Open System For Wood Boiler

[WStayton]

Hi Guys!

  Another thread has opened up what, maybe, is a new (at least for me!) problem.

  My wood fired boiler is going to be circulating water that is stored in three each about 425 gallon tanks lined with EDPM material, and insulated with 5.5 inches of closed cell foam.

  The bottom of the cover will also be "covered" with the EDPM material, and will mate against a piece of 1/2" x 5.5" plastic material screwed down to hold the liner(s) for the tanks in place.

  Everything will be closed up a tightly as possible, but it will be far from sealed.  I started out thinking that this was a good thing, since it would allow for expansion of water as it heated, without having to have any other provision except filling it a little (1/2" or maybe 3/4") "short" to allow room for expansion.  Therein lies the problem, as the water in the tanks is alternately heated and  cooled, SOME air will be alternately drawn in and expelled.

  Will this system be an automatic built-in problem for the boiler???

  I had figured that I would have to have some sort of chemical additives in the water to minimize rust and corrosion - such as commercial boiler water products, but is this going to be enough with the ready access to SOME air on a c0ntinuing basis?

  I had toyed with the idea of "floating" a sheet of polyethelene on top of the tanks, in direct contact with the water, but polyethelene is slightly permeable to oxygen, so this would only reduce the exposure, not completely eliminate it.

  So what do you think, am I building a problem, and should I go back to considering heat exchangers in each tank to take care of heat transfer from the wood furnace and allow it to have a closed, slightly pressurized system???  Maybe 5 psi?

  And, while we're looking at it, is this open source of hot water going to be a corrosion problem for the baseboard heating units that will be fed by the hot water in the tanks???  They ARE copper, but even copper will corrode!

  I'm not big on doing things that are not necessary, but I'm even LESS big on doing things over because I didn't do them right to begin with, to the tune of a few thousand dollars!  <grin>

  All opinions are hereby solicited!

Regardz,

Wayne Stayton

[mike90045]

If your boiler is not copper or brass, you will have electrolysis occurring in the system. What is your interconnect pipe ?  With dissimilar metals, the less noble one, is destroyed. It matters not that steel boiler is 80' from the copper heater, electric current will still flow, no additive will stop it, only a less noble metal (zinc) will help, or a calibrated anodic protection charger.

[WStayton]

mike9004:

  Internal plumbing, between components, will be Schedule 40 PVC, everywhere that it can be exposed to light/traffic/abrasion and barrier PEX for the hidden/covered part.

  The boiler is definately cast iron.

  In the "real" world, cast iron boilers are used everyday with copper baseboard units - BUT these are closed systems, so there is NO oxygen to contribute to the rusting problem, and some sort of chemical additive is often added to the mix.

  I WILL install sacrifical anodes ("zincs") but, as is explored in another thread, getting these "into" a boiler so that every part of it is covered is problematic - I don't have a 1" diameter arm that is six feet long, with four universal joints in it!  <grin>

  What I'm searching for here, is whether or not a more-or-less open system is a long term solution to the heating equation.

Regardz,

Wayne Stayton

[Tom Reed]

Please us CPVC if it's for hot water. Regular PVC will melt at fairly low temps. Did it where I used to work.

[WStayton]

Tom:

  Now I am confused (AGAIN, so whats new?)!

  I though schedule 40 (heavy wall) was approved for hot water - or am I laboring under yet another, more, already, misconception/misunderstanding???

  Since I haven't bought anything yet, it easy to fix, at least!

  I'll do some more loking - thanx for the heads up!

Regardz,

Wayne Stayton

[WStayton]

Hi Guys!

  Well, I see where I made my mistake - Schedule 40 is approved to 140 F, which would cover all normal domestic hot-water.

  However, in my system, the hottest water is going to be just sub-boiling, so schedule 40 won't work!

  CPVC is rated to 200 F or 220 F, depending on which website you believe, but since my high temperature's will be low pressure (5 to 7 psi, at most) I THINK I am safe with the 220 F rating.

  Thanx for the "heads-up", Tom, its lots better to have a red face here on a forum than to have a red (scalded!) face behind the wood boiler some night!  I have talked about using schedule 40 in half a dozen different postings and you are the first person who has caught it!

