's the "coil in the water" type though so doesn't have the PHE & pump. The heating is great, no need for a bypass, no "hissing" rad valves due to a mismatch between boiler sizing (and hence flow rate demands) and the heating system. SWMBO has thoughts of a log burner - whether we get one with a boiler in it I'll have to think about (it'll only be a small one, and the pipe runs would be awkward).
I also put on in our rental flat 12 years ago now. It's been great
There would have to be some very persuasive reasons for me to ever go back to a combi. Combis are popular because it allows builders of modern shoe boxes to save a few sq feet of footprint off an already tiny house, and pinch a few more quid off the build costs. As far as I can tell, those are really the only pros. On the con side, it means "uninspiring" DHW flow rates due to power limit of the boiler, while due to power requirements for DHW, the boiler is pretty well guaranteed to be grossly oversized for the heating load. And when it breaks down you have nothing - no heating, no hot water, no options. At least with the thermal store you can have a backup immersion heater which will keep you in hot water (as long as you aren't too lavish with usage) and some heating.
Of course, plumbers love them too as they don't have to think or actually design anything - bang it on the wall, connect 5 pipes, collect cheque and walk away.
As a landlord there are other benefits. If (or rather, when) the boiler decides it wants a holiday, the tenant can put the immersion on and carry on until the engineers deign to turn up (I'm waiting on them for another property, and they say they are fully booked up for 2 weeks - though it would be different if it were a "no heating or hot water" situation). Plus, by shifting a lot fo stuff that would be inside a combi outside of the gas appliance, I am able to tackle issues myself without needing to worry about the safety and legal issues that mean I have to leave it to the "experts" to fix the boiler - often having to diagnose the problem myself and tell them what to replace (it seems that a lot of youngsters in the heating engineer trade have SFA by way of diagnostic skills).

* I was servicing for a mate who was rallying. Another competitor came into the pits with a very hot engine and his radiator split at one end. The flash boiling of the coolant caused a small blast that peeled open one header of his radiator and rolled it back so that you could look down the tubes of the core.

 

[oscar21]

Thankyou so much for your help, I am going next week to connect it all up so will report back if it all works.

 

[Simon47]

I've just had a quick look, and on this page, there's a video showing how a loading unit works - that may be the 4th pump.

 

[oscar21]

The 4 pumps are :-

1. boiler to cylinder
2. C/H and UFH circulating pump
3. UFH dedicated pump after manifold
4. DHW circulating pump. (this just pumps round the cylinder/heat exchanger