Pressurized Storage with DHW - can this be done

[mpilihp]

Hi again, im back with another crazy thought. I really like the simple design that can be done with pressurized storage but I also like the ability of having a DHW coil in the storage tank with the unpressurized setups. Has anyone done a pressurized tank storage with a DHW coil in it? IE weld a flange onto the top of the tank that allows connecting a DHW coil onto? A boilermate DHW tank and pump can cost $900-1000, so paying someone several hundred to weld on and pressure test the tank would be cheaper. Last year one of my company's larger buildings was getting a new boiler/heating system and they were welding big 6" black pipe, putting on 90s and Ts and so on and using a sort of xray device to check the welds before pressure testing the system. Im thinking this company would be able to do the welding and pressure testing of the tank.

What/how do you need to clean the tank to ensure there isnt any gas vapors left in it before welding? I know when they do welding on gas tanks they fill it with water, do the same thing there??

Then you would have the issue of how to switch your DHW source from the storage tank to your backup but sounds like it can be done.

Could even install two flanges, one down low on the take to preheat the DHW going into the top coil....

~ Phil

 

[Nofossil]

That could certainly be done - no question. Just a matter of calcualting the cost. To be effective, a preheat coil needs lots of surface area distributed around the tank. That might be a little difficult to acomplish, and if you use copper you have to be sure it never touches the steel.

There are two other approaches:

1) Use a SuperStor or other indirect DHW tank set up as a zone in your heating system. Use the existing storage circ - only need a zone valve for the SuperStor.

2) Use a vertical sidearm connected to the top and bottom of the storage and to the top and bottom of the DHW tank. In order to do this, the tanks would need to be near each other, and the top of the DHW would need to be higher than the top of the storage. Anytime storage is hotter than DHW, you'll get a pair of thermosiphon loops that will slowly bring the DHW up to the temperature of the storage. I haven't tested this one, but I'm confident it would work just fine.

 

[Bob Rohr]

A couple of ideas I have been working on. I want to wrap some external HX around my 500 lp pressurized tank. The Rheem Ruud Solaraide tank has copper coils wrapped around the outside for a HX. they work great and also provide the double walled protection required in some areas.

Here are two ideas. One is solar panel copper absorber plates fastened with double sided tape. I have had this up to 180F for extended periods now. So far so good.

The other is a girdle that fastens with some coil springs to keep it tight to the tank. aluminum heat transfer plates wrapped around copper tube for better surface area.

If I get brave enough I may try to weld some stud pins on my LP tank to fasten a coil at the bottom for solar input.

hr

 

[mpilihp]

Wow thats a neat idea putting it around the LP tank, let us know how it works out.

I was thinking of using a spirl type DHW coil such as this: http://www.boilersupplies.com/tankheaters/vaillant.html

If a welder would put a flange on for several hunderd $$s it would be alot less than an indirect hot water tank.

NOfossil, I like your second idea of the thermal siphon loop, thinking it could be done with an old electric water heater?? My only concern would be on recovery, how quickly would the tank reheat, after the Teens are off and out of the house im sure that would work fine for the two of us but with kids....

THanks for the suggestions/ideas!

~ Phil

 

[Nofossil]

NOfossil, I like your second idea of the thermal siphon loop, thinking it could be done with an old electric water heater?? My only concern would be on recovery, how quickly would the tank reheat, after the Teens are off and out of the house im sure that would work fine for the two of us but with kids....

~ Phil

I'm very sure that the recovery would be slow. Might not be a problem for two reasons:

1) You'll need a mixing valve on the DHW tank outlet anyway, to prevent scalding. Much of the time, the DHW tank will be at elevated temperatures, as high as the storage gets. In that scenario, a 40 gallon DHW tank will last for days.

2) If the DHW gets cold enough to demand heat, then your primary heat source should kick in anyway, whether that's electric, indirect fossil, or indirect from storage. In any event, the thermosiphon loop doesn't make recovery worse than it would have been. I just don't think it could be the ONLY way to heat the DHW, though in practice it might be all that you would need.

 

[mpilihp]

Sounds good, ok well my other question was do you think a electric HW tank could be used? Would need to connect to one connection on the top and then utilize the drain on the bottom to get the siphoning effect working correct?

How much higher do you think the DHW tank would need to be above the storage??

 

[Nofossil]

Sounds good, ok well my other question was do you think a electric HW tank could be used? Would need to connect to one connection on the top and then utilize the drain on the bottom to get the siphoning effect working correct?

How much higher do you think the DHW tank would need to be above the storage??

I think common practice is to tee off of the pressure relief on the top and the drain on the bottom. Ideally, the DHW tank would be entirely above the storage. I suspect it will work as long as the midpoint is even with the top of storage, more or less.

 

[mpilihp]

Thanks its a good idea and can be done failrly in-expensive.... Hopefully I can find a tank around central maine for a reasonable price.

~ Phil

 

[Bob Rohr]

A couple issues with those thermo siphon "long tubes" They will track pretty close to whatever temperature your boiler runs at. You MUST have a listed ASSE mix valve, check the Powers website to learn the difference between 1016, 1017, and 1070. This is a serious scald issue, not to be taken lightly.

Also if you remove T&P;valves and install a tee, the temperature probe must be long enough to reach through the tee and into the actual tank, top 6" of it. They make extended probe style T&Ps;for this use. Again, a serious safety issue, especially with elevated tank temperatures.

Some HW tanks have additional un-use ports on the top or side, look for them before working with the T&P;port.

higher operating temperatures will extend draw down as Nofo mentioned. Higher temperatures do cause minerals to precipite out more quickly. Allow a way to acid flush that HX.

hr

 

[mpilihp]

Im just brain storming, im aways off from doing anything with DHW, still trying to wrap my head around it all, I was missing something NoFossil was saying and there needs to be some sore of a coil in the tank for DHW or a HX which adds cost and complexity and still makes the idea of putting a coil in the storage tank look good to me. Yes would need to dig in deeper into the threeway valve to regulate the temperature so not to cause scalding.

THanks

~ Phil

 

[Bob Rohr]

yeah, the coil right in the tank would be an excellent method. excellent heat exchange, no additional circulator required. If you can get the connection installed in the tank, and find a coil for the right price, go for it.

I am not a big fan of those thermo siphon tubes, yes they are simple, and used to be cheap to build They do have mind of there own and really should have a pump or zone valve and temperture to make them safer, in addition to fail to cold, scald protection. I've noticed some of the OWF manufacturers have dropped them from their catalogs. Possibly a liability issue or advice from their legal team. Soft tissue lawsuits are common and expensive to defend. The burn victim typically wins.

hr

 

[mpilihp]

That will be my plan if I can find a 500gal tank, saving up so if I find one I can purchase it and not miss out on one. Missing out on one that I dont know about doesnt bug me as I dont know! Im working up a parts list for doing the plumbing to get the wood boiler connected to my oil boiler, it will be in parallel and for now will keep the oil boiler belly hot until I add storage. Im adding Tees and ball valves so I can easilly attach a storage tank and a indirect hot water tank in case I dont do the coil in the storage if I end up not able to find a propane tank. Also adding Tees to later connect my garage heat (Radiant, tubes already run) and a hot tub-dreaming alittle)

~ Phil