Replace 1960 80% boiler?
I have a burnham holiday boiler no. 8-60 series 2 that recently combustion tested at 80% efficiency. I think the boiler was probably manufactured in 1960 based on the boiler no. It's 172k input and 140k btu output.
All advice is that if the boiler's over 15-20 years old it should be replaced. Why? If it's 80% efficient and has been reliable for 50 years, why drop $10k for something only marginally better? (I'm not interested in ModCon because the extra cost of servicing and shorter lifespan make them uneconomical for my limited needs).
Presumably a new one would be smaller (my heatloss calculation is 90K btu Max) and have sealed combustion, electronic ignition, and perhaps less jacket loss (though hard to tell). Does anyone have any real world experience with the savings expected with a newer smaller boiler?
Right now we're using about 1150 CCF of gas per year or about $2300. I estimate that about 250 CCF of the 1150 are for DHW.
If my replacement age samples for resi. HVAC equip. are representative, your unit has outlived 90% of other units. My sample avg. age is 25 yrs, and, correcting for the sample size, the true replacement age for all HVAC equip. is between 21 & 29 yrs.
Originally Posted by dstrutto
Note that replacement age is less than or equal to wearout age and that normal wearout exhibits an increasing failure rate for the unit's components.
As long as it is likely to fail gracefully [for a few bucks you might be able to get monitoring devices which warn you of problems] and not catastrophically, I guess you could keep it.
From another angle, if a new unit is $10k and a catastrophic failure of your existing boiler would cause $100K damage, you should get a new unit if the likelihood of a catastrophic failure is more than 10/100 = 10%. The $10k would then be considered an insurance premium against a catastrophe. Of course, existing homeowner's insurance will change this equation.
Calculation methods are notoriously inaccurate, and trend toward the high side. I'm betting your true design-day heat load is under 75K. Download the FSA calulator and select your boiler & HW heating types and the city with the nearest heating season climate as yours and it'll give you a much more realistic & reliable heat load and operational efficiency estimate based on boiler modeling developed at the Brookhaven Nat'l Labs. You can also insert different boiler & HW types and compare them to your existing boiler's performance. I think you'll be surprised.
Originally Posted by dstrutto
It's a buggy piece of software- expect it to crash regularly. But it's modeled/calculated results are far more accurate than most.
Combustion efficiency isn't a good measure of the actual system efficiency- it's just the beginning. Has someone retrofitted an automatic flue damper? If not, subract at least 5% from any presumed AFUE. If the system is atmospheric-drafted rather than sealed-combustion/direct-vent (and I'm sure it is), subtract another 3-5%. Standing pilot instead of electronic ignition? In your case (based on your CCF/year use) subtract another ~5-7%.
I'd be shocked if you're getting any better than 70% true efficiency out of it, and a right-sized direct-vented cast-iron boiler would deliver a true 83-86% (depending on model.) Even with old-skool high-temp radiation (baseboards/radiators whatever), a modulating-condensing boiler would deliver ~90% and cut your fuel use to under 800CCF. If you add more radiator area or tighten up the place to where your existing radiation can deliver the heat at 130F or less on design-day a mod-con will average in the mid-90s.
If you're heating hot water with a standing-pilot atmospheric drafted tank you would improve your overall efficiency hanging an indirect-fired tank off the old beast, cutting your water heating standby losses in half (and losing the extra pilot.) Hang it on a mod-con and you'll have gone from ~55-60% efficiency to 85% efficiency on the water heating.
The EF numbers in hot water heating are very squishy, since they're based on 62gallons/day use, and with tank heaters volume and storage temps are everything. Very high volume users use more CCF, but have higher efficiency. The standby loss is roughly the same for a particular temp, but there's a huge difference in standby loss between 120F storage and 140F. Also not included in the EF test is fuel used by the space heating system to make up the heat lost by the extra flue-drafting and air infiltration necessary to supply combustion air for the HW heater (assuming it's not direct-vent). This adds something like 3-5% to the total fuel use.
I wouldn't be surprised if you couldn't cut your fuel use by at least a third, and possibly half with a mod-con & indirect. Short of that, a perfectly sized direct-vented cast-iron boiler + indirect would cut it by at least a quarter, and maybe a third.
If your heat load IS under 75K, (and if you're brave enough to design it yourself) using a ~$500-1000 direct-vented tankless HW heater as a boiler and a "reverse-indirect" as a heating system buffer can put you somewhere between cast iron & mod-con efficiency. (This violates the warranty of most tankless units, but Takagi actually markets their product line as dual-use. A ~$600 Takagi Jr should be able to handle your loads with ease.) If your radiation temp requirements are over ~130F it takes a bit more plumbing & design work to get there, but if it's 130F or lower it can be dead-simple to design with.