   Upon further reflection, I'm wondering if I should have hard copper for the very high, just off of the boiler, temperatures.  Hard copper costs more but it only costs more ONCE!!!!

  Also, I suppose the outlet pipe for the solar water heater should be CPVC at least, and maybe Copper, since that can reach boiling too!

  Thanx for the heads up - much easier toi fix with the stroke of a pen than with a mop and bucket and hacksaw!!!  <grin>

Regardz,

Wayne Stayton

[Tom Reed]

Well it just happens that I've had the pleasure of replacing a bunch of melted, sagging 2" sched 40 pvc pipe with some nice sched 80 cpvc. Fortunately all that spilled on our case was about 60 gal of distilled water.

[WGB]

Open system boiler treatment:
http://hawkenenergy.com/store/individual-parts/water-treatment-certified.html

This is just first one I grabbed, many more out there!

[WStayton]

WGB

   Thanx for the link - but I crawled around all over that link and I can't find anything that says that the product you indicated was for open systems.

  They also show lots of barrier PEX and pumps and assorted hardware, so what makes you think that that particular chemical/treatment is for an "OPEN" system?

  It would seem to me that using barrier PEX on an open system would be sorta like nail-polish on a mule - It looks good, but does absolutely nothing!  <grin>

  Or, did I miss something IMPORTANT here - AGAIN? <smile>

Regardz,

Wayne Stayton

[cognos]

You'll need some kind of feedwater treatment to protect any metal in the system exposed to water.. That's what boiler feedwater treatment chemicals are for - protection from dissimilar metals corrosion, hydrogen pitting, etc.

In the trade, in large boilers, we use de-ionized water, and re-use condensate - essentially distilled water. Since you are not creating steam, you need to just start with DI water. DI water does not conduct electricity, it's only when it has additives and contaminants in it that it enables corrosion.

You say "open sytem." I'm going to assume you mean an atmospheric, non-pressurised system. Yes, there will be oxygen gas available for absorbtion at the interface of your surge tank. An easy - and very effective -  way to control this at the interface is to "seal" the surface of the interface with an inch or two of oil - linseed will do. Float it on top. This will minimise evaporation, and stop oxygen absorption at the interface. It will also decrease the requirement for make-up water, to replace feedwater lost to evaporation.

Start with de-ionised water. This will solve 99% of what you are concerned about. You may have to buy it.
Limit anywhere that the water can be exposed to air.
Seal the expansion tank with a gas-impermeable oil.

[WStayton]

Hi All!

  I have done some thinking and some looking (Google) and have come to the following conclusion:  I SHOULD have a closed, slightly pressurized, boiler system because:

a) It will rust out if it is an open system.

  I base this on the fact that EVERYBODY tells me that I have to have barrier PEX in the system if I want ANY steel/iron parts to last.  It would seem to me that having barrier PEX and having an open system is just an exercise in failure.  What good does it due to have oxygen impervious PEX if your system has an open exchange of oxygen to the air?

  Most (in fact, all that I have been able to find!) systems use a closed loop that quickly uses up the oxygen that is dissolved in the fill water and then operates in an an-oxic environment.  No oxygen = no oxidation.  Poof, problem solved!!!

  Most systems use some sort of "zincs" in the closed water of the system, but it looks to me like this is just to guard against the entry of vary small amounts of oxygen in make-up water that is introduced due expansion and contraction during normal heating-cooling cycles.

b) A closed system will allow me to use R/O water as the make up water which will eliminate a lot of the scale/oxygen/rust problems.  The volume of the boiler that I am going to use is slightly more than 20 gallons and I don't believe that my heat exchanger(s) will have a volume of more than ten gallons, so the total volume of R/O water required is less tha one days output from a 100 gal/day system, which is what I intend to install – although I'm not sure that I am comfortable with having it be a direct feed with the usual back flow preventing device(es), since these can fail and there is really no way to know, until you fall over from being poisoned by the water! <grin>  Maybe some sort of make up tank with a sight glass that I can open a valve and fill manually when it gets a little low?

