I know this is not a solar question but

Charlie from wmass

it is something you guys will come across more than the fossil fuel guys do. What are the pros for a pressure vessel for thermal storage for a wood boiler system versus an atmospheric storage tank. I have a bias towards pressure vessels but I am willing to have my opinion changed.

EddieG

Modern Hydronics

Which I am now reading has a section on this topic, with pros and cons. Pressure tank has to be rated, so usually a little more pricey. Along with being bulky and difficult to get into the building.  The non pressure tanks can be assembled in place, but have to have water added on occasion. I will look in the book tomorrow to see the other pros and cons listed. I have only used a non pressure rated model myself. It was on a Tarm Wood Fired about 10 years ago. System works well and homeowner is pleased. I took up quite a bit of space in the basement! 

Charlie from wmass

Thanks Eddie

I will put Modern Hydronics on my book list.

Bob Gagnon plumbing and heating

Non- pressurized tanks

I like my non pressurized tank. It was cheap and big. I simply framed a box

8' X 8' X 4' and lined it with plywood and one piece heavy duty roll rubber roofing with NO seams. You can make the heat exchanger a large as you like, and it should last many decades. I talked to a solar installer who has seen many installed in California in the 70'S and never seen one fail.



Thanks, Bob Gagnon

Mark Eatherton

Caution...

Some woody bio mass boiler manufacturers will NOT allow their systems to be a "''closed" loop system. It avoids them having to comply with the ASME requirements.



As for water loss, if you make a good tight fitting lid, and install a condensing vent (think old time copper still) water loss is minmal if at all.



ME

Bob Gagnon plumbing and heating

Tight Fitting Lid

That was one mistake on my tank, I built it in a corner of my basement and it's hard to get around it to seal it. What do you mean Mark, about the condensing coil, I never heard of that. It sounds like it would look cool too.



Thanks, Bob Gagnon

Karl_Northwind

baby oil

I have thought about putting a tiny bit of baby oil (think a tablespoon) on top of the water surface to eliminate evaporation. 

seems simple enough, but check for compatibility with the tank material.

the thing I have seen with atmospheric tanks is that people on the solar loop don't put a large enough heat exchanger loop in the tank, and so the solar loop runs at much much higher temps than necessary, reducing efficiency.



K

Mark Eatherton

Like this Bob...

I suppose a person could make it out of plastic as far as that is concerned. Anything that causes the vapors to condense and roll back into the tank is good.



ME

Kevin_in_Denver_2

Mineral Oil

We've tried a few ounces of mineral oil at the top of an atmospheric tank, but it doesn't seem to solve the evaporation problem for more than a few months.



I haven't done it yet, but a reasonably priced precision water pressure regulator should be more reliable than a float valve (or a homeowner) to maintain the water level of an open tank. It must be mounted at floor level and connected to a bulkhead fitting. I would use the Watts 1/4 215M1 0-8 It should be able to provide make up water to within a few inches of the desired level at all times. 150 bucks. http://www.watts.com/pages/_products_details.asp?pid=772



By the way, I like ME's solution as well, just watch out for the revenuers.

Charlie from wmass

The tanks will be for customers

I would not feel comfortable making the tanks. If it was my own house or a buddy's house it would be one thing. What if you capped the tank with Parrafin wax?

Ironman

Depends...

On if the wood boiler itself is pressurized, how much storage it needs and is it being connected to a boiler or scorched air?



If you're connecting an unpressurized wood boiler to a pressurized hydronic system, then I would look at a large indirect. If it's large enough, it would provide storage as well as the hydraulic isolation you'd need with the internal coil.

Kevin_in_Denver_2

Paraffin gets messy

We tried paraffin before we tried mineral oil. After things get hot, when they drop back down to room temperature, you have a real mess on your hands if you ever have to service anything in the tank.



Paraffin vapors are flammable when the liquid gets hot (that's how a candle works) .

hot_rod

if the boiler is a pressurized style

A lot of wood burner use old LP tanks for pressurized storage. Plan on at least 300 gallons. 500 is better yet. storage allows you to run good, long, hot burn cycles. also a longer non-burn "un-load" cycle.



Here is one company that offers refurbished LP tanks. Remember expansion tank size with large volume systems



Also Tom at www.amerciansolartechnics.com has a lot of experience with un-pressuried tank production.



Be careful with EPDM bladders there are different grades an not all can handle continious use above 180F.



Plenty of pros and cons with either type of storage.



hr

Mark Eatherton

Off topic, kinda....

Karl et al,



Many years back, I was taking a tour of our local sewage treatment plant. In speaking with the operators, I told them that we typically put veggie oil into the traps of our floor drains to keep it from evaporating and causing sewer gas to enter the building. He said that he would prefer that we use MOTOR oil, because it was easier to get out of the water when it hits the plant.



It seems that motor oil will not hydrolyze and go into suspension in the water, like vegan and baby oil does. Personally, sound like an icky gooey mess if a person ever had to get back into the tank, plus some bladders are not compatible with hydro carbons.



As I said, a person doesn't HAVE to use copper. PEX would work as well. Mightn't even be a bad idea to leave the vent somewhat trapped in the bottom end...



Although I've never seen it, I've heard of people using thing that float on top of the water to avoid evaporation. Things like Ping Pong balls and Nalgene chips, and those shipping peanuts.



ME

Bob Gagnon plumbing and heating

Questions

Kevin, did you use mineral oil in a rubber roof lined tank? I want to make sure I don't damage the liner. Where do you get mineral oil from? I have an open coil where I pump water through a flat plate heat exchanger, would mineral oil be alright going inside the flat plate?  Mark I still don't understand the purpose of the condenser coil, will that help evaporative loss from a loose fitting lid? I agree with Karl about the size of the heat exchanger coil being big enough, I made mine out of 6 coils of 1/2" X 60' coil copper, and it really works good, a couple of hours of sunshine and it really charges up my tank very well. With solar you have to make hay when the sun is shining.



Thanks, Bob Gagnon

Bob Gagnon plumbing and heating

180 Degrees

Hot Rod, would a wood boiler get a large tank like this, up to 180 degrees easily?



Thanks, Bob Gagnon

Kevin_in_Denver_2

Mineral oil should be OK

you can get a pint of mineral oil at Walgreens for $6.



It's pretty benign stuff so I don't think it would affect EPDM even at 180F.   Definitely OK with all metals.



I haven't looked up the flash point.



Since it always floats on top, it doesn't get into any of your pumps.



Bob, it looks like your tank is really hard to put any sort of lid on.  Maybe you could fit puzzle pieces of 3" XPS floating on the top of the water.