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pressurized vs. non-pressurized storage

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by chuck172, Mar 2, 2011.

  1. chuck172

    chuck172 Minister of Fire

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    I'm in my third year of heating with my Tarm and I never suspected that my Domestic hot water system is lacking. According to Chris @Tarm:
    "Pressurized storage tanks do not normally have domestic hot water coils in them. Most options being used in North American are simple steel tanks, either factory fabricated for the purpose or modified LP tanks, etc. In most cases these tanks do not have a heat exchange coil in them and it is difficult to add one. This means that the heat exchange for domestic hot water production has to take place outside of the tank, typically this is done with an indirect water heater. The heat exchange coil that is built into the indirect is sized assuming 180 degree supply water temperature. This is not a problem when you are heating the indirect with the wood boiler or fossil fuel boiler, but when you are heating the indirect from storage, you will be using lower temperatures than that and the performance of the indirect drops off quickly. Even if the tank starts out at 180 degrees (or more), once you start to get below, say 160 degrees (150?), you can’t expect much hot water production from the indirect. This is all perfectly practical, but it does mean that you need to fire the boiler frequently enough to maintain high enough tank temperatures to meet your domestic hot water demand when you are running off of storage for a significant portion of the time, like in Summer.

    An unpressurized tank system normally incorporates a heat exchange coil for domestic hot water production. It is easy to ‘oversize’ this coil so that it has enough surface area to produce acceptable amounts of domestic hot water at lower thermal storage tank temperatures, say down to 120 degrees tank temp. This coil is normally supplied directly from the well or city cold water supply and feeds into a more traditional water heater. The cold incoming water is heated, or pre-heated, before it gets to the water heater and the water heater only fires when the pre-heat from the thermal storage tank is insufficient. In an ideal, but expensive, scenario, the in-tank domestic hot water coil feeds a normal indirect water heater. This allows you to pre-heat the incoming cold water as described and simultaneously heat it via the coil built into the indirect. This gives even more heat exchange surface area and so allows for even lower tank temps. One of the things that people are looking for when they are thinking about heating domestic hot water with the wood boiler in the Summer, in my experience, is long coast times between wood boiler firings. I would suggest that the unpressurized tank system I described means the difference between firing once or twice per week and firing two or three times a week with the pressurized system I described.

    There are of course many variables including storage volume and how much hot water is needed (retired couple or four kids and a ton of laundry?). Also, there are clearly other options like shell-in-tube or flat plate heat exchangers that can be used with pressurized storage to take advantage of lower tank temperatures. And, of course, if you are living in Europe , you can just buy a pressurized storage tank(s) that have over sized coils built in!

    As I say, none of this is a hard and fast rule about pressurized vs unpressurized storage, but given the market as we see it and the technology available here, I think they are good rules of thumbs. I will be very interested to hear others’ thoughts on this, it is something that is very important to the industry and consumer and something that we continue to struggle with". Chris.

    I have a 45 gallon superstore that seemed to work well with my pressurized storage. Now I'm wondering if I can upgrade my DHW system. I plan on burning all summer.

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  2. Nofossil

    Nofossil Moderator Emeritus

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    It's a good discussion. My current candidate for best of all worlds:

    1) Pressurized storage, to start. Much better performance, more usable heat from same size tank.

    2) Indirect DHW tank, plumbed as a zone in the heating system so that it can be heated with wood or backup heat source.

    3) DHW tank mounted near but above pressurized storage with sidearm heat exchanger plumbed to thermosiphon from storage. That SHOULD result in raising the temp of the DHW tank to the temp of top-of-storage automagically, but allowing DHW to remain hotter than storage if storage heat is withdrawn for space heating purposes.

    Attached Files:

  3. chuck172

    chuck172 Minister of Fire

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    Chris (from Tarm) claims that the coils in the indirect hot water tanks are designed for 180* boiler water temp. and are somewhat lacking when used with lower temperatures as seen with our wood-boiler pressurized storage systems.
  4. Singed Eyebrows

    Singed Eyebrows New Member

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    When IPS built my tank I had a 3' diameter 2 turn stainless domestic coil put in. Even with this easy install it was about $300.00 for the loop. I know my tank is not typical, it is an option though, Randy
  5. ewdudley

    ewdudley Minister of Fire

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    +1

    And I would note that the power of the sidearm will increase with elevation, so if the sidearm was above the storage tank and the DHW tank was above that in a first-floor closet, then so much the better.
  6. dzook

    dzook Member

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    We just finished a project with a Vedolux 37 where the owner used a 1000 gallon propane tank and had it modified with two domestic coils installed into the top to the tank. the boot/flange that held the coil was mounted almost horizontally into the tank but right at the top. With two domestic coils, doubling the surface area, the tank seems to give good domestic water with the top of tank down to 140deg.
    BTW the lines going into the domestic coils were redone neater after this photo was taken.

