Tank in tank thermal store with 2 primary connections -issue

[Mustardshots]

Hi
I bought an ACV SLE 240 tank believing from their website that it would have multiple connections (the plus model) and having found a cracking deal. Well, as ever if it's cheap there is a reason: these were clearance units and were not the plus version (and are no longer sold) - only coming with 2 ports to the primary (external) tank.


My original plan was simple: I use more heating at night and less hot water. I use less heating in the morning but lots of hot water. Being a tank in tank I had two independent volumes of water and a lot of energy that could be used in different ways. one set of ports with pumped boiler feed, another set of ports for out and return to rads. That way the energy from the primary could be used towards the radiators and the secondary energy would also help heat the primary. The boiler being independent kicks in to keep the cylinder hot enough. Caveat: I am no pro so this is all theoretical and I am aware that people generally have massive tanks to do this so don't expect huge benefit.

Now I only have two ports to the primary tank it seems I have limited options: 1) send the cylinder back and pay through the nose for what I wanted in the first place 2) use the boiler and 3 port valve as per the suggested layout to heat either cylinder or rads (which I guess doesn't give me what I want) 3) concoct some cunning plan. And it's this last option that I'd like some help with.

As you can tell I am no expert - and I am no doubt going to be told so in no uncertain terms. The only thing I came up with was a two pump set-up which on second thoughts I'm not convinced works. I've added a picture to my gallery but I'm afraid I don't know how to link to that. Looking at the scribble version - if the boiler pump moved to the other side of the rad connection and ran marginally slower.... I am tying myself in knots - can anyone help, or am I rally going to have to bite the bullet and return the tank?

Many thanks

Pete

 

[Norcon]

Don't return it. Use it as its purpose is intended and it will be fine.
Its an unvented cylinder though and subject to install regs.
The heat exchange can pull a massive 53kw at peak so probably the largest in its class giving supreme performance before going direct.

 

[Mustardshots]

I'm trying to avoid using just the boiler to drive the CH direct - I want to use some of the energy in the cylinder but maybe I'm making life too hard. From my understanding of the installation diagram that isn't possible.

Reason is that I am (for now) stuck with elec boilers and cheap rate electricity over night sounds too good to miss: Kind of wet storage heating.

Thanks for your advice

Pete

 

[SimonH2]

I think I'd look at teeing the boiler and heating to the one set of ports. As you've hinted at, there is then the problem that one pump will cause flow round the other circuit - so you may want to use motorised valves to shut them off when the pump isn't running. Alternatively, you'd need non-return valves with enough pressure required before they open that they won't open when the pump in that branch is off.

Fit a modulating pump and TRVs on all the rads, you can then ditch the room stats and control each room individually with it's TRV.

Biggest problem I could see with this is that over a day you could need quite a bit of heat and end up with no hot water. To that end, I'd suggest a second stat higher up on the tank would be in order, set to turn the boiler on (even on peak rate lecky ) when your stored hot water gets down to some minimum acceptable level.

 

[Mustardshots]

Simon - thanks for that.

I came up with something quite similar I think a few days ago - I've uploaded a schematic which explains it I think. But basically 2 pumps and two 2 port valves:

One pump + valve for the rads, another pump running faster than rad pump and pulling from boiler again with it's own valve. The idea there being that to start with the rads pull from the cylinder until the cylinder stat calls for heat. Then the boiler pump kicks in and as it throws more water out than the rad pump has an appetite for it can supply the rads and the cylinder. If it's just hot water then the rad valve is closed and the boiler supplies as and when.

It's much easier to understand if you look at the schematic - I will attempt to add the schematic here:

Tank in tank with 2 ports solution?

• Mustardshots
• 29 Nov 2011
• 1

How to get a thermal store out of a tank in tank cylinder with only 2 ports to the primary tank...

Thanks again for any input/improvements or otherwise!

Pete

 

[SimonH2]

Yep, that's about what I had in mind.

But dunno about the "faster pump" bit. The pump speed (or rather flow rate) should be set to suit the requirements.

Ideally you need the boiler to only ever put fully hot water into the top of your cylinder so that it heats top down. This may require a TMVin order to get it right.

The CH flow rate will be variable - and if it's not less than the boiler can produce then you'll struggle to even maintain temperature let alone actually heat the store !

