My first wood boiler, storage and data monitoring

I think its in the 15 gallon ballpark, of expansion. Not a whole lot, but its about 10% of boiler volume.

I ran my home built boiler(120 gallon) last year with temp swings from 180-150. My system is open. I filled the boiler to just under "full" then fired it up. I let the boiler heat up and burp out the expanded water from the vent line. After a few days all the expanding that would be going on just rose and fell in the boiler tank, no more water came out. When I opened the maintenance cover the water was just under where it started.

I should add that I designed a "bubble" into the top of my boiler tank to avoid the rust ring in the majority of the boiler tank. So my rust ring could actually rot through its loaction and I could still run just fine.

If you can measure your remaininh space in your water jacket on the boiler you can do the math to figure out how many gallons to expanded to your "full" line.

If you are calling Central Boiler anyway it may be beneficial to ask them their advice on your project. Worse case they say they don't recommend it and then all your out is the breath it took to ask.

Good luck to you.

brack86svo said:

The issue I am running into now is.....

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A "simple" problem, likely, and also relatively simple to solve, maybe. It's all about the math. BtuH moving through pipe = deltaT x gpm x 500. I will assume that you have 1-1/4" inside diameter supply and return pipe. If your pipe diameter is smaller, you have a significant problem. If it is larger, your problem may be less.

I will assume you have flow rate of about 12 gpm, that being the recommended maximum flow rate with 1-1/4" pipe. Do you know what your flow rate is? Do you know what your pump head is at 12 gpm? The 007 pump curve will tell you the flow rate if you know the pump head. Pump head can estimated/calculated based on pipe size, pipe length including fitting equivalents, and flow rate. The 007 pump curve shows that 007 will move 12 gpm at pump head = about 7 feet. If the pump head is > 7 feet, less flow; if < 7 feet more flow. What is your pump head at 12 gpm?

Assume maximum flow rate of 12 gpm. Assume deltaT = 20F. Then btuH that can be moved = 20 x 12 x 500 = 120,000. Any output of your boiler > 120,000 btuH will result in the boiler having rising temperature because all output is not being moved out of the boiler. Temperature will rise to the idle point, then boiler water will gradually cool to the restart point, repeat.

Now assume deltaT = 30F. Same calculation: 30 x 12 x 500 = 180,000 btuH. This looks close to your described situation, 150F water coming back to the boiler. But you still experience idling cycles, particularly as return temperature increases above 150F.

The solution is either to increase your flow rate which might not be easy, or to reduce your boiler output as return water temperature rises above about 150F. You can reduce boiler output by making sure that when you load the boiler you have plenty of storage capacity to absorb the boiler output as the wood load burns down. And as the wood load burns down, boiler output falls, and at 12 gpm and with deltaT falling below 30F you still can move the boiler output without idling.

More to the point. Your boiler has a rated output of 170,000 btuH. This would likely be high burn maximum output, with output falling as the wood load burns down. The problem is now clear. Your system will not move sufficient water at deltaT < 30F to handle the boiler output. The solution is not to overload the boiler so that boiler output is falling as return temp reaches 140-150F and the full reducing boiler output can be moved to storage without idling.

With care in loading your boiler you may be able to raise storage to your desired maximum temperature without boiler idling. Do a search on this forum for "weighed wood burns" and you will find an extensive discussion on how to do this.

Is the piping actually 1" inside diameter? If so, I calculate close to yours at 12 gpm: 60 feet with 1" steel pipe = 7.22' of head; about 7.6' feet of head for plastic pipe. But note what this means on btuh carrying capacity: 180,000 btuh at 30F deltaT, 120,000 btuh at 20F deltaT, 90,000 btuh at 15F deltaT.

Your boiler is rated at 170,000 btuh and at the temperatures you mentioned, 12 gpm may not move your boiler btus at higher outputs, which means idling.

The 007 pump curve shows closer to 11 gpm at 7.22' of head: 165,000 btuh at 30F deltaT, 110,000 btuh at 20F deltaT, and 82,500 btuh at 15F deltaT. And if the pump head is greater, the numbers are less.

Edit: On my second install attempt on my Tarm, I used 1" steel pipe. Too small for boiler output, and I had a problem similar to yours.

I may have missed this, but can you set up the boiler circ to pull cool water from the bottom of storage rather than pushing hot water into the top? It *might* help to use a Viridian circ set up to vary the speed to give you the temp that you want going into storage. That way the actual flow rate will be faster when the boiler is producing a lot of heat, and slower when it's not. It would also compensate for the situation where you're returning hotter water to the boiler (when you don't have much stratification in storage) - it will run faster to keep the boiler outlet temp from getting too high, which should reduce the chance of the boiler idling.

The whole 'pumping away from the point of no pressure change' concept is not carved in a sacred tablet carried down from a mountain somewhere. It's a design concept that addresses two things: First, that scheme does a better job extracting air from the system as bubbles come out of water more effectively at lower pressure. Probably not a big concern in your situation.

The second issue is cavitation. The boiling point of water drops as pressure drops. If you have a big circulator and you're running high temperatures, you can get a low enough pressure on the inlet side so that the water will flash to steam, causing cavitation and severe wear on the circulator impeller. Typically a concern with larger systems, though some folks here have managed to produce this effect in residential installations.

This is a design consideration on closed systems, where you typically have an expansion tank and air separator, and you'd ideally place the circulator just downstream from them. On an open system, it's a bit different. To reduce cavitation risk, you'd ideally have your circulators lower in the system (as you do), and have them on the cool leg rather than the hot leg.

If you decide to go with the Viridian as I mentioned above, it would be ideal to place the temp sensor near the boiler outlet, even if the circ is in the basement.

