Open-loop water systems - water chemicals?

Ken Samson

I have a few questions that I have not seen covered in any of a variety of books...

I'm designing a cement slab radiant system. I intend to use four plastic storage tanks, 500 gallons each, for thermal storage. The heat source will be a wood-burning stove that has stainless steel water heating tubes. I may add additional copper heating tubes on the outside of the wood stove to gather more heat for water heating, as opposed to space heating.

Due to the use of the plastic tanks, I intend to keep the total pressure of the system low, and basically run an open-loop system. The pumps and valves will be bronze. I hope to use standard PEX, as there will be no ferrous components to the system.

The intended maximum temperature of the water coming out of the wood stove and being stored in the tanks will be 140 degrees F. A boiler loop will include a connection to the radiant loop through a three-way mixing valve to form an injection system. The three-way valve will be adjusted to be around 85 degrees during system startup. A zone valve will sit just in front of the mixing valve. The system is intended to operate with both loop pumps running all of the time, preventing serious thermal shock to the floor or other parts of the system.

I would prefer to not run a glycol solution to keep the cost down (2000+ gallons of water), and because I should have no freezing problems given the locations of the equipment.

With an open system, I trust I should adjust the water PH to about 7 to prevent other problems. I can test the water using simple aquarium PH test kits.

What chemicals should I add to the water to adjust the PH if I find it is too high or low?

What, if anything, should I add to the water to prevent bacteria from growing?

With open systems, I'll have to watch the system concentrations as water leaves the system. Do you have a suggestion for monitoring the water composition to ensure that I don't get too much calcium, for example, in the water? Or should I just drain off a few hundred gallons every once in a while and re-fill? (I would use it to water trees, depending upon the other chemicals I need to add..)

Should I run the water through a carbon filter system before filling the hydronic system? This would get rid of the chlorine (a good thing for the plastic components of the system). My concern of doing this relates to the second question, where I don't want to be growing algae in the walls of the plastic pipe under the cement.

Open-loop systems seem to be less common. This may prompt someone to add a chapter in a book somewhere to cover these questions.

For any help I would be grateful!

Ken Samson

hot_rod

Good questions, all of them

First the plastic tanks. I searched all over for tank that could handle constant 140 at ANY pressure. 140 seemed to be the upper limit on large unpressurized tanks. How will you keep the wood boiler from overshooting that 140? You really want to burn any wood fired devise HOT, to run efficiently, and minimize smoke. It gets real hard to control their output temperature maximums when they burn hot anfd fast. It takes some clever, and involved piping to be able to dump excessive BTUs quickly and safely, I've found.

I too, tinkered, and struggled with open tanks and the corrosion, bacteria, and related odor issues. The tank was in my shop. I finally gave up and went to pressure rated steel tanks.

Glycol is not a good idea in an open system. The oxygen inhibitors would be used up quickly, then the corrosion fighting ability is gone.

I highly recommend barrier pipe, should someone, someday want to use a cast boiler. The cost difference is well worth it.

The outdoor furnace manufactures sell an inhibitor to be used in their units. Keep in mind, however these are outdoors and any smell merely vents away with the smoke.

You would have to choose you chemicals carefully. At that ideal bacteria temperature some chemicals may be providing an additional food source and actually increase that problem.

As you mentioned a chemical treatment in an open system would need to be ongoing, perhaps expensive. Evaporation is another issue with hot water in an open system, as well as humidity.

My go to guy for hydronic related chemical advise is Scott at 417-860-2998.

But I still would recommend finding a way to use HXs to isolate the wood boiler and go with a closed system.

hot rod

Ken Samson

Closed system


The stove rated output is 135,000 BTU/hr, and has four stainless steel water heating tubes in it, rather than being a water-jacket wood-fired boiler. I went this route for a variety of reasons, the largest being that a water-jacket boiler does not usualy burn as hot and I'm in a resadential area, so I wanted a clean, hot burn. Next, I have designed the pump and tubing such that I can pull the full rated output fom the stove, plus a safety margin. I'm working on a way to regulate the water flow so that I can get the desired temperature water out of the unit (140 degrees F). If others are interested I can post some results here later. Sometimes I think I got my electrical engineering degree so that I can support my home projects better. You raise some good points, but I'm sure I can keep the temperature under control, as I've oversized the storage tanks and engineered the system to be able to pull off more heat than the stove can produce. Thanks for the contact number, I'll be giving him a call. Anyone else have any comments?

hot_rod

Your piping

willbe critical. What is your load at design conditions? There may be days when the entire output of your wood boiler will be needed in the building. On these days you would need to be able to disconnect from the storage tanks.