 

[Dan Robinson]

Is it just me that thinks this is more than a little barmey?

 

[Mustardshots]

It might be barmy - I think Simon's point regarding the boilers is critical. I'm using 2 Amptec boilers which need a high flow of water across them to prevent there being no more than 14 deg C (from memory - may be wrong) rise in temp across the boilers. I suspect that this is where I open myself up for criticsm - I have no idea what temperature drop to expect from the rads but would assume that from cold it would be much much more than 14 degrees. Then you end up pumping luke-warm water back in to the top of the cylinder. Ok, the Secondary tank has a mass of hot water with energy that can heat the primary as well, but eventually that will cool to the point of having no hot water and not enough energy to aid the heating circuit.

These ACV tanks are supposed to be super fast heat up but it's all a bit of guesswork now. Maybe it's time to call it a day on this and look for another solution?

Thanks both for your advice/comments

Pete

 

[Dan Robinson]

There are several problems I foresee. Not least your boilers will battling to keep up. during dual command cooling the dhw.

There will be issues with cross flow as well.

As they are electric boilers, there is absolutely zero point in trying to do what you are trying to do as there is no modulating boiler to try and maximise the efficiency of. You will do better to have a secondary buffer tank (as you mentioned kind of) to heat the heating off the off peak electricity. It would have to be open vented really - although there are a couple of ways around it.


I am wondering if there is a header arrangement you could have with a NRV... but my judgement is clouded at the moment.

You shouldn't have the rad return where you have it either.

It is totally the wrong cylinder to have bought for the application you have in mind.

 

[SimonH2]

On the whole, I concur with Dan_Robinson - it's really the wrong tank for the job.

Do you have room for another cylinder ? If so then all you need is a cheap pre-insulated DHW cylinder as a store for your heating. A couple of 2 port valves* to direct boiler flow to one or both cylinders according to their thermostats - and a TMV to control boiler outlet temperature.

The TMV goes in either the flow or return, and simply circulates the water round the boiler until it's hot enough to effectively heat the cylinders top down.
In the flow from boiler to cylinders, connect the boiler flow to the common port, the "hot" connection tees into the boiler return, and the "cold" connection goes to the cylinders. While the boiler flow is below the setpoint, the water circulates out via the "hot" port and goes back to the boiler. When the flow is hot enough, the valve starts directing it to the cylinders and the return flow to the boiler will be a mix of heated and unheated water. See this diagram (but ignore the plate heat exchanger).

Heat bank basic arrangement

• SimonH2
• 12 Aug 2011
• 3

Basic arrangement for a heat store. No control of primary flow in DHW PHE risks destratification...

It's also possible to put the TMV in the boiler return - in this case it's set to control the inlet temperature to the boiler, and assuming a constant flow and heat input, that will control the outlet temperature.

For the heating, I'd suggest using a TMV as shown to get a more steady radiator temperature - but you'd want your tank connections at top and bottom, this diagram also does DHW. Otherwise, you could have them scalding hot to start with, and slowly cooling off as the store depletes. Use TRVs on all rads, and do NOT have a bypass - that way, a modulating pump will only move the water the rads need to maintain room temperature and you won't unnecessarily mix the water in the store. Ideally, you want to have the store with hot water in the top, and cold in the bottom. Done right, there'll be a boundary layer that moves up as you use heat, and moves down as you replenish it. If you allow it to get mixed up, you'll end up with a tankful of lukewarm water and not get to extract as much energy from it.
If your rads are adequately sized, then under low-medium heat demand, you'll find the return temperature from the CH is not much above room temperature.

Your situation is different to most people's. Most are trying to maximise condensing boiler efficiency, you are trying to maximise energy storage to take advantage of cheap rate lecky. To that end, you want to be heating the stores as hot as you can reasonably get during the cheap rate period, and maximising the energy extraction at other times.

* I'm not familiar with these boilers, if they need a pump run-on period then you may also need a bypass to allow for this when both valves are closed.

 

[Mustardshots]

Hi Simon - thanks for that.

I totally get the TMV where it is - very logical I just hadn't thought of it. However I've struggled to find something that performs what's required. Closest I've found so far is a Solar Transfer Valve by Reliance. That can be set up to 70 degrees (I've assumed the thermal store would be as hot as poss - 90 - 95 degrees) so partly answers the issue of cooling far too quickly. Have you come across anything I've missed? There must be somethign for Thermal stores but so far I've drawn a blank.

