parallel hot water tanks - can switch to single as well as dual use?

[taffyinhighlands]

I've got a DHW system with 2 storage tanks run in parallel - everything is well set up for the 2 tank operation, but other than completely removing one tank, and draining it to avoid legionella risk, is there a way I can keep both hooked up but only run the single tank (system was installed for 14 ppl, but usually only 5 there now) for most of the time, bringing back online with other one only when its needed?

thanks in advance

 

[Ric2013]

In parallel... when you open a hot tap some of the water comes from one cylinder and some from the other cylinder (in theory?)

Are you aware how unusual a setup this is?

 

[taffyinhighlands]

In parallel... when you open a hot tap some of the water comes from one cylinder and some from the other cylinder (in theory?)

Are you aware how unusual a setup this is?

Have asked around up here and it doesn't appear that unusual - everything is pretty well balanced with the same lenght pipe runs into and out of each cylinder, just basically doubling the tank capacity. I can just shut both the heating and tank outflow circuits off, both by the electric valve as well as isolating valves, but that leaves both the heating circuit and the tank itself just stagnant. If I motorised an additional set of valves on the tank outflow I could in theory achieve the aim of being able to switch, but how toi prevent legionella etc? - only solution I could think of was to completely isolate and drain both circuits in the 2nd tank.....

 

[SJB060685]

Turn cold supply off to the first tank and install a level valve on its hot out before it tee’s into the main hot water pipe. Drain than cylinder of water to prevent legionella bacteria buildup.

 

[Ric2013]

Thanks for the diagram, Taffy. I still maintain that running them in parallel is a bit silly, but I can see why it might have been done, and I have to admit that, done correctly, it should work (and obviously does!).

Turn cold supply off to the first tank and install a level valve on its hot out before it tee’s into the main hot water pipe. Drain than cylinder of water to prevent legionella bacteria buildup.

In practice, that's probably good advice. In theory, though, isn't the pipework to the valves a dead-leg if we're talking legionella, and when you refill, you then need to be careful where the flushed water ends up going?

What if he (or she?) did as you suggested on both cylinders, didn't drain either of them, but instead alternated the one in use on a weekly basis? Would that be a valid alternative?

 

[SJB060685]

There is a potential short leg of legionella risk yes. Short of completely remove and repipe I can’t see you not having that risk. You then have the problem of repiping again if you want to reinstate the second cylinder.
We had several customers with similar hot water systems but they were stately homes over multiple floors, many bedrooms and en-suites. It’s not your common domestic setup and I image the OP lives in a large dwelling.

 

[taffyinhighlands]

Thanks for the diagram, Taffy. I still maintain that running them in parallel is a bit silly, but I can see why it might have been done, and I have to admit that, done correctly, it should work (and obviously does!).


In practice, that's probably good advice. In theory, though, isn't the pipework to the valves a dead-leg if we're talking legionella, and when you refill, you then need to be careful where the flushed water ends up going?

What if he (or she?) did as you suggested on both cylinders, didn't drain either of them, but instead alternated the one in use on a weekly basis? Would that be a valid alternative?

thanks - guess thats on option, can just see them forgetting!

 

[taffyinhighlands]

There is a potential short leg of legionella risk yes. Short of completely remove and repipe I can’t see you not having that risk. You then have the problem of repiping again if you want to reinstate the second cylinder.
We had several customers with similar hot water systems but they were stately homes over multiple floors, many bedrooms and en-suites. It’s not your common domestic setup and I image the OP lives in a large dwelling.

Thanks - yeah, I was concerned about the same, so I think its going to be drain and isolate with new elbows replacing the current Ts

Its an old farmhouse, but has 6 bedrooms, but was rented out for upto 14 guests and the system designed so they never ran out of hot water. No need now a single family lives there I guess.