Boiler Size/Replacement

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Zachary Smith

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Hi,

My boiler started leaking water a few days ago, and after reading all the helpful posts I thought I'd post to get all of your educated thoughts. I also seem to have a unique boiler and thought everyone would be interested to see it. My heating guy has indicated that there is no likely ST fix to get through the winter and has started calculating what the new system should be.

House
- 1820's Federal with approx. 1/3 un-insulated. 1/3 w/ 1920s insulation, and 1/3 w/ 1960's insulation
- Windows are single pane with exterior and interior storms
- Around 4,000 sq feet
- Separate Propane instant HW heater
- Existing HW heating
- 49 ft Baseray Cast Iron Baseboard (370 BTU/hr/Linear ft)
- 35 ft Baseray Cast Iron Radiant Radiators (Inset, 20 inch tall, 5 inch deep, 187 sq ft at 240 BTU/Hr/Sq ft)
- 10.5 ft Classic 3 Tube Cast Iron Radiators (3 ft tall x 9 inches wide)
- 5 ft Smaller 3 Tube Classic Cast Iron Radiators (2 ft tall x 9 inches wide)
- 4 ft Tiny Classic Cast iron Radiator (1.5 ft tall x 4 inches wide)

Current Oil Boiler
- H.B. Smith 2500A/2500L 25 Mills
- 450,000 MBH
- Estimated circa 1960's
- Approx. 3 ftx 3ft x 4 ft tall
- See pictures
- Believe it's a universal pump/circulation
- 10 Taco zone valves for diff rooms, each feeding off main HW feed
- Believe it has an outside therm to adjust water temp
- Exhausts through existing chimney
- Dampness around base a week ago is now expanding and small trickle can be seen coming out of corrosion near base

Walked through the house this morning with my HVAC guy as well as someone who appeared to be from the wholesale heating supply company FW Webb. I am waiting for them to do the full calculations on my radiators and provide their full recommendation for boiler and size but had a few questions for everyone.

Due to the repeated advice on this and other forums that a heat loss calculation is REQUIRED to size the boiler properly, I asked if they would be doing that calc along with figuring out the radiation capacity. They replied that they could, but didn't need to.

I pushed again, citing the advice on here and my desire to prevent buying an overly large boiler and the resulting short cycling. They indicated that I have a large circulation system (see details above) and a huge amount of radiators that would require a large boiler, the boiler size will be determined by that rather than by heat loss.

Questions:

1. My guy refused to give a rough estimate until he did calcs but the FW Webb guy offered a prelim size guess of around 375,000-400,000 MBH. The current boiler is 450,000 MBH and doesn't seem to short cycle. My house is very large (>4,000 sq ft), very old (1820), very drafty, but does this seem really large?

2. They mentioned Buderus as a possible brand, and think they may have mentioned the G315 series. Does anyone have experience with those or thoughts? I want an oil unit that could be switched to gas in the future and prefer reliability to super efficiency.

3. Is it possible they are right that the system can be sized just based on the current radiators? I trust my HVAC guy to a ridiculous degree but their response doesn't jibe with the thoughts on this board. He indicated that my system has to move/heat so much water that short cycling shouldn't be an issue.

4. What is the benefit to my heavy cast iron baseboards vs regular base boards. He indicated they actually throw off less BTUs but radiate the heat better???
 

Dana

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Even a 4000' tent doesn't need a 450MBH boiler. Even half that size is probably oversized for the whole-house load, and LUDICROUSLY oversized for the radiation on any single zone. Your current beast doesn't short cycle because of it's excessive thermal mass, but being ridiculously oversized for the house load it's standby losses are a large fraction of the total BTU input. Any new boiler won't have nearly the thermal mass, but you'll probably be saved by the thermal mass of your high volume radiation as long as it isn't oversized for the whole-house load. With smarter controls to manage the available thermal mass you can still get away with some amount of micro zoning, but it may need some tweaks.