Reverse-indirects can be a great way to add thermal mass to any heating system that needs water temps above 125F, and can max out the efficiency of a cast iron boiler (new or old) by providing a minimum burn length, and increased duty cycle. Old-skool high mass boilers often don't hit full steady state combustion efficiency in under 5 minute of burn (and the first 3 minutes are TERRIBLE), and anything that you can do to lengthen the burn will get your operating efficiency closer to it's raw combustion efficiency. Anything under 10 minute burns on a high-mass boiler is a short cycle. If they're under 5 minutes it's as-operated AFUE will be double-digit percentages below it's combustion efficiency. For more see:
Nice post Dana. manufacturers and IBR have been fudging efficiencies for years. The most noticeable transgression is putting 1/2" of insulation in the jacket to get ratings increases. 80% means nothing except combustion efficiency and has nothing to do with system efficiency.
Yeah, combustion efficiency specs are as meaningless as line out of This Is Spinal Tap:
Originally Posted by Peter Griffin
"The numbers all go to eleven. Look, right across the board, eleven, eleven, eleven and... "
This amp goes to eleven...
This boiler goes to 80%...
And Navien tankless water heaters go to 98%...
And jadnashua's boiler is rated 94%...
None of which means a whole lot without the context.
Combustion efficiency is just an upper bound that the system will never reach. How it really performs depends on a lot of other factors. But you can get much closer to the theoretical with right sizing, low boiler mass, boiler modulation, condensing heat exchangers, and sufficient thermal mass to the load to keep burn cycles long enough to be efficient under part-load, and sufficient radiation to have optimally low return water temps.
And none of that is likely to describe a circa 1960 cast-iron beast ('ceptin' maybe the thermal mass of the load, if it's cast iron radiators, not convectors.) Most boilers of that ilk I've run into are running 55-60% efficiency in-system, but some make 70%. None break 75%.
Actually, where mine runs most of the time, it is sitting at 98% on the curve.
Originally Posted by Dana
Sizing and setup is critical to maximum efficiency. no one system is 'best'. gaining the knowledge to maximize efficiency is only one part of the equation, as sometimes, that imposes certain use constraints. Understanding that also takes a lot of study. Optimizing a system to your desires and needs is complicated. Most rarely end up that way. It would be nice if it were simpler, but currently, it isn't.
Nothing operates at 98% system efficiency with the exception of a large fire burning in the middle of the living room. There are always standby losses, piping losses, loss against electrical cost etc.
On a normal day, my supply is running at 125-130, and the return is 70 or so...if I believe the spec sheet, at those values, it is 98%. At a worst case, it drops to the mid-low 80%, but I don't think I ever have needed to run there. Anyway, it is a lot more efficient than the last boiler setup I had - in the order of 40% less fuel used to maintain the same lifestyle.
When the indirect calls for heat, I have the choice of a 7 or 14 degree differential...I set it for 14, so it runs long enough to be 'in the zone' longer. The input goes to 190, and the return is in the order of 100 and rises as the tank reheats.
I'm not worried about standby losses in this situation, as I'd want that area warm anyway, and there's enough so I don't have to use a radiator or vent to provide it. It's never been too hot, so it isn't all that much.
Oh, I know Ja, it's just that the manufactures throw a lot of numbers out there that really have very little meaning. All things being equal though, there is more to efficiency than the number the service tech gives you (that one just measures combustion efficiency) and numbers the manufacture gives you which measure efficiency under sterile lab conditions. anyway, if the boiler is more than 20 years old, chances are a new one will do you a world of good.
It seriously hangs around 100F for much of the burn on the return when it's just making hot water for the indirect (no dilution from other radiation returns)?? This I've never seen! But if you have a substantial buffer in series with the return, maybe...
Originally Posted by jadnashua
As the return temp rises, where does it finish, at the end of the DHW burn? (I'm gonna guess it's closer to 150F than 105F. :-) )
I believe you'll see 98% raw combustion efficiency out of it with 70F return from radiation, and lots of slab-radiant runs in that zone most of the season. With the distribution & boiler all inside of conditioned space and with sealed combustion, that's truly 98% fuel efficiency in heating mode. (Sounds like you've got this dialed pretty good!) Depending on how much pump you have going to make that happen, you're likely to be at least 96-97% efficient on total energy inputs. (ECM pumps & zone valves, anyone?)
But yep, 40% reduction in overall fuel consumption woud be within the normal range if you went from a 3x oversized 78-80% AFUE cast-iron boiler + standalone HW tank to an appropriately sized mod-con + indirect. Good move!
And that, dstrutto, is exactly why you might consider getting rid of a "perfectly good" 50 year old boiler that's still running at 80% combustion efficiency. If you're using 1150ccf/year now, how long would it take to pay off if you only used 700-800? It kinda depends on what you're paying for fuel now, and what you expect to prices to be over the next decade vs. the cost of money. It's clearly not a short-term investment, but how are your other investments doing, after taxes, eh? (Lessee assuming a new system cost of $10k and annual savings @ $1/ccf, $350minimum, it's a 3.5% after-tax return. It's not necessarily a no-brainer for you, but for people with 2x the heating bill of yours it can be. But you can probably invest $5-6K for an 85% AFUE right-sized system and get return of at least $250-300 too.)