The rest - the idea of adding an external tank for heating - makes total sense. One development though, the suppliers have come back to me now with a multi-port version of what I have which would make life so much easier.

Thanks once again

Pete

 

[SimonH2]

I totally get the TMV where it is - very logical I just hadn't thought of it. However I've struggled to find something that performs what's required. Closest I've found so far is a Solar Transfer Valve by Reliance. That can be set up to 70 degrees (I've assumed the thermal store would be as hot as poss - 90 - 95 degrees) so partly answers the issue of cooling far too quickly. Have you come across anything I've missed? There must be somethign for Thermal stores but so far I've drawn a blank.

For my thermal store I put the TMV in the boiler return - it's just a standard one made for bathroom/sink use and doesn't go all that high. By being in the return, it doesn't need to go as high as in the flow.
Honeywell do some valves that might be applicable, but I gather they are eye wateringly expensive

One development though, the suppliers have come back to me now with a multi-port version of what I have which would make life so much easier.

Yes indeed.

Just be aware that you are going to need a blending valve (just another name for a TMV) on your DHW supply. If you are heating the store hot enough to store a useful amount of energy, then it's going to be far to hot to be safe. That may have come with the cylinder, it usually does with a thermal store.

 

[Mustardshots]

For my thermal store I put the TMV in the boiler return - it's just a standard one made for bathroom/sink use and doesn't go all that high. By being in the return, it doesn't need to go as high as in the flow.

That's the bit I'm not getting - I do apologise.

To my understanding the Thermal mixing valve jobby is stopping cold water going into the cylinder by circulating it around the boiler circuit until it reaches a chosen temperature. The standard TMV's I've found are there to prevent water getting too hot - surely that's exactly the opposite of what's intended? Hence me suggesting the Solar valve as it's function is to ensure water is at least a certain temp before being allowed to pass. I am sure I am missing something fairly basic here so apologies in advance.

Your point on the hot water mixer is noted and there is indeed one that's included with the system.

Many thanks again Simon

Pete

 

[SimonH2]

To my understanding the Thermal mixing valve jobby is stopping cold water going into the cylinder by circulating it around the boiler circuit until it reaches a chosen temperature. The standard TMV's I've found are there to prevent water getting too hot - surely that's exactly the opposite of what's intended?

All these valves are is a thermostatic element that moves a valve disk - if the element is cold, it opens one port to common, if it's hot it opens the other port to common. Obviously, for a lot of people, the commonest (perhaps only) installation setup they've seen is as a mixer to control maximum temperature at the hot water taps - but this is just one way of connecting it.

Using it in the boiler return works in just the same way as using it for a tap - connect hot and cold water to the inlet ports, and it will regulate the temperature at the outlet. Set it to (say) 14˚ below your desired boiler outlet temperature, and by the time the boiler has added 14˚ you'll get out more or less what you want.

Turn it around though and feed water in through the common port, and it will now act as a diverter. If the water is cold it will go one way (recirculate back to the boiler in this discussion), if the water is hot it will go out through the other port (to the cylinder(s) in this discussion).

So the same valve can be used as a mixer or a diverter, the difference is in how you connect it. In general this is true of most valves - you just need to watch out for things like inlet strainers and non-return valves which are common in TMVs sold for DHW installations.

While I've talked about these valves as though they are like a switch (either one way or the other), in practice they operate over a range. So if (for example) it was set at 60˚, it might be hard over one way until about 55˚, about half way at 60˚, and not hard over the other way until the temperature is up to 65˚. So in this case, over a range of about 10˚ the valve is in an intermediate position so it will mix (in mixing mode) or partially divert (in diverter mode) the flows.

 

[Mustardshots]

Got you - makes total sense now.

So I've just got to look for a valve with the hightest temp mixing point I can find and crank it right up. I'm guessing there will always be a compromise in terms of what is sent back to the cylinder to prevent a couple of pipe's worth of water spinning around the boiler circuit until it reaches the sweet spot. Has to be better to get almost up to temp water into the tank than for the rads to keep on eating away at the thermal store.

Is there a generally accepted rule of thumb? Again - many thanks for your input

Cheers

Pete