Do some of your own napkin math on this- HVAC contractors (even good & well-intentioned ones) tend to be too conservative and with 10 indivitually controlled zones and the lower thermal mass of the replacement boiler you're already in short-cycling risk territory, and oversizing increases that risk pretty rapidly.

Start by running a fuel-use based heat load calculation to put some stakes in the ground on your actual 99% heat load, and don't oversize the boiler's DOE output by more than 1.4x from that number, which will have you covered even during Polar Vortex events. You may have to do some interpolation if you normally keep parts of the house under 60F for long stretches of winter. If you're on a regular fill-up service that stamps a "K-factor" on the billing slips, a few wintertime K-factors would be sufficient for ballparking it.

Start adding up zone by zone the square feet equivalent direct radiation (EDR) of all your baseboard & radiators, as well as the total. Do NOT just multiply the EDR' x 170 BTU/hr to size the boiler- that would be an absolute maximum size even if it were all operated as a single zone.

The benefits of the cast iron are radiated heat comfort compared to convected heat out of fin tube, but also the thermal mass of the higher water volume and cast iron, which helps suppress short-cycling on zone calls.

Buderus G215s are decent boilers, but at ~130,000 BTU/hr of DOE output even the smallest one might be oversized, which is why you really REALLY need to run the load calculations to get this right.
 

Zachary Smith

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Thanks Dana. Appreciate the help a lot.

I worked out the heat loads below and they seem unreasonably low.

62,233 BTU/hr load at 62 degree base temp (using 1.4 sizing factor)
61,093 BTU/hr load at 65 degree base temp (using 1.4 sizing factor)

From:12/21/17 to 1/25/18 (36 days)
HDD @ 62: 1,566
HDD @ 65: 1,675
Estimated 249 gallons used at 138,700 BTU/gal for 34,589,999 BTUs total
Estimated 80.5% Efficiency based on note from maintenance 4 yrs ago
2 Degree design temp
27,844,949 Net BTUs used over period

See anything I may have messed up?
 

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60K is not an unreasonable load number for a 4000' house that's been tightened up and insulated some (really!)

What did you mean by "using 1.4 sizing factor"??

For base temperature, use a number 5F cooler than your average indoor temp. (For newer better insulated houses it's more like 10F lower).

I've got to run right now- I'll take a peek at your math tomorrow, but the answer is not insane. See if you can come up with the EDR feet numbers.
 

Zachary Smith

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The 60K load reflects multiplying the 44k actual load by 1.4x to get equipment sizing per the website you linked.

Using a 55 degree base (We set house to 60 and turn up one zone to 68 for 4 hours after work), I get:

46,761 actual load and 65,466 using the 1.4x factor.

No rush and hate to make you check my math, just puzzled that my boiler potentially ~9.6x oversized based on rough calcs.
 

Zachary Smith

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Thanks again, I worked up the following. Just question whether I got the radiant radiators right, seems high but there is 36 ft of them.

Type - BTU Rate
- EDR Feet sq - BTU/hr - Note

Baseray Cast Iron Baseboard - 9-7/8"x 2.5"x 49 linear feet - 480 BTU Rate - 49 EDR - 23,520 BTU/hr - Rate per mfr
Baseray Cast Iron Radiant Radiators (inset) - 20"x 5"x 36 feet - 240 BTU Rate - 432 EDR - 103,680 BTU/hr - At 180
H.B. Smith Radiators - 3 Column - 36"x 9"x 35 sections - 170 BTU Rate - 175 EDR - 29,750 BTU/hr - At 180
H.B. Smith Radiators - 3 Column - 26"x 9"x 18 sections - 170 BTU Rate - 68 EDR - 11,475 BTU/hr - At 180
H.B. Smith Radiators - 4 Column - 19"x 4.5"x 28 sections - 170 BTU Rate - 63 EDR - 10,710 BTU/hr - At 180
Total 179,135

Just reread your comments and actually followed your instructions below...

