Two hot water tanks - joined.

[Chris McBrien]

My CH system and Immersion hot water system uses two tanks. One tank is directly above a solid fuel stove that feeds the indirect heating coil. This tank does NOT have an immersion heater fitted. The base height of this tank is about 2.5 meters above the ground. My seconf hot water tank is under the stairs and at ground level. This tank IS fitted with an immesion. The distance between the two tanks is about 30 foot as the pipe flies.

These two tanks are connected together by two pipes and I've fitted a transfer pump in the loop so that in the winter when I have the fire going full blast I can have one bath from the water provided by the immersion heater and then turn the pump on for ten minutes to transfer the water from the FIRE tank to the IMMERSION tank and let others have a bath quicker.

A slight problem occurs in the summer when the fire is let to go out and there is only cold water in the upper FIRE tank. The hot water from the lower IMMERSION tank now percolates its way to the higher, colder tank. Heating it aswell.

It is impossible to set the two tanks at the same hydrostatic height in order to stop this natural transfer.

Any ideas Guys?

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[Andy Hall]

First of all, with this somewhat unusual arrangement, hopefully you have made sure that there is a clear vent path from both cylinders to the feed/expansion tank. This should be an individual one from each and anything else that you add should not block this in any way.

I can think of two solutions:

- A zone valve arranged in series with the pump and powered by the same supply. When you turn on the pump, the valve opens as well.

- A swing check valve in series with the pump. It remains closed unless the flow from the pump opens it. Convection would not be enough.

[Rick]

One tank, with both coil and imersion. A possible problem with this is overheating of the water ?

Rick

[Aidan]

The two hot water storage tanks should be connected in series, i.e.,

i) the cold feed from the cold water storage tank goes into the lowest cylinder at the base. ii) The hot water outlet goes from the top of the lower cylinder and connects to the cold feed at the base of the higher cylinder. iii) The hot water outlet from the top of the higher cylinder connects to the HW distribution system.

For all practical purposes you could regard this as one cylinder. Put the immersion heater into the upper cylinder.

The DHW pump should have a bronze impeller, NOT a standard CH pump. I'd consider connecting a CH pump to a pair of closely-spaced tees on the return from the upper cylinder, and connect this to the indirect coil of the lower cylinder to heat it.

[Aidan]

gravity circulation between a thermal store and a hot water storage cylinder on the floor above. It didn't work. Gravity circulation/convection exerts sufficient differential pressure to keep the swing-check valve open; in fact, it made no discernible difference.

A spring-check valve may have worked. I'm going to amend it to plan B, a NC zone valve.

Yes, I'd recommend that. Fitting the immersion heater to the upper tank is the best plan, IMHO.

[Chris McBrien]

First, regarding the dual venting of both tanks.

In all fairness I did this system 22 years ago and first time I only properly vented the one tank. I ASS-U-ME'd that the two connecting/transfer pipes would allow the air back into the unvented tank... WRONG ! The first time I drained the system (during the installation) I popped my head into the loft and 14.7 psi on the outside and 0psi on the inside of the tank had done its job and crushed the tank. We/I learned from that and put a hole through 23 inches of Granite to get a vent pipe through and back to the cold water source tank. Problem solved.

The motorized zone valve, switched with the pump, seems to be the logigal answer. Beleive it or not I did buy one some seven years ago but.....

If I put the immersion into the upper FIRE tank I would not have any hot water in the lower IMMERSION tank unless I used the transfer pump. The idea was to get a second load of hot water into the IMMERSION tank from the FIRE tank quickly, rather than switch the immersion on again. The whole problem is caused by this 23 inch Granite wall that virtually splits the house and the distance between the two tanks. I have to have the FIRE tank right above the fire so that the pipe run to the indirect heating loop is fairly short.

Thanks for the "Swing Valve" pointer, never knew they existed. Why only BSP fitting? Are they more of an industrial device.

Thanks again guys. This is a great board.

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[Doctor Drivel]

connecting/transfer

Firstly terminology: tank = cold water storage tank cylinder = hot water storage cylinder

Do don't mention where the hot water draw-off comes from and how the two cylinders are vented.

I would do this:

1. Have the FIRE cylinder as a "pre-heat" for the IMMERSION cylinder
2. No pump between.
3. Take the cold feed of the cold water storage tank into the bottom of the FIRE cylinder.
4. From the top of the FIRE cylinder, DHW draw-off, take an open vent over the cold tank
5. Take a pipe from the top of the FIRE cylinder, DHW draw-off connection, to the cold feed of the IMMERSION cylinder.
6. From the top of the IMMERSION cylinder have a vent pipe over the cold water storage tank, and from pipe take the hot draw off to the taps. So, eaxch cylinder has a vent pipe each.
7. Remove the two cyldiners connection pipes.
8. Remove the pump.

- Both cylinders will be vented, so no implosions.

- No natural circulation between the cylinders

- One cylinder feeds the other. This acts as preheat in winter and pre-heat in summer too as the water in the FIRE cylinder will be warmer than the water in the cold water storage tank

Simple no need for pumps.

