Boiler sizing - loss due to oversizing?


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Old 01-10-09, 11:40 AM
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Boiler sizing - loss due to oversizing?

OK - I apologize upfront for another sizing question (but you have all preached the importance of proper sizing).

I am going from a 1.5gph nozzle size (245btu/hr boiler, 38 yr old)

My heatloss is something ~70k - 80k btu/hr. If I jack it all to extreme assumptions I can get as high as 95k btu/hr.

I will put in a 60gal indirect (current have a tankless coil combined with a superstor 45gal storage tank). Yes this might be overkill but SWMBO is weighing in on this one.

ACTUAL OIL used over the last few years has been ~1700 gals annually. Peak periods (roughly a month) are as high as 13gal/day. Note this is input.

I have two choices in front of me:

Option A) A buderus 115/5. Gross output ~136k. USED 7 yrs(comes from a long standing local company with full warranty).

Option B) A new smaller size buderus the /3 or /4 (I would lean towards the /4 for sizing given the homework I have done to date).

Note both would have full controls (logomatic), OTR, the same piping, etc.

Option A is somewhere between $2000 and $4000 cheaper at the end of the day. (not posting total costs but the relative difference is the real question, and I know its a big range but the install estimates vary a lot).

Ignoring the fact its used (I know this may not be possible for some of you... ) - I would need to see some pretty significant efficiency gains to make option B pay out in less than 5 years (give me some efficiency guesses so I can play fuel cost assumptions)

I looking for more experienced guesses than mine.... what do you think the efficiency (and thus savings rate) might be simply from having the smaller capacity boiler??
 
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Old 01-10-09, 02:39 PM
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There is nothing written in stone but I would guess a total of 40% if properly installed. Why such a big indirect. What is the hot water demand? Unless you have hot tubs, whirlpools or spas' a 40 gallon should be more than required. Check this link for sizing.
http://www.comfort-calc.net/indirect_sizing.html
 
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Old 01-10-09, 02:49 PM
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Thanks rbeck.

The larger indirect is just since we sometimes runout with the current SS 45 gal storage. I think it could be a different system problem than storage size and could be talked into a lower size tank.

My question was - what would you guess the efficiency difference would be between a new 115/5 size verses a 115/4 or even 115/3 size boiler. All would have the controls and reset.

Do you think the 115/4 would see 40% gain over the /5 or did you mean relative to the original system?
 
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Old 01-10-09, 03:54 PM
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Your true heatloss is probably still overstated. It would be really hard to guess without knowing how much thermal mass you have.

With enough thermal mass and controls you could overcomes some of the inefficiencies of having an oversized boiler.
 
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Old 01-10-09, 04:07 PM
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thermal mass

Now that you mention it, I will end up with this un-used storage tank (my prior tankless coil storage tank). When I offer the concept of tying in this unused tank with the boiler circut to create thermal mass - I get a glazed blank look.......

but you are suggesting there are ways of mitigating a modest oversizing. (essentially adding thermal capacitance and system monitor/controls). It makes sense in theory - I just wasnt sure about practical implementation. (if I wasnt careful I could spend too much time tinkering with this.........)
 
 

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