Z1 - 480 Sq ft - 85 EDR - 0.2 ratio
Z2 - 449 Sq ft - 100 EDR - 0.3 ratio
Z3 - 226 Sq ft - 68 EDR - 0.3 ratio
Z4 - 192 Sq ft - 198 EDR - 1.0 ratio (sun room all windows)
Z5 - 385 Sq ft - 78 EDR - 0.2 ratio
Z6 - 355 Sq ft - 76 EDR - 0.2 ratio
Z7 - 182 Sq ft - 72 EDR - 0.4 ratio (hottest room in house)
Z8 - 388 Sq ft - 126 EDR - 0.3 ratio
Z9 - 385 Sq ft - 60 EDR - 0.2 ratio
Z10 - 355 Sq ft - 63 EDR - 0.2 ratio
Unzoned - 400 Sq ft attic - 0 EDR - No radiation

Some #s changed slightly as I remeasured things and found a radiator I missed!
 
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Dana

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The 60K load reflects multiplying the 44k actual load by 1.4x to get equipment sizing per the website you linked.

Using a 55 degree base (We set house to 60 and turn up one zone to 68 for 4 hours after work), I get:

46,761 actual load and 65,466 using the 1.4x factor.

No rush and hate to make you check my math, just puzzled that my boiler potentially ~9.6x oversized based on rough calcs.

The 60K isn't the "load", it's the ASHRAE recommended boiler output capacity. The load is the 44K number.

When you re-calculated using base 55F to reflect your 60F average indoor temp, note that 60F is 8F cooler than the code required capacity for 68F at the 99%$ outdoor design temperature. So it needs another 8 heating degrees tacked to that 46,761 BTU/hr number to reflect what the load would be at 68F indoors. So without running the actual numbers, guesstimating in my head the load at 68F indoors would be something like 51-52,000 BTU/hr (?) (I'll let you do the actual arithmetic) and a max boiler size of 52K x 1.4x= 73K. (DOE output, not IBR.)

There isn't much point to calculated the output of each radiator at 180F- the total EDR is what you're looking for. The heat load doesn't track the room's floor size either- it's more about exterior surface area (walls, attics, foundation walls) and exterior surface type (insulated wall, window, uninsulated wall, insulated attic.) Corner rooms on the top floor have a lot more heat loss per square foot than a room on the first floor with only one of the 4 walls being an exterior wall.

Woring backward from your 180F total 179,135 BTU/170BTU per EDR' you have about 1054 total EDR', and a 99% design load of about 52,000 BTU/hr @ the code-min 68F indoors. That's a ratio of ~50 BTU per EDR', which takes an average water temperature of about 120F. That would be a great temperture number if it were a natural gas fired condensing boiler, but it means with a right-sized boiler will need a system bypass branch with a thermostatic mixing valve to ensure enough boiler output is mixed with the return water so that the water entering the boiler is 140F or higher. (With water cooler than that there is corrosive condensation on the heat exchanger and in the flue.)

The flow requirements of that much radiation is probably a lot more than the smaller boiler can handle if pumped direct. Setting it up primary/secondary with a smaller pump on the boiler loop can be done in a way to keep the entering water temp at the boiler up to temperature too. There is some design work to be done here, not gross hackery. Throwing another monster-oversized boiler at saves the installer from having to do any thinking or calculations, but would be absolutely the wrong thing to do.

If Buderus, the G115WS/3 puts out 74,000 BTU/hr, which is the right range. The Energy Kinetics System 2000 EK1 Frontier with the 0.68gph nozzle puts out 83K, which isn't ridiculously oversized, though probably more than 1.4x the 99% heat load.

Run your math multiple times to be sure. Based on the description of the house I would have expected ~60-80K for a load, not ~50K. From the fuel use number I suspect it's pretty air-tight, and an efficient shape, basically a rectangle without a lot of corners bump-outs dormers, etc. and a window/floor area ratio that's lower than the ~15% typical of new construction. (Window/floor ratios of 10% or less are pretty common in early 19th and late 18th century houses.)
 
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Zachary Smith

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Thanks Dana. I worked out the revised numbers below.