[Pete C]

Hi,

I'd have thought a sprung single check valve is enough to stop convection, if not try a differential bypass valve as used in CH:

cheers, Pete.

[Chris McBrien]

The two pipes used to transfer the water between the two CYLINDERS were already insta= lled when the house was modernised back in 1981. Only one cylinder was installed then= and that was the IMMERSION cylinder. The house heating system then was electric stor= age radiators... =C2=A3=C2=A3=C2=A3. I then installed a big cast iron Bont ESSE IV (=

12kWatts) solid fuel room heater with a water jacket. This had to fit into the 23 inch t= hick Granite wall. The cylinder for the FIRE had to be right above it. I then "Teed" the= two existing pipes into the new tank so that I could transfer the FIRE cylinder water t= o the IMMERSION cylinder. There is no need to transfer hot water in the other directio= n. OK the cold water from the IMMERSION gets transferred to the FIRE cylinder.

I will go ahead with the zone valve as I do have one. There are a pair of free contacts in t= he zone valve that can be used to switch the pump on when the valve is open

Thanks again guys.

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[Andy Hall]

contacts in the zone valve that can be used to switch the pump on when the valve is open

Good point on using the aux contacts - that is the right way and in a more conventional design they are used to give the demand signal to the boiler and indirectly the pump; this all happening once the valve has opened.

Another thing that you could perhaps do is to fit a cylinder thermostat on the FIRE cylinder and use that to open the valve....

[Andy Dingley]

Can you mount the immersion tank _above_ the fire tank ?

With my experience of solid fuel waterheating, I've never been short of heating capacity when the fire was lit and I've often been short of HW storage after the fire had gone out. I'd have no problem with the fire always heating both tanks, with thermosyphon circulation between both. Obviously having the immersion in the upper tank and discouraging circulation would be an improvement from that point too.

Another solution might be to rename the upper tank "single", the lower tank "extra" and fit an immersion into the upper tank. Use the pump when you want extra HW storage, otherwise just use the upper tank for everything.

[Doctor Drivel]

He said he can't do that.

[andrew]

On a similar note I wish to add 2 solar panels to heat dhw. These have been rescued from a swimming pool dismantling. I have been discussing it with people on uk.environment and posted some pictures of what I think Ishould do to make use of the panels and take the opportunity to plumb in a wood stove back boiler.

Currently I use gas to heat a 60ltre cyclinder in summer. Some concerns have been expressed that this will be causing waseful short cycling of the boiler. I wonder what evidence there is for this. It has been suggested that the short cycling would make the boiler less efficient than an offpeak immersion, with a given daily loss from the tank of ~2kWhrs(t).

To make any sensible use of the solar I will need increased cylinder capacity and this can only easily be fitted one floor below the current cylinder, this makes a direct connection to the wood boiler more practical.

I have put a shematic at

and the main picture of the panels is followed by the proposed schematic and the possible roof site for the panels, this is overdue for repair already.

AJH

[Chris McBrien]

I can put an immersion into the FIRE cylinder but my bathroom and shower are right next to and above the IMMERSION cylinder so that's where the main hot water usage is. The kitchen sink is at the other end of the house and tends to use the hot/warm water from the FIRE cylinder as it is closer. The FIRE cylinder only gets really hot when I turn off the CH pump so that all

12kWatts of the solid fuel stove goes into the indirect heating loop of the FIRE cylinder. Then the water is worth transferring to the IMMERSION cylinder to get a saving on its use.

I've looked at the BES website mentioned above and because the sring valve is so cheap I'm going to give that a try first. If it fails I'll go back to plan (a) and fit the zone valve.

Great to bounce these ideas around... Chris.

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[Aidan]

If this is so, it suggests that you have the two cylinders connected in parrallel. If so, when you turn on a hot tap, you will get some water from both cylinders simultaneously. If one is hot and the other cold, this will be a problem.

The DHWS pipework from the 2 cylinders should be connected in series, see above. All the hot water supplied should be coming out of the top outlet of the FIRE cylinder

You shouldn't have any valves in the open vent pipe from the top of the cylinders, zone valves and check valves included.

[Pete C]

Hi,

I'm aware the convection is in the same direction that the water is pumped. The pump will be able to overcome the resistance of the check valve or bypass valve, whereas the convection currents won't.

Another way to do it would be take the hot output from the top of the immersion tank to the same level as the bottom of the tank or below, before going to the other tank.

If you want to fit a zone valve, by all means do so :)

cheers, Pete.

[Chris McBrien]

There are NO valves, check or zone, in the vent pipes of either the FIRE cylinder or the IMMERSION cylinder.

My very initial mistake, 22 years ago was to omit the vent pipe on the FIRE cylinder as I ASS-U-ME'd that the transfer pipework would allow the vent on the IMMERSION cylinder to take care of matters. As I said earlier, I did learn from that mistake.

Must go for a Pint. Its been a long day.

Chris.

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[Doctor Drivel]

I still don't know how the cylinders are vented and how the hot draw-offs are configured.

[Doctor Drivel]

That is why it is a silly idea