Plumbing question - thermal store pipes

[julianf]

We have a gledhill thermal store here.

The Rayburn thermosyphons into it. The dhw runs through a big coil within the store and is, effectively, heated in demand.

There is a thermostatic mixing valve at the end of this coil, just to bring the temperature back down to non-danger levels.


What I want to do is fit a flat plate heat exchanger (3/4" threads) between the output of this dhw coil and the thermostatic mixing coil.

Why? Because if the store is not hot, I want to be able to turn on the oil fired boiler and pump hot water through the plate heat exchanger and heat the dhw (post coil).

Any remaining heat in the thermal store will get scavenged and partially preheat the water passing through the coil. The thermostatic mixing valve (post coil, post plate heat exchanger) will bring the water back to safe temperatures again before it goes on to the taps.

I've thought about this quite a lot, and this is what I want to do. This question is not "is this a good idea" - I mention the plan simply for background! : )


My problem -

What on earth is this connection -

Some sort of 26mm compression fitting??

The output from the store is actually 22mm and then it's adapted up. But its all factory brazed together and I don't really want to be messing with it.

Basically I just want to insert my 3/4" parallel fphe terminals between that 26mm pipe and the mixing valve. I wasn't expecting oddness.

I'm hoping it's not odd at all to someone who knows and the can just tell me that I need xyz fitting and it will all be good!

Thank you!

 

[gaz1]

whats your model gledhill thermal store?

oh and its not advisable to run it that way

 

[julianf]

It's a torrent. Vented. Rayburn is direct via 28mm. 3 coils - dhw, boiler, and solar.

I don't intend on using the boiler coil at all. If the Rayburn is lit then I don't need the CH rads on anyway. If it's not lit, then I'll run the CH direct from the boiler. I don't ever want to heat the store from the boiler.

I've found another tmv knocking about here -

It measures up very similar to the one that's currently in use. I've taken it apart and found that it's thread is 1" BSP, it's a, umm, parallel (?) connection (the one with the washer) rather then a compression.

So I'm thinking I need some sort of flat ended 1" BSP pipe to mate up with the nut / flange on the store, and then bring this to my plate exchanger. And then do the same in reverse to get back to the TMV.

Here is the plate exchanger -

It's a touch over 12" long and about 3" deep, so overkill, but it's what I have.

 

[julianf]

I'm guessing I could probably use a 1" / 22mm fitting like this -

...if the threaded end was thick enough and I could flatten it nicely.


Then bend up 22mm copper to wherever my plate exchanger was, and then use a 22mm to 3/4 tap connector.

On the other side, tap connector to 22mm pipe, and then... I don't think 1" tap connectors even exist? So I'm unsure of how I would join back up to the TMV.

I guess I could just screw a 1" female over the TMV, take that to 3/4 male and somehow, and then use a 3/4 tap connector again.

The serviceability would be maintained from the parallel fittings, even if the direct connection to the TMV was threaded.

I reckon there must be a more elegant solution though.

 

[gaz1]

still not enough info as theres a model number

https://www.gledhill.net/product-category/thermal-storage/

 

[Gareth J]

Slight tangent bit hopefully you don't mind.

What sort of boiler do you have? I'm aware that oil boilers used to do the hot taps have to have either an integrated or external hw store (presumably to stop short cycling). Is that still the case? Or can you get more "direct" ones now that can cope with the sort of demands of short duty cycles?

Asking as am currently working out thermal store options of my own!

 

[julianf]

still not enough info as theres a model number

https://www.gledhill.net/product-category/thermal-storage/

What is it that you are actually trying to find out from the model number?

 

[julianf]

Slight tangent bit hopefully you don't mind.

What sort of boiler do you have? I'm aware that oil boilers used to do the hot taps have to have either an integrated or external hw store (presumably to stop short cycling). Is that still the case? Or can you get more "direct" ones now that can cope with the sort of demands of short duty cycles?



Asking as am currently working out thermal store options of my own!

Ours isn't a combi. It's a standard unit, so won't have that small tank.

I don't think the tank is anything to do with duty cycle though - I think it's just a small, highly insulated, reserve that can be used for hot water whilst the main exchanger is heating up (ie the boiler firing)

Our setup won't have that. So the exchanger on the thermal store won't get hot until the whole boiler is up to temp. There will be a greater delay, and, probably, less efficiency.

A better way to do it would be to feed a combi dhw input from the store. Gledhil used to make a small thermal store for retrofitting solar that worked in exactly that way. But, I suspect, most combis have a max input temp.

But, yeah, I don't have a combi, so I'll have the inefficiency and delay. But I'm happy to suffer that.

 

[Gareth J]

Ah, fair enough. Just checking there wasn't something that got around that. Personally, am considering an electric inline booster to top up warmish water from the store. But plans change...

 

[gaz1]

the tank provides extra info to its design build

what you dont understand is using a lower power coil to transfere heat to the cylinder can have other effects

returning really hot water back to a wood stove can effectively boil your water to steam and then you have real trouble as the wood stove can explode due to steam pressure

 

[julianf]

the tank provides extra info to its design build

what you dont understand is using a lower power coil to transfere heat to the cylinder can have other effects

returning really hot water back to a wood stove can effectively boil your water to steam and then you have real trouble as the wood stove can explode due to steam pressure

The tank has two input coils, one designated a solar coil, so, I assume, good for solar.

It has a boiler coil, that I'm not intending to use.

The Rayburn, as previously mentioned, is direct, via 28mm.

There is no possibility of the Rayburn exploding. Even without the heat leak radiator, a boiling event causes the system to vent. I know this with certainty, as I've not yet bothered to connect in the heat leak.