Without a more detailed plumbing diagram I wouldn't even hazard a guess, though if you pull water out of an otherwise sealed tank, more water has to be drawn in to replace it. An open system isn't *that* different from a closed system in that respect.

The lack of stratification may also be caused by the way the plumbing is configured. Incoming hot water is injected straight downwards. With sufficient velocity, that can result in pretty complete mixing from the get-go. That's a reason why the Viridian might help - most of the time it will be running at less than full speed, resulting in less mixing in storage.

Can you take some readings during the charge and discharge cycles - maybe top / middle / bottom - and see if you get mixing during charge, discharge, or both?

What do you have for zones & a house circ?

If you could slow that load flow down you'd likely see better storage action. Switching to an Alpha circ was one of the best things I ever did. I might also consider running to the plenum exchanger first, or breaking them into separate zones.

I feel the frustration in the OP's situation. I was there when I first set up my Tarm, using 3 oil tanks in open storage with a 140,000 btuH rated boiler and first 3/4" and then 1" steel pipe and a 007 circulator. Very similar issues.

Slowing down the flow may help with less mixing and more stratification in storage but even with cooler return water the idling issue may be worse. Typical 1" pex is 0.875 inside diameter, not 1", and I assume Thermopex is the same. Flow may be about 8 gpm, assuming head is based only on 60' of pipe, and it likely is less ("It has a 007 that pumps through a sidearm and then a plate exchanger for the water heater. From there it goes through a heat exchanger in my air plenum to heat the house.") At 8 gpm btuH capacity is only 120,000 at deltaT=30, much less than the 170,000 btuH rating of the boiler.

Also, the two oil tanks for storage probably have a capacity of about 275 gal each, 550 gal total. This is fairly minimal storage for a 170,000 btuH boiler. Assuming minimum usable water temperature is 120F, maximum storage capacity from 120F to 185F is about 300,000 btus. To prevent idling some very careful boiler loading may be required to keep both the burn rate and total burn output on the low side of the boiler rating. This may cause other problems for the boiler, as best boiler operation in my experience is allowing a boiler to operate at maximum burn rate through the burn, meaning high burn on loading and then gradually reducing output as the wood load burns down.

I would like to find an easy solution to the problems in this situation, but that easy solution escapes me at this point.

One other thing is likely raising the temp your house/load circ kicks in at. I can't see you getting much heat out of the water when it is only 120, especially with a plenum HX - so the load circ at that point is only serving to mix your storage & mess up stratification. I would try raising that to the 140-150 area - along with maybe trying to slow load flow. But you likely can't do much on that end with a 007. Plus when winter really hits, you might need some of that flow back to keep your house warm - without knowing how the plenum HX performs as is. Which would be where having it before your DHW - or separate loops - would come into play.

You might be stuck with the idling issue with the underground piping you have - balancing wood loads will likely be key.

One more quick one.

I have heated my DHW all summer from storage, a couple of times now. The first tries at that, while I still had a 15-58 3 speed load circ, were pretty disappointing because it messed up my stratification too quick. Just flowed too much. I could only go maybe 2-3 days before needing to fire again. The situation was made worse because I only had a sidearm for a HX. I added a FPHX to the sidearm (and a small B&G Ecocirc on the DHW side of the HXs), and swapped the 15-58 for the Alpha which allowed me to throttle the flow on that circuit down to minimal (I also slowed my baseboard zone flows). Not sure what it actually is, but the digital readout on the circ says 1gpm when just the DHW zone is flowing. I could now go a week between firings, and the stratification remains. During that exercise, I also discovered that the 15-58 was also pulling some flow through the now-cooled boiler - also compounding things. Don't think it was a lot (which is why I didn't realize it was happening for quite a while), but it didn't help anything.

So my lessons learned from all that - an Alpha circ is awesome [HIGHLY RECOMMENDED] for multi-zone loads using a single circ (likely also for any system with a single load circ), and heating DHW all summer with wood might not be worth the hassles & extra component costs of doing it and should maybe be down on the priority list a bit when planning a system. (I have used my electric elements for it the past 2 summers, even with what is now a pretty good wood heating setup for it).

[I also ballpark figured that DHW BTU requirements are about 1/30 of my house heating BTU requirements, so don't see the need to priority it with an adequately sized DHW tank - random tidbit.]

(Guess that wasn't so quick....)

Spot on. When charging DHW from storage (I have an indirect DHW tank with an internal finned coil) I run the charging circ at 10%, which is probably less than 1gpm. DHW recovery is a tad longer, but it does wonders for stratification and really extends the useful life of storage.

In my case I'm using a 007 with a variable speed control. I'd use an Alpha, but the US versions don't provide an external speed input :-(

brack86svo said:

So you run the Alpha through your DHW exchangers, then through your baseboard at the 1 GPM? I don't have a zone valve to separate my DHW from my heat exchanger in the plenum but, I'm not really sending much heat into the house if the blower isn't on, just some radiant into the plenum.

I probably will not separate the DHW and house heat into separate zones this year, since I don't want to drain the system down but, I have valves at the crics, so I may order that pump and play with the flow to see what I can get to work.

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The baseboards are on 4 other separate zones - so as they open up, the pump pumps more. Not sure now that I think of it what my baseboard flow might be. Guessing 2-3 gpm? Also a bit suspicious of accuracy of the Alpha display, but not suspicious enough to try to measure my flows otherwise. My baseboard zone flows also get aided some by convective flow (the upstairs ones at least), which also might throw the pump display off a bit. Just so happened by chance that all of my zones ended up with areas needing the most heat being the first in the zones (and ones not needing a lot at the ends), so if the water in the rads is cooler at the ends of the zones, it all seems to work out anyway. Or maybe the guy who designed it 20 years ago really nailed things - seems he generously oversized my baseboard too, which I am quiet grateful for now with the changes I made since.