Expansion Tank Question on Wood-fired boiler

Eric Johnson

I'm having a problem with an Extrol 60 expansion tank on my wood-fired boiler system. I have it installed on the 1" supply line. It screws into a cast iron assembly with an air scoop at the top. The tank hangs vertically below the supply line.

I don't think it's ever worked right. At the moment, my system pressure drops to zero when the water is cool (100 degrees) and runs up from between 15 to 25 psi when the water is hot (190-200). I have been able to keep the pressure down by venting air out the bicycle air valve at the bottom of the tank, but now I think it's full of water. The pressure doesn't stay down for long, in any event.

Short of removing the tank, how do I know if it's shot? Or, for that matter, how would I determine its status if I took it off? Can I restore the air by drawing a couple gallons of water out of the system and then pumping the tank back up with a bike pump?

Thanks in advance for any suggestions.

Eric Johnson

Dan Peel

Expansion tank

Letting the precharge out of your tank only worsens the problem. Your #60 should have a precharge (before system pressure) equal to your cold fill pressure - typically 12 to 15 lbs. The acceptance volume of a #60 is about 2.8 Gallons. With your high temperature system anything over about 80 gallons of total system water will require adding another tank. Enjoy...DAn

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Eric Johnson

Thanks.

I'm using a flat plate heat exchanger so the total volume on the wood-boiler side is less than 50 gallons.

The tank came with a pre-charge of 12.5 pounds. I can't figure out why it never worked.

Anyway, do I need to remove the tank to restore the pressure? If it doesn't work when I do that, does that mean it's nfg, or could there be some other factor I'm not taking into consideration? Is there a way to test the tank when it's removed from the system?

Thanks for your time.

Dan Peel

charging

You can recharge the tank in place by reducing your system pressure to atmospheric and adding air into the tank. If the tank holds air pressure then the bladder is probably OK. At 50 gallons of fluid you should be just fine, no reason it should be giving you issues unless your system is trying to flash to steam, that can sure increase your closed loop pressures in a hurry...... Dan

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hr

Dan hit on an important

thought. That being it is not uncommon to let a wood boiler run away and overheat. With a low, or no precharge, you may have steamed the sucker, especially running 190 setpoint. Fairly easy to overshoot that.

hot rod

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Eric Johnson

steamed

Well, steaming is certainly a possibility. Forgive my ignorance concerning hydronic systems, but is "steaming" something that can be fixed with a re-charge like Dan suggests, or does "steamed" equal "trashed?"

hr

If the tank

had a low air charge, or no air charge the boileing point is reduced. At 0 psi (sea level) it would boil and turn to steam at 212° At 5 psi I think water boils at 227° again at sea level. At 5000 feet elevation water boils at 220° with a 5psi pressure, or 202°F with 0 psi on the system.

If you lowered the precharge to say 5 psi and the boiler boiled and flashed to steam you may have pop ed the relief valve. Which is why you pressure gauge reads zero. You have probably lost some water in the system if this happened.

Good idea to have a low water cutoff switch on these boilers. Prevents the pump and blower or damper from operating if you lose the water.

I also pipe the relief valve line into a bucket on all my wood boiler installs. Then I know if someone has let it overheat and over pressurize Good way to save glycol, also.

hot rod

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hr

If the tank

had a low air charge, or no air charge the boileing point is reduced. At 0 psi (sea level) it would boil and turn to steam at 212° At 5 psi I think water boils at 227° again at sea level. At 5000 feet elevation water boils at 220° with a 5psi pressure, or 202°F with 0 psi on the system.

If you lowered the precharge to say 5 psi and the boiler boiled and flashed to steam you may have pop ed the relief valve. Which is why you pressure gauge reads zero. You have probably lost some water in the system if this happened.

Good idea to have a low water cutoff switch on these boilers. Prevents the pump and blower or damper from operating if you lose the water.

I also pipe the relief valve line into a bucket on all my wood boiler installs. Then I know if someone has let it overheat and over pressurize Good way to save glycol, also.

hot rod

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Eric Johnson

Hey, thanks.

The relief valve hasn't lifted.

I do the bucket under the relief valve line thing for the reasons you described. I really like the low-water level shut-off switch idea for the pump and blower. The system has a domestic water feed, but about half the time I close the valve because I don't want to lose all my glycol in the event the relief valve opens. Then I lay awake at night worrying about burning out the pump in the event that excessive pressure blows the relief valve. So this thing would go in right above the pump, I'm guessing.

Back to my Extrol 60: If I steamed it, did I cook it?

hr

I'm still not clear

in your first post you mentioned the pressure dropped to zero. Either you have a small leak or you relief has at some point lifted. If at one time the system was pressurized. Keep in mind air will purge out after a system heats and circulates for a few days. You need to re boost the fluid, glycol mix in your case.

With the system at 0 psi, recharge the air in the tank to 12 psi. Then refill the glycol and pressurize to 12 psi. You should be good to go if the tank is sized correctly. Glycol in a system may require a larger expansion tank, you need to run the nunbers to know for sure.

If you have a glycol system I really would not have it connected to a auto fill valve. Disconnect this, add a LWC and watch your pressure.

Yes the pressure will climb when it heats, but it needs to stay below the relief valve pressure. If you start at 12 psi and it starts running up over 25 psi or so your expansion tank is probably undersized. Many 30 lb relief valves start to dribble around 28 psi.

A glycol refill system can be added if you need to maintain pressure, although a leak free system should not need one after it has been completely purged.

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Eric Johnson_2

charge & purge

Thanks. Sounds like my system needs to be re-charged and purged. To me, that sounds like a fun way to kill some time over Thanksgiving.

I suspect that the leak you suspect may be the bleeder on the top of the scoop. I'll have to re-read the documentation, but at the moment, I think it's wide open.

Also, I appreciate the advice re: glycol and my domestic water supply. I'm using heating system antifreeze (notoxic), but I still don't want it seeping back into my domestic water supply. The LWC is a much better solution.

I've used pressurized wood-fired boilers for about 10 years, and I've always had the situation where when the system temp is way down, the pressure drops considerably, climbing back up to around 20 or so when the water heats up. Sounds to me like some of this is normal from what you say, although my parallel gas boiler system sits on 14 psi regardless of water temp.