  There is the question of how big of heat exchangers do I need to have in the three tanks.  As I envision it, this will be made up of a four foot baseboard copper radiator,  that will just fit, cornerwise in my 3.75'ish ft inside square tanks.

  Four a heat exchanger made up of a piece of finned tubing,  four feet long, I figure that this has the capability to transfer about 12,000 BTU/min assuming that the heat transfer in water is five times what it is in air – which seems reasonable to me since water is about 350 times as dense as air and its specific heat capacity is about 4 times that of air on a mass basis.

  However, the 1/12 HP circulator pump that I planned on using, is rated for about 30 gpm and with the heat exchanger(s) I think that they will be lucky to circulate about 20 gpm, or 160 lb/min – so with a delta T of 10 deg F / tank, I can  dissipate about 5,000 BTU/ Min.  The boiler is rated at 2,167 BTU/Min, so it looks, if all my assumptions are right, like I won't create a water powered rocket that will launch through the roof!!!

  The actual heat dissipation of finned tubing immersed in the stored hot water is a scientific wild a$$ed guess – but if anything, the actual heat transfer would be, I THINK, more than I have allowed and even if it is half what I have guessed, it is still enough!

  Does anybody see any BIG holes in this analysis???

  Inpuit is hereby solicited!


Cognos:

   Since the baseboards, and the engine cooling water will both be dumped into the three tanks and then flow by gravity from one end to the other, I don't really think oil is a good idea - if only 1% per day of it gets stirred in and moved along, eventually it will all be in the end tank with nothing in the first tank.

  Note the engine cooling water has its own heat exchanger on the engine, so the water circulated to the tanks does NOT go through the engine, only through the engine heat exchanger.  The engine water system is a 7 pse pressurized system, similar to an autotomotive system except that it uses a heat exchanger not a radiator and since it is designed to run with raw sea water (salt or fresh) I don't think corrosion is a problem.  The heat exchanger does have marine "zincs", BTW.

  Thanx for the input, everybody!

Regardz,

Wayne Stayton

[WGB]

WGB

   Thanx for the link - but I crawled around all over that link and I can't find anything that says that the product you indicated was for open systems.

  They also show lots of barrier PEX and pumps and assorted hardware, so what makes you think that that particular chemical/treatment is for an "OPEN" system?

  It would seem to me that using barrier PEX on an open system would be sorta like nail-polish on a mule - It looks good, but does absolutely nothing!  <grin>

  Or, did I miss something IMPORTANT here - AGAIN? <smile>

Regardz,

Wayne Stayton

All that company sells is open steel outdoor wood boilers.
Central Boiler is also an open system, they also sell same type of water treatment.
All open system OWB companies that I know of sells water treatment.
Water treatment is also important in close systems too, but is a huge issue with open.

[WStayton]

WGB

   About a closed system:

   I THOUGHT that if you have a closed system that you have filled with distillled or R/O water, you just need to monitor and make sure that it stays in a pH of 7 to 8 (slightly basic) and you're good to go - supposedly the water turns slightly dark and just sits there at that state with no oxygen to to oxidize anything.  All you then have to do is occasionally monitor and confirm that the pH stays slightly basic and everthing is okay.

  Is this not true?

  Or did I miss something AGAIN???  <grin>


  On a slightly different piece of the puzzle, I'm contemplating how to hook the system up to the R/O water source without jeopardizing the R/O system.

  My thoought is to have a small (i.e. 5 gallon'ish) pressure water tank at the high point in the system with a sight glass on it, so I can see how much water is actually in the tank, and then just refill it with a manual on/off valve when it gets a little low.  The pressure water tank comes with a "bladder" which separates the air from the water and doesn't contribute to oxidation and also insures that the air is not slowly moved out of the tank and into a "bubble" somewhere that obstructs the flow through the system - Anybody see any holes in this solution to the problem?

  I'm only anticipating pressurizing the system to something like 5 to 7 psi, so a 150 psi rated domestic water tank ought to be plenty safe for this application, no?

  Thanx for the input WGB!

Regardz,

Wayne Stayton