Two smaller Boilers instead of One large Boiler

[sdrobertson]

I don't want to hijack someones thread so I thought I would start a new one. Looking for input on the pro's and con's of installing 2 small boilers instead of one large one. My thought is that you could burn one during the shoulder season and fire up both during the cold weather. You could rotate the boilers when you only needed the output of one so this should save on boiler life (this makes sense to me but maybe I'm wrong as to longevity). The boilers could be placed side by side so the footprint wouldn't be much larger than a big boiler. The piping could be shared so that cost wouldn't be much more and the price of the two boilers over one large one wouldn't be that much more especially if you could extend the life of the boilers by rotating firings. You could size your storage properly to fire the boiler as you needed as when its really cold, you would double your heat output with both firing. You would always have a back-up in case of something happening to one boiler until you could get if fixed. I think a smaller boiler is more efficient burning full-tilt than a larger one burning 1/2 loads. Looking at Nofossil's efficiency numbers compared to my 60, I would say that two 25's would put out more output than a 60.

I don't see a downside to this - opinions please.

Thanks,
Shannon

 

[DenaliChuck]

I think its a great idea, except for the expense.

 

[sdrobertson]

I just looked at the prices - and your right, I was thinking there was more difference between the same line of boilers but different sizes. On the other hand, I doubt the price will go down so if one boiler will last 25 years do you get 45 to 50 out of two for? Unnamed source april sale price Eko-25 $5850.00 x 2 = 11700.00 / EKO-60 $7150.00 - Difference of $3550.00 for the extra years and efficiency. I wonder at what point someone would be money ahead as the storage would cost roughly the same.

 

[steam man]

While sitting here baby sitting two large laid up marine boilers, it made me think how being cold is the worse thing for the boilers from the dewpoint, rusting, etc. My take is that a properly cared for boiler that is used could last a lot longer than me. I don't think the extra expense is worth it if you figure that you almost need two of everything. Consider that you would need to clean two units including the stacks. Yuck...I think the only time I would use a multi-boiler setup is with a large building that may have large variations in load that is used all year long and also could use the redundancy.

Mike

 

[Nofossil]

I think I'm with Steam Man on this one. There are some boiler components which wear out as a function of burn time (refractory components) but I think the steel wears out by calendar time, whether it's used or not.

The dual boiler approach that makes sense to me is a small wood boiler next to a fossil backup boiler. Ideally, the wood boiler would have just enough capacity to meet your peak load, and the fossil boiler is there if you need it so that you can go away for a few days or be lazy during the shoulder seasons.

 

[EricV]

I tend to agree with nofossil and steamman. 1 boiler running near full capacity would be the most efficient burn regardless of shoulder season or not since your heating the storage. It would run less hours per day at full bore in the shoulder season but still running at it's peak.

If longevity is a goal I would take the difference in cash and put it in a bond or some other safe investment and in 25 years or so you would have the cash for a new one and you would be buying 25 year newer technology.

Or, you can buy two, store one in my shop until you need it back and I'll keep it nice a warm for you. haha

eric

 

[DaveBP]

I believe multi-stage boiler setups are common in commercial buildings. Siegenthaler elaborates on several ways they can be hooked up and controlled. I've seen photos of banks of small natural gas mod-con boilers all in a row on the floor of a large boiler room.

Solid fuel boilers would have a lot more manual input required unlike the microprocessor magic that fossil fuels use. Computers can't chuck wood like a woodchuck can.

Seems to me they could easily cost half again what a single monster boiler would. There must be a long term money efficiency payback (beside the failsafe aspect of redundancy) even after considering the financing costs. It might be changing but long term savings are still a higher corporate priority than simply being "green".

Hard to know how it shakes out with fuel prices fluctuating like they have in the last 12 months.

 

[BrownianHeatingTech]

Multiple-boiler systems are great. If you need it.

If you don't, and you are using storage, it's just added expense, for the most part.

Joe

 

[Gooserider]

Definitely not the pro that some of the others here are, but my impression is that because of storage, wood boilers are essentially semi-independent from the heating demands of the load - you heat the storage, and the storage heats the load, so it's not a big deal that your actual heat demand will change with the seasons. Nor does it really matter how big the boiler is, as long as it's big enough, and the storage is large enough to store a full burn's worth of excess heat.

In the case of large buildings running dino-boilers, they aren't using storage so they need to match the boiler output more closely to the load. Since the load changes with the temperature, it can be easier to match the load by having multiple smaller boilers, running as many as needed at any given time, not to mention the redundancy benefits...

Gooserider

 

[cguida]

Can somebody explain the efficiency cost of running a large boiler with half a load of wood? Wouldn't it be just a shorter burn time, with no other consequence, or am I missing something?

A situation where two boilers in parallel might make sense -- a school or other public building with a large heat load during the day, and very much less during the night. If the goal is to displace fuel oil, then haveing the extra wood boiler capacity on hand when there was somebody there to stoke it (them) would help. Might even cover the extra cost.

 

[Duetech]

sdrobertson,
On any given installation that I have seen posted here nearly half the cost is in all the added stuff that is needed to get the heat out of the boiler. Though piggy-backing would reduce some costs there would be boosted cost for isolating the individual systems and combining the two systems the most obvious to me would be all large plumbing versus what is actually needed for the smaller individual system. The storage for two boilers could be in one tank but I think using two boilers to charge the tank would be less fuel efficient unless you went to two heat exchangers (atmospheric) or supply lines (pressurized). In either scenario mixing would seem to be more likely than stratification because of high volume transfer rates. Another thought is the volume of wood of two smaller boilers versus the capacity of the larger boiler. The biggest warning that I got with my boiler is down time corrosion due to condensation and I would tend to lean towards keeping any idling boiler (cold) heated above ambient conditions and that would be an added draw on storage. Last thought is that in combined systems if there is a "combined system" leak that demands repair both boilers would be down at the same time. But finding a smaller unit for a good price......

 

[TreeCo]

It seems to me instead of two smaller boilers the money would be better spent on a large storage tank and a large boiler.

Here's a large tank I saw on craigslist but no one seems interested. It is a bit large but man what a heating flywheel it would be!

I have no interest in the tank but it sure seems like a lot of tank for the money. I guess it would need it's own outbuilding!

https://www.hearth.com/talk/threads/37570/

 

[Piker]

The only benefit I can see to installing two smaller gassers is if you didn't install any thermal storage. At some point in the season, everyone's boiler is oversized... being able to shut half of your system down during the shoulder season would help a tiny bit with efficiency, but I can't say that the extra cost could be justified. I do tend to think that a boiler will last longer if it idles less. Hotter temps in the combustion chamber and heat exchanger will produce less condensate, less soot, less creosote, and ultimately less acids to corrode the steel. For most situations, I think money is better spent installing storage than extra boilers.

cheers

 

[Como]

http://www.windhager.co.uk/casestudies/index/view/casestudiesID/27/

Pellets, but the principal is the same. Also have a look at their brochure that has more on cascade systems.