Contractor recommendation....

[kuribo]

Can anyone recommend a contractor to assist with the install of a gasification wood boiler with 1000 gal storage, propane backup, in floor heat? I am located in SW Wisconsin, near Prairie du Chien. The local people won't touch it as it seems it is too outside the box for them.
Pex is installed, most of the equipment is here.

I have an Econoburn 200 boiler and according to my calculation:

gpm= btu/hr / (delta T x 500), at 200,000 btu/hr and a delta T between boiler out and supply return of (180F - 80F) 100F, my gpm to storage would be 4gpm. The contractor who came out and walked away said that my 1" pex supply and return to storage (60 foot run each way) wasn't capable of transporting more than 115,000 btu/hr. Dale at Econoburn said I could never move the boiler output with 4gpm and that it would go into shutdown. Even if the max output is closer to 300,000 btu/hr, that would only be 6 gpm. 1" pex seems to be rated for around 10 gpm....They keep talking about gpm, I keep telling them it's about delta T. What do I have wrong????


Would really appreciate any suggestions. Thanks.

 

[stee6043]

Your boiler isn't designed to operate at 100 degrees delta T. You should be operating more in the range of 20-40 degrees.

You'll also want to de-rate the boiler output by 20-30% when you do your math. Rarely will you approach 100% output when we consider wood condition, draft and all of the other variables associated with burning wood.

Silly question - what is the heat load of the house? Your design discussion should start with heat load, not output.

Another silly question - what temperature are your floors designed to be used at? Will you be mixing/injecting? I don't think many floors are designed to take the full output of a boiler (180). They are usually mixed down to a useable temp at a specific flow rate.

And last silly question - are you planing to send heat to your 1,000 gallons of storage separate from your heating loads? You didn't mention storage vs emitters.

I had 1-1/4" plumbing to my storage tanks and it was nearing the top end of capacity when my boiler was really cranking. I think you're going to want to rethink your plumbing strategy to storage if the 1" pex is your main line. Double 1" lines for storage and perhaps single 1" line going to your loads, assuming your loads are "normal".

 

[kuribo]

Thanks for the reply. Let me answer your questions below....

The boiler won't be operating at a 100F delta t, but rather 40F delta T. The return from storage is mixed in the boiler loop so that the return to the boiler is always 140F.

Heat load for the house is calculated as 80,000 Btu/hr. It's fed from storage. All in floor hydronic. Design point is 100F supply, 80F return. 180F from storage mixed down to supply the 100F for the floors.

Storage is plumbed with a 2 pipe set up so boiler supply can be sent straight to mix valve for zones and excess feeds into storage.

How many gpm are you sending to storage and at what delta T? 1" pex, according to the math, should be sufficient to send 200,000 btu at a 100f delta T to storage.

 

[E Yoder]

How do you get a 100° Dt feeding into storage? At least some of the time it would be hotter than that, correct?
I don't do storage on the boilers I work with, so maybe I'm missing something.

 

[maple1]

Thanks for the reply. Let me answer your questions below....

The boiler won't be operating at a 100F delta t, but rather 40F delta T. The return from storage is mixed in the boiler loop so that the return to the boiler is always 140F.

Heat load for the house is calculated as 80,000 Btu/hr. It's fed from storage. All in floor hydronic. Design point is 100F supply, 80F return. 180F from storage mixed down to supply the 100F for the floors.

Storage is plumbed with a 2 pipe set up so boiler supply can be sent straight to mix valve for zones and excess feeds into storage.

How many gpm are you sending to storage and at what delta T? 1" pex, according to the math, should be sufficient to send 200,000 btu at a 100f delta T to storage.

It is about moving heat away from the boiler. With a 40dT thru the boiler, that is 76,800 btu/hr at 4gpm.

 

[kuribo]

I get a 100F delta t to storage by pumping 180F water to storage and returning 80F water from my floors.

40F delta T through the boiler is 180F out minus 150F in at 10gpm is 200,000 btu. The boiler loop circulates 10 gpm through the boiler. Closely spaced tees supply 180F water from this loop via a temperature set point variable speed circulator to storage: 180F to storage and 80F back. The variable speed circulator only returns an amount of 80F water such that when mixed with the 180F water through the boiler the boiler return is never less than 150F.