Personally I feel the output ratings on some wood burners are a tad optimistic. Maybe with a belly full of dry hardwood and a perfect burn temperature you may get the listed output. It takes some serious wood tending to keep them in this "ideal burn range" I've found.

Mine is listed at 135,000 output. My calculations show more like 80,000 actual based on wood consumption and actual heat generated.

As you know you store more btus in hotter water, and that wood burner really wants to run hot temperatures to burn clean. It may work out better with smaller pressurized tanks storing 180-200 degree temperatures. Less dollars involved in storage capacity, insulation need to keep the BTUs contained. Wide delta t's are fun to calculate around. If in fact you need 80-85 degree temperatures and can store 180 that a big delta t to play with.


I'd like to see your piping layout for the project. I like tinkering with solar and wood fired heat, and enjoy seeing other ideas

hot rod

PJO

My two cents...

Ken,

I'm not nearly as knowledgeable as HR, but nonetheless totally agree with him :-) Between the extra storage and the plastic tank holding 140F (or more) water, it's a 2000 gallon sized can of worms...

One more reason to not do it this way. If you say you will not reach 140F, then you will almost always be in the most favorite zone of temperature for bacteria...making your task even harder.

If you want to spend the money I could help you a bit with the sizing of a disinfection system that could work, but you are better suited to spending your time and money on say, a 500 gallon low pressure/high temp. system that holds water at 200F or so...there's enough capacity there for at least 90% of your season.

A good disinfection system will cost you money upfront and in chemicals (or UV lights) for the life of the system. It will also require periodic maintenance and monitoring.

Convinced yet? How about what will happen if it fails and your away on vacation for two weeks...or you have to drain and clean it every summer.

Anyway, as you can tell I think you should heed HR's advice - especially because he has tried it himself.

Take Care, PJO

hot_rod

Two more cents

When it comes to heating with wood and radiant heat is the desired end goal I have tried to encourage clients to consider a masonry fireplace. Aside from being the cleanest wood burning device available, the comfort rivals that of a well designed radiant floor system.

Your neighbors will appreciate it also. One, perhaps two, quick, hot fires and that's it every 24 hours.

hot_rod

Storage tank comparisons

A gallon of water stored at 200 degrees and run down to 100 will give you (200-100) X 8.33 = 833 btus

So 500 gallons would store 416,500 btus

Take your example of 1000 gallons and use a stored temperature of (140-100) X 8.33 = 333,200.

So it would be less space, less gallons of water, and less insulation using one 500 gallon tank. A 500 gallon tank, not unlike those LP tanks you see in yards are pretty easy to find and inexpensive, pressure rated, and stamped, and easily insulated.


In addition using wide delta tees would allow decreased pipe sizing between your boiler and storage.

Store it hot and run it down as low as possible. Food, or water, for thought.

hot rod

joel_14

Hot Rod!

Hey do you know about H.S. Tarm???? They have been popular up here for decades and those guys make a neat big tank that is colapsable to fit through doors. it is designed to be a storage device for thier wood boilers so it'd gotta be rated at 140 + right???

hot_rod

Seen that tank

on one job. Clever design, great for retro fits.

I thought the workmanship was poor on the copper coil HX however. Sloppy assembly green slimey solder joints everwhere. The homeowner commented he could have done a nicer job soldering than this! Too bad the rest of the equipment was top notch.

I notice Tarm offers a pellet/ corn burner now. That entrigues me. Much easier to store, the fuel, and control the burn on these unit, it seems.

Plenty of currently "landfill bound" raw material available to squeeze into pellets!

hot rod

Scott25

good questions


With the system that you are designing I see multiple concerns with the chemistry of your water. The first being dissolved oxygen. With an open system the amount of dissolved oxygen will be a major concern. This dissolved oxygen will greatly increase your chances for corrosion problems unless properly addressed. Secondly, due to evaporation, water hardness must be taken into consideration. When water evaporates it leaves behind all minerals (calcium, magnesium, silica, etc.) and these will "cycle up" and plate out on heat exchanger surfaces. Thirdly, is biological growth. The elevated temperatures of an open loop heating system are the perfect petri dish for experiments that you don't want to perform in your house. Biological growth is eminent if left untreated.

I can help you make some informed decisions regarding your water chemistry if you would like. Please feel free to email me at anytime.

Scott