When you say -

what you dont understand is using a lower power coil to transfere heat to the cylinder can have other effects

I really don't understand what you are saying? Or how it is relevant? I'm talking about putting a plate heat exchanger on the dhw coil output to boost the dhw temperature. I'm not talking about heating the store at all. No heat is returned to the store from the plate exchanger.

I've thought about this quite a lot, and this is what I want to do. This question is not "is this a good idea" - I mention the plan simply for background! : )

I appreciate any comment that stops me from killing myself, obviously, but I'm not sure this is really the case here?

 

[julianf]

Ah, fair enough. Just checking there wasn't something that got around that. Personally, am considering an electric inline booster to top up warmish water from the store. But plans change...

The normal way is to heat the whole store from the boiler, but then you effectively need to keep the store hot from the boiler, due to the delayed heat up time, and also suffer the losses from it.

And, of course, if it's hot, then the other inputs are not going to be as efficient.

It may be that the standing losses from the boiler are so very much greater than those from the store, that heating the boiler to heat the fphe becomes less efficient than heating the whole store. But I seriously doubt that.

I suspect some manual intervention will be required. But, in the context of a life of chucking logs on a Rayburn, this is acceptable. Someone comming from a mains gas / combi boiler background would probably find it unacceptable.

What I mean is, I hope / suspect the boiler will remain off unless, for example, someone wants a bath, and it's winter (so no sun) and the Rayburn has not been lit recently (ie an infrequent scenario). Or it's winter and we have been away, and want to give the house a blast of heat before lighting the fires.

So the boiler is supposed to be a backup, rather than a usual source.

An electric solution would work. For some reason I think it would never get turned off here though. And, plus, I have the boiler... And if / when we sell, people look down on properties without oil or gas heating...

 

[Gareth J]

All makes sense to me.

And, of course, if it's hot, then the other inputs are not going to be as efficient.

Yes, this is something that seems difficult to sort out properly. Have been reading up about some types of thermal store which encourage stratification or even have physical baffles in them to allow the low grade heat inputs to be presented with fairly cool water while allowing the hotter sources to heat a portion over and above dhw temps.

It's all a compromise though and working with what you have has to play a part.

I'm not totally sure as I've only read about them in passing but I think the electric inline heaters I mentioned don't turn on unless the hw its below it's setpoint so in effect would only use juice if the hw wasn't hot enough. No idea if it'd cheaper or more expensive than a short cycling boiler through a hx though.

 

[julianf]

I'm not totally sure as I've only read about them in passing but I think the electric inline heaters I mentioned don't turn on unless the hw its below it's setpoint so in effect would only use juice if the hw wasn't hot enough. No idea if it'd cheaper or more expensive than a short cycling boiler through a hx though.

Yep, I'm certain that's the case. I meant that it would always be on here and noone would even try and use anything else... So all our setup would suddenly be electric.

I don't know why the mentality would be different with the oil boiler - no logic at all really.

I'm not too bothered about hot water comming out the taps all the time. Most of the time someone turns on a hot tap it's to wash their hands after the loo etc. and they (myself included) never even wait long enough for the hot to come through the pipework. All our washing up goes in the machine.

If I'm a proper skint flint I might even consider adding one of those timed buttons to the boiler on - like in blocks of flats, for the lighting.

If you have no kit currently, maybe check combi boilers to see the max input temps too. That would be simple enough, just plumb the dhw from the store to the combi. Combi won't fire until its fphe gets too cool.

It's what I'd do if I didn't have a boiler alreaddy. So basically I'm trying to do the same external to the boiler that I have... Which, even if it's a bit more lossey, will still be a load cheaper than getting another boiler.

 

[Kayos]

I get what you are trying to do, I dont think it will work how you imagine

Plate to plate exchangers are very inefficient, I've used then to separate old (leaky) systems from modern pressurised boilers and the rule of thumb is 3 x capacity to size them but in reality it's more like 4 x. So if you have a 30kw boiler you need a plate that's 90 to 120kw

 

[julianf]

You know more than me about these things, but -

Some thermal stores use plate exchangers on the dhw feed, rather than a coil - ie they have a pressure switch, and a pump, so pump from the top of the store, through the exchanger, to the base, whilst the dhw runs through the other side of the exchanger.

These stores have visably smaller exchangers than the one I have.

Oil fired combi boilers use them in the same sort of setup. Pump the dirty water through one side, the dhw through the other. A quick Google image search for "oil combi boiler" shows stacks of them. All with smaller exchangers than I have....

Aside from that, do you think my choice of fittings to deal with the 1" parallel connections will be ok? Is there a more elegant way to mate up to this mixing valve, other than what I proposed above?

 

[julianf]

Eg -

The exchanger I have is much larger than the one in the combi above, and, probably, a similar size to the one on the side of the thermal store.

Again, I have all the parts except for the pipe connections. I'm going to give it a go. My question was as to the pipework, rather than the concept.

 

[Kayos]

The oil one looks like a storage combi and uses the plate as a pre-heat rather than continuous heating

If I'm honest, I'm not a fan of thermal stores, buffer tanks and all the other stuff as it's mostly used in the wrong situation and doesn't perform as it should

As for the fittings, the only combo that doesn't work is parallel male and taper female, I'm using ptfe cord at the minute as it's a good balance of ptfe and the liquid sealers

 

[julianf]

Ok, so i reckon on going from the female nut -

to this -

to 22mm pipe, to -

to

and then back to -

then to 22mm pipe, and to another of these -

which will screw into this -

which will then screw back onto the thread of this, with some ptfe, i hope -

Im not that keen on that last one - 1" parallel to 1" parallel with tape, but i guess it will work. I think the pressure regulator for the setup is limited to 3 or 3.5 bar, so its not vast pressure or anything.

 

[julianf]

I guess it would be simpler to use one of these to couple back up to the thread on the mixing valve -