    Attached Files:

  7. Jeff S

    Jeff S Feeling the Heat

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    I wish I thought of it before I insulated my tanks but I think if you wrapped your propane storage tanks with several loops of copper or maybe even pex you would have a effective way to preheat your domestic water.I believe the circumference of a 500 gal. tank is close to 10 ft.You could easily wrap several hundred ft.around one of these things for a large reserve and since all would be insulated ,the water in the tube should be the same temp as the tank.All that would be left would be to temper it.
  8. tom in maine

    tom in maine Minister of Fire

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    If you have a SuperStor in your system, this can draw off your pressure tank and it would allow you to heat the Superstor with relatively cool or warm water.
    As an example, if your pressure tank is only 118F you can pump that tank water into the SuperStor and heat it to about 118F. It might take a while, but it is do-able and you do not need to modify anything. An external plate hx will do the same thing with a regular water heater tank. This scenario gets around the limitation of using an in tank coil in a pressure tank. Most pressure tank coils are not large enough to perform very well, at say 2 gpm, when the tank is cooler than 130F.

    All this being said, an unpressurized tank does not suffer this same limitation since we can install whatever size heat exchanger we want.
    Our standard DHW hx will deliver usable hot water at 3 gpm with relatively cool tanks, being below 120F.
    We wind up dealing with a lot of apartment building and motel systems that require very large gpm ratings for solar DHW systems. It is a simple matter to install a very large hx in an unpressurized tank to fit any flow rate. They can be simply retrofitted also. Just pop the cover and install whatever you need.

    I know that a lot of you like the perceived economy of old LP tanks, but for convenience and service and simplicity (read that very small expansion tank), an unpressurized system can offer a lot.

    Maybe it is not your cup of tea, but it does work very well if properly designed.
  9. jimdeq

    jimdeq Member

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    Chuck172, I have a sidearm heat exchanger off of 1200 gallons pressurized storage. I dont think I have it plumbed correctly because it sure seems like it takes a long time to recharge. It runs off a Grundfos Alpha. The sidearm is 48" of 1.5" inch copper with a 3/4" inside. It runs off a zone valve. I have a stainless tee threaded into the bottom of my propane fired hot water heater. Sidearm goes into the side off the tee and A419 aquastat set at 130 is threaded into the end of the tee. The problem is that it seems like Alpha is constantly pumping to the sidearm. I know it is working because water heater rarely fires,but it seems like I could do something more efficient. Any thoughts anyone?
  10. in hot water

    in hot water New Member

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    There are numerous tanks that are approved for higher than 180F in the coil. The tank we sell is approved for up to 200F in the coil as well as the tank temperature.

    But you really should not have to drive them that hot to get acceptable performance. Most indirects, regardless of the brand will have 15- 18 square feet of HX coil surface. Some use finned coils, some are smooth, some use corrugated stainless.

    If you really need more performance you can buy dual coil indirects and series the two coils. We offer a tank with 36 feet if 1-1/2" diameter lower coil, with an upper coil option of another 30 feet. With a large diameter coil you can move a lot of flow thru the coil. We have tested ours with a 14 gpm flow in the coil. 14 gpm of 180- 200F boiler input should cover just about any residential DHW application you can throw at it.

    Some of the reverse indirects used to be rated with 200F boiler supply to get the high output numbers. Make certain to have a good listed thermostatic mix valve if you plan on running those high temperatures or with those side arm, long tube type HX that run wild, with no temperature limit. it's possible to get your DHW up close to boiler temperature, that can be extremely dangerous!

    hr
  11. chuck172

    chuck172 Minister of Fire

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    But how do the indirects (superstor) do when the pressurized storage temps drop to say 140*, and I'm looking to heat dhw to 120*?
    Is there generally enough square feet of coil in the water heater tank to efficiently do the job?
  12. Rick Stanley

    Rick Stanley Feeling the Heat

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    I'm gonna guess, no. But we'll see what the heating guys say. I have an oil boiler with a dhw coil in it. I've been keeping that boiler at 140 all winter with my Garn. Dhw production SEEMED ok and heating SEEMED ok. I ran out of wood a few days ago and am now using oil and keeping the boiler at 180. There's no comparison. Waay better showers (2 family house) and the house is warmer.
    Anyways, I don't know why an indirect would work any better............?? bU t wHa t dew eyE nO
  13. tom in maine

    tom in maine Minister of Fire

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    Yes, the circulator might run longer to heat up the tank, but as long as there is a reasonable temperature differential,
    it will do the job.
  14. Nofossil

    Nofossil Moderator Emeritus

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    Indirect DHW heaters usually have a coil sized to provide a heat load of around 100K btu/hr at 180 degrees to match the output of a typical fossil boiler. That also provides rapid recovery after your teenager has taken a 45 minute shower.

    You can run cooler water through and still transfer heat, but it will take exponentially longer as the supply temp approaches the DHW temp. It helps to run the circ at really low speed to make the most of whatever stratification you have in storage.