 

[maple1]

We don't know much about your entire system, or anything about any other head losses through your boiler loop or your storage loop and what they would add up to - but just based on info posted, IMO you should be able to move the heat away if your boiler loop can flow 8gpm and your storage loop can flow in the 3-4 gpm range. That is assuming net boiler output ballparked at 20% de-rate, and not allowing any kind of cushions for things like a narrowing dT at times - it assumes a constant dT of 40. Which is pretty wide. Mine runs happily right around 20, but there are also times when my return temp is closer to 150 than 140 with my loading unit. It would also require a burn start when storage is fully depleted, and a shut down immediately when storage is fully charged - in order to get all the way through charging with no boiler shutdowns. I.e. as soon as one lap gets through storage - which should take about 4-5 hours assuming all output going to storage - you will have no room to store any more heat. So stopping the burn short of full storage or having really good dumping capability would be pretty important - it would be a fine line.

 

[maple1]

I get a 100F delta t to storage by pumping 180F water to storage and returning 80F water from my floors.

40F delta T through the boiler is 180F out minus 150F in at 10gpm is 200,000 btu. The boiler loop circulates 10 gpm through the boiler. Closely spaced tees supply 180F water from this loop via a temperature set point variable speed circulator to storage: 180F to storage and 80F back. The variable speed circulator only returns an amount of 80F water such that when mixed with the 180F water through the boiler the boiler return is never less than 150F.

150 or 140? 150 would be 30dT.

 

[stee6043]

This is not easy to follow....and might be why you've had a hard time finding a contractor .

One math issue you may be overlooking - the recirc required to keep the boiler at 140 when returning 80 from the system would be significant. Nearly half of your circ's flow output will be consumed by recirculating (waste) hot water from the 180 supply to get the 80 degree return up to 140 before going back into the boiler. So if you're flowing 10gpm at your primary circ you're going to get less than 5 +/- to the system. Not ideal. I'm not aware of any folks around these parts intentionally designing a boiler to run in a constant state of mixing at the boiler. To be efficient you really want to keep your return (from the system) above 140.

If you're using a proper mixing system for your in-floor heat you shouldn't be returning 80 back to your boiler. At each zone (or every few zones?) the 80 will mix with the 180 to push 100 thru your floor. The balance is going to mix with your return and head back to the boiler. I think the pro's would typically tune the flow (gpm manipulation with ball valves or fancy pumps) thru each zone to get the boiler return temp they need/want (140 +/-). With the dedicated mixing valves at each floor zone the 100 degrees at the floor is a sure thing and almost independent of boiler output so long as you have sufficient flow and temp to maintain the mixing.

I have a suggestion - post a sketch of your system. This may prove very helpful.

 

[kuribo]

Yes, i am assuming the tanks stay well stratified and that I would fire the boiler when the tanks are close to fully depleted. It would indeed require that I weight the wood load and take care not to get into a situation where the boiler is still putting out heat with storage full. The temps would coast up at the end of the burn ideally so the delta T doesn't collapse as the last gallons of 80F water leaves storage and is replaced with 180F water. It would be a rather fine line but others here are doing this I believe....

The math says 10gpm or so in the boiler loop (180F output, 140F return) and 4-5gpm (180F supply 80F return) through the 1" pex to storage. The head losses through the primary loop are minimal, through the loop to storage I calculate at around 12 ft or so. It seems to me that the 1" pex should work. I don't understand this comment above: "So if you're flowing 10gpm at your primary circ you're going to get less than 5 +/- to the system. Not ideal." Why not? It's all I need.....

A mixing valve mixes down the 180F supply with 80F return water from the floors to say, 100F, which is supplied to the floors. The supply temp to the floors could be set up with an outdoor reset to vary the supply temp to meet weather conditions. The surplus return from the floors, at 80F, goes back to the boiler via storage.

Appreciate the replies....

Here's a schematic.....

 

[maple1]

One factoid that you may or may not want to consider is that running all the time at max boiler output temp will cost some efficiency. I am quite sure I saw noticeable wood savings when I started not charging my storage so high - I think I get the most btus from wood to system when running the boiler as low as possible, for as long as possible, which is 140/160 in/out. What temp does your boiler shut down at? You might be running constantly close to the edge there too?

 

[kuribo]

I think the people at econoburn told me the shutdown temp can be user set within a narrow range, like 180-195. Something I will have to double check. With the low temps needed for in floor heat, I can make it work with an upper temp of 170 if need be.

 

[maple1]

That would then have the catch-22 of narrowing your boiler dT, requiring more flows. Then you might get up against it with your 1". Still thinking though that it should move the heat unless there is more head in that loop somewhere - although I admittedly didn't check any pex flow charts. Sending such low temp water back to the boiler mix loop will play a big part, yes.

 

[stee6043]

I think you're going to want P2 on the load side of your circuit. P1 will supply heat to storage. Also, your circs should always be on the return side, not supply.

You can control P1 with the boiler and P2 with your home t-stat or floor sensors. This is a pretty typical setup.