Wood boiler not enough?

[cschwinn]

Plus if you're really feeding an 80,000 btu per hour load a 320 gallon tank would be more of a buffer tank than a storage tank and stratification wouldn't be such a big deal to begin with. Definitely worth doing though, it would help insure than when the boiler is running it can run flat-out hot and clean for an hour or two at a time.

--ewd

Is there a formula somewhere that would allow me to know what to expect from a 320 gallon tank? I am concerned that I would go through all this work and still find that my reserve is not big enough. The boiler struggles to keep up at full speed and I want to avoid that if possible.

 

[cschwinn]

Problem is it is 20 inches to tall

Is there a possibility of excavating a couple of feet for clearance ? Pour a new pad and allow space for insulation piping etc. Same gallons as horizontal but less of a footprint.

Will

Nope, tubing in the floor. Unfortunately not an option...

 

[ewdudley]

Is there a formula somewhere that would allow me to know what to expect from a 320 gallon tank? I am concerned that I would go through all this work and still find that my reserve is not big enough. .

These might show up in Those Useful Tidbits but here they are: A btu is the energy to heat one pound of water one degF, and a gallon of water weighs as close as you please to 8.333 pounds per gallon.

One thing these imply is that gallons per minute (gpm) times deltaT degF times five hundred equals btu per hour. For instance if you had 3 gpm going to a section of baseboard and it goes out at 175 degF and it comes back at 150 degF, then btu per hour would be 3 * (175 - 150) * 500 = 3 * 25 * 500 = 37500 btu / hour.

And back to your storage tank: 320 gallons is 2667 lbs of water, so that's 2667 btus per degF of deltaT that you can store in the tank. If you can go from 185 degF down to 135 degF that's 50 degF deltaT, or 50 * 2667 = 133000 btu, give or take.

You say your head load is 80000 btu per hour, so that's an hour and forty minutes of storage flat-out at 80000 btu per hour.

Which sounds discouraging. But is your heat load really 80000 btu per hour? If you ever heated the system with fossil fuels exclusively you may be able to go back and figure a good estimate of the amount of fuel you were using in the middle three months of winter and use that to figure average heat load. If your load is actually more like 35000 btu per hour, now you've got close to four hours of storage.

Also since you have a mix of baseboard and radiant, you can feed the baseboard first and then mix the baseboard return water down for the radiant loads and perhaps get you storage return temperature down below 100 degF. This increases your storage deltaT (in the what-if example) from 50 degF to 85 degF, which gives you about 225000 btu storage, or about 6.5 hours storage at 35000 btu per hour (assuming your real heat load is as low as 35000 btu per hour).

But in any event 320 gallons is plenty to at least help solve a creosote problem caused by short-cycling of your current boiler. The unit would be able to run hot and clean at full load for an hour or more and then could shut its draft off completely for a good while and then run at full load again when the tank is depleted. This avoids having so many smokey start-ups and periods of smoldering limited draft.

The boiler struggles to keep up at full speed and I want to avoid that if possible.

As others have suggested above, the fact that your boiler can't keep up without storage doesn't sound right. If the fuel is going in the fire box and the ashes are coming out, then there are a few things to look at.

First is inefficient burning with smoke going out the stack. Storage may help this by allowing you to tune the unit burn correctly at the rate you choose to burn at.

Second would be excess combustion air, meaning you're burning clean but you're mixing in a lot of extra air that is cooling your exhaust gases and limiting the amount of heat you can extract from the mixture.

Third would be too much fire relative to your exhaust-to-water heat exchange surfaces. This would mean high flue temperatures with heat going up the flue before it has a chance to heat your water. You would need to limit the rate of burn and tune it to run clean to solve this problem.

--ewd

 

[cschwinn]

As others have suggested above, the fact that your boiler can't keep up without storage doesn't sound right. If the fuel is going in the fire box and the ashes are coming out, then there are a few things to look at.

First is inefficient burning with smoke going out the stack. Storage may help this by allowing you to tune the unit burn correctly at the rate you choose to burn at.

Second would be excess combustion air, meaning you're burning clean but you're mixing in a lot of extra air that is cooling your exhaust gases and limiting the amount of heat you can extract from the mixture.

Third would be too much fire relative to your exhaust-to-water heat exchange surfaces. This would mean high flue temperatures with heat going up the flue before it has a chance to heat your water. You would need to limit the rate of burn and tune it to run clean to solve this problem.

--ewd[/quote]

Thanks for the feedback, your three suggestions above sound reasonable, I just don't know how to go about testing/ruling them out. I will have to think about that. As far as I can tell, the recovery time for the boiler (7-9 minutes) is to long that the previous charge of hot water has already cooled down to room temperature. I'm hoping that a steady load of hot water can avoid this happening. The well insulated storage tank should help in holding that steady charge and allow the boiler to help the storage recover 20 gallons at a time. Hopefully I'm not missing something with this plan!

 

[cschwinn]

In fairness to the boiler manufacturer and for feedback to all of your great thoughts/assistance with my issue, I wanted to post an update. I talked with a local rep who supports the boiler, and found someone that can be helpful! Anyway, after sending pictures he noticed that the return piping came in the side of the boiler rather than in the back (allowing better water flow from the back across the firebox and out the supply). I switched it and am noticing some benefit although have to wait until colder temps to really test it. Seems like it was a worthwhile change as I am extending my run time already. From my observations, even an extra 30 seconds to a minute will help tremendously. Now onto the second step which is storage. I am contemplating a DIY experience...considering fiberglass, propane tanks or some other structure.

Thanks again for the input!

 

[Willman]

considering fiberglass,

Tanks should be able to handle high temps. not certain if fiberglass can do that. I would go with the steel propane tanks.

Will

 

[Singlemalt]

I have 2400 ft. of pex radiant and 50 feet of baseboard.

Is this one loop? If not, how many loops do you have? and how long are they?

What size circulating pump are you using?

 

[Urban Hillbilly]

I have had similar problems. The recomenations you are getting are probaly better than mine, but I found a simple fix. I have a Franco Belge wood boiler from the 70's tied into my natrual gas system. The wood and gas boiler use the same circulator signaled by the thermostat or a strap on aquastat on the wood boiler. I just added a second aquastat on a timer switch. This allows the wood boiler prime the whole system with a base line temperature. You will figure out how long this takes through trial and error. Once the timer switch turnes off the system should be primed, at which point the second aquastat circulates at a hotter temperature. Keep in mind, this is just what I had to do to get my old heap to heat the place.