47,000 - Actual Load per Fuel Use (60 Degrees in Late Dec-Late Jan)
54,000 - Load Needed by Code (68 Degrees Req. Inside)
76,000 - ASHRAE Recommended Boiler Output Capacity (DOE)
926 - Estimated EDR
157,420 - Estimated Radiation (170/EDR using 180 Degree Water)
58 - BTU Load by Code/EDR

Unfortunately, I won't have a nat gas hookup until next year and will need to replace the boiler soon.
 

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A load of 54K is credible, and a boiler that can deliver 76K won't leave you cold until the next ice age, or maybe it'll lose a degree or three during the coldest hour of the coldest day of the century. So now you have the right order of magnitude pretty firmly staked out. There maybe quibbles that in a wind the actual load could be 25% higher than the 54K, number (and that's still covered with 1.4x oversizing), but not 50% or 100%. AFUE testing presumes 1.7x oversizing, which would be ~92K. Boilers with a DOE output over 100KBTU/hr need not apply.

If you look at the nomograph on p.2 of that radiator document you'll see that even at design condition outdoors the radiation would deliver that much heat at 125F. It may need to be higher than that in some room or another, but probably not 10F higher.

With the total thermal mass of water & iron in the radiation the average water temp simply can't hit the ~150F+ it would need to be returning 140F or higher water back to the boiler without absolutely roasting you out of the house. There may be some thermal bypassing plumbing in the current design, or perhaps it's a sufficiently old-school high-mass boiler that wouldn't need it, but those usually test pretty low, in the 65% steady state range or lower.
 

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Even though it's hard to find reasonable selection choices in the menus, running a room-by-room Manual-J using an online tool such as loadcalc pr coolcalc is a useful sanity check. Those tools tend to overestimate, often by 30% or more, but never by 100%. You've described the place as "drafty", but run the numbers using different air-tightness/infiltration settings to get a feel for the range.

One thing about "drafty" is that it can usually be cost-effectively corrected for not huge money (but way more than a family outing at Mickey-Ds) using blower door and infra-red imaging directed air sealing, which will increase comfort and lower the heat load by quite a bit on houses that are truly leaky.

As lousy as rules of thumb are, a tighter than average house with R13 in most of the walls and R20 in the attic with clear -glass (not low-E) double panes or clear glass storms over wood sash will usually come in under 20 BTU/hr per square foot of conditioned space @ 0F, often under 15 BTU/hr per square foot of conditioned floor area. If your ~4000' house is measured by fuel use at 54K @ +2F that's 13-14 BTU/hr per foot of floor area, which is usually not a very drafty house.
 
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Unfortunately, I won't have a nat gas hookup until next year and will need to replace the boiler soon.

If you're thinking of possibly converting to gas, that will narrow down your boiler choices a bit. A pretty good not-too-expensive triple pass cast iron boiler like the Biasi B10 series is sold with both oil or gas burners, and fairly easy to convert. With a natural gas burner the same boiler puts out a bit less than with an oil burner, but it looks like the four plate Biasi B4 might still be a reasonable fit, at 94,000 BTU/hr out with oil, 90,000 BTU/hr with gas:

B10-SPEC-CHART.png


At 90K out it'll still hit it's AFUE numbers even without resorting to heat purge economizers, etc. Biasi ships the Carlin EZ-GAS conversion burners on this boiler, a part that runs ~$650 at internet pricing. The installation is almost trivial once the gas plumbing is in place.

Converting other simpler type boilers is similarly easy, but not all oil boilers would be as amenable to a conversion. With the B-4 there's not much thinking or calculation to do for the conversion- they already have it spelled out.
 

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Thanks Dana. I may have overstated the draftiness of the house. There are around seventy windows And 3/4 are older single pane wood sash windows but All windows have both indoor and outdoor storm windows. Also just spray insulated approx 3/4 of the uninsulated walls in the oldest section when I replaced siding with new quartersawn clapboards. I guess it can’t be too drafty if I only used 250 gallons a month or 44,000 BTUs/hr in the worst of winter.

I’ve heard good things on the biasi, and being able to do oil now and gas next year is ideal. I’m getting a lot of informal recommendations for Energy Kinetics as well, as a good option to start oil, switch to gas, and run low water temp. Any thoughts on pros/cons between the two?