    In reference to other post on long time to heat DHW with sidearm - that's certainly true. Sidearm HX has VERY little surface area and relies on thermosiphoning for the DHW <-> sidearm transfer. Works well when you have hot water flowing through the sidearm anyway as you would when heating other zones. Pretty slow if that's the only reason you're running a circulator. That's the reason form my thermosiphon approach above - slow heat transfer, but operates 24/7 as needed with no circulator. I don't have that setup, but that's what I'd do if I were starting over.
  15. tom in maine

    tom in maine Minister of Fire

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    "You can run cooler water through and still transfer heat, but it will take exponentially longer as the supply temp approaches the DHW temp. It helps to run the circ at really low speed to make the most of whatever stratification you have in storage."

    Since this hardware is already in place, it should not cost much to try it with the temps discussed.

    A thermosyphon is great if you can locate all the hardware properly.

    OR one could consider a proper unpressurized tank with a DHW coil. ;-)
  16. leaddog

    leaddog Minister of Fire

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    This would work IF you had the tanks vertical and you wraped the coils at the top where you had the high temp. BUT if the tank is horizonel you would tend to try and make the tank all the same temp and loose stratification. Not a good thing. A horizonal tank could have 100* water at the bottem and 180* at the top.
    leaddog
  17. Grover59

    Grover59 Member

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    I just use a flat plate hx, I have an electric hot water tank with the electric coil disabled, and I use the thermostat on the tank to start the pumps to the hx when the dmh tank temp gets low. I have set the thermostat on the dmh tank at 120 this gives me about 140 at the faucet. My 750 gal non pressurized wooden freak show tank may get down to 120 and I still have plenty of heat for dmh.
    I love flat plate heat exchangers they seem to give you a lot of flexibility may not be as efficient, but they really seem to work well. I don't have any coils in my big tank, I just use dip tubes to move the water, I don't seem to see many others that do it this way.

    Steve
  18. bupalos

    bupalos Member

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    Regarding the external heat exchanger, that's a very interesting idea. Someone needs to try it. I definitely think pex would be better bang for the buck there.
  19. Chris Hoskin

    Chris Hoskin TarmSalesGuy

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    anyone ever have any problems with a flat plate clogging because of sediment or mineral build up when being used to heat domestic?
  20. Hydronics

    Hydronics Feeling the Heat

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    I have often wondered how well a flat plate would work as a side arm -basically replace the sidearm with a flat plate placed at the bottom of the DHW tank and a copper tubing riser tied into the top of the tank. By placing it low on the DHW tank you'd get the greatest delta T and therefore the greatest change in temp / density which would maximize flow through the domestic side of the hx. Even the smallest flat plate has more hx area than most sidearms. Thoughts?
  21. Grover59

    Grover59 Member

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    I can say this thermosiphoning happens quite easily my loops to and from the tank are long and up and down and I have to put check valves in to stop the flow. Most all of the loops have zone valves now so it is not a problem. Even my floor heat would circulate through the 300 feet of pipe quite easily. Knock on wood but at this time I have not had any problems with crud in my hx, I have heard some people have, I am sure a strainer would be a good idea particularly in the dmw loop where you could get rust and I do get rust.

    Steve
  22. dpsfireman

    dpsfireman Member

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    There is another way to stop unwanted convection and that is by using heat traps. The beauty of this setup is that when the zone valves are closed they stop convection. However, when the power goes off and my valves open, it takes a while but it will still convect supplying some heat when the power is off and I don't have the generator running. They work great and there are no moving parts!

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  23. Bad Wolf

    Bad Wolf Feeling the Heat

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    Has anyone tried a hybrid system? If you took a propane tank for a presurized system and installed a access something like a small "man-way". That is cut, say a 10" hole in the tank, take a piece of 10" pipe and contour one end to match the curvature of the tank, weld a flange with holes into to the other end, match the curved end to the tank and weld. You could then install a prefabricated DHW coil into the tank. The system only runs at 14 PSI so I would think a good welder could ensure it would be solid.
  24. ewdudley

    ewdudley Minister of Fire

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    Sure! There've been a few hearth guys who have reported success with this idea.

    Many if not most of the European-style purpose-built pressurized storage tanks incorporate a DHW coil in the top (and optionally a solar coil in the bottom), typically with internal duct/baffle systems to let the cooled water fall away to the correct stratification level.

    Here's a nice field expedient version at post #5 of this thread:

    http://www.hearth.com/econtent/index.php/forums/viewthread/72019/#835793

    And post #3 of this thread, where the water is pumped from storage, or from a fossil fuel boiler, to small buffer with a DHW coil:

    http://www.hearth.com/econtent/index.php/forums/viewthread/66601/#760078

    --ewd
  25. chuck172

    chuck172 Minister of Fire

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    How about stacking a small tank with a coil in it on top of the large storage tank. I'm using a 500 gallon propane tank for storage now. Going with the "hybrid idea" as Greg proposed, how about setting a 50 gallon tank with a dhw coil, ontop of the 500 gallon. Stratification would guarantee the 50 gal. tank the hottest water. Size the coil to provide all the dhw needed.

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