Also struggling to understand the full implications of having 2.5x the likely load in existing radiation. Since that means I could run more efficiently at 120-130, is the necessary bypass piping relatively simple/cheap to solve the negative impact of low return temps to the boiler? Can that be simply resolved with a Biasi or EK?

I’ve managed to get a basic grasp of boiler sizing due to your kindly help, but the complex piping in my basement looks like a spiderweb to me and have a limited sense of how it’s really designed. Appears to have large wide diameter asbestos insulated feed running around basement, smaller diameter copper feeds off of that running up to all 10 zones, small diameter returns from each zone with Taco zone valves.
 

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I'm not sure how easy it is to convert an EK System 2000 from oil to gas. They sell both gas & oil models, but it's not clear to me what's involved in a conversion. Their heat exchanger design is quite unique, and they have one of the most sophisticated control systems in te industry. With a Biasi it's dead-obvious & easy, the design is an evolution of fairly well established boiler designs, and it's comparatively inexpensive.

A system bypass may not be necessary if plumbed primary/secondary if they do the pumping math, even with 100-120F water in the radiation. Even if pumped direct system bypass branch protection is pretty standard when dealing with high water volume radiation systems. Running the radiation at a lower, more constant temperature with much smaller temperature over/undershoots is more comfortable than the hot-flash followed by the chill that can happen with high temp operation with a boiler grotesquely oversized for the zone loads.

The high volume of the large diameter manifold probably keeps it from short-cycling on zone calls. With a smaller boiler with it's own pump tee-ing into that large diameter pipe in two locations fairly near each other would allow the boiler to operate at a much higher temperature than the fat distribution loop. The short section of fat pipe between the tees between the boiler is where the higher temp and lower temp water mixes, but the flow through the boiler can be much lower than what' circulating in the fat pipe. This is the simplest version of what's called "primary/secondary" piping- the loop to the boiler being the "primary", the loop the radiation being "secondary". There are variations on the theme. If what you currently have is one huge loop, what you currently have or what would may make sense in the retrofit with a much smaller boiler would be more similiar to the "series primary" configuration, on the left:

siegenthaler-presentation-small-41-728.jpg


If instead of a fat-pipe loop it's a fat supply pipe with the zone valves that isn't directly looped to a fat return pipe where all of the radiation returns terminate that works too.
 

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My plumber just called with a verbal rec of Buderus g215, 225k btu despite just 900EDR or 153,00o BTU in radiation. Also wants to replace all of my 30 radiator vents, which are working fine, with new automatic vents. Seems to be suggesting a 130-160 degree water temp range based on outdoor reset. Also doing bypass piping rather than primary secondary setup. Estimated 3-4 days with 3-5 guys working.

While I’m waiting for the actual write up and exact details, disappointed the $18k bid seems based on how nice my house is rather than the right size/setup/way to do things. Thankfully I have another bid pending and another guy walking through Monday.
 

Zachary Smith

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Just received written bid, quoted below:

We shall provide and install one Buderus 215 six section cast iron boiler with a Riello F-10 oil burner and associated electrical and piping to existing around boiler. We shall also install 1-SX60 expansion tank, 24 Automatic radiator vents, 1-Watts Pressure regulator and back flow preventer, 1 -Tekmar or Taco modulation control, Tac 00e Veraridian Variable speed circulator with isolation flanges, flue pipe, 1-by pass loop, 1-1” Taco Zone valve with transformer for den / entrance room, draft regulator, High limit, piping, electrical and associated labor. This price does not include domestic hot water production.
If Automatic radiator vents are not desired, deduct ($xxx)
This price does not include a stainless steel liner that I recommend with these boilers but that are not required if the chimney has an internal clay liner. This quote does not include disposal of old boiler and associated piping.

Just curious, normal to not get breakdown of specific materials and labor? Thanks!
 

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Most contractors put just one number on it, to avoid having to dicker endlessly over individual line items and end up taking a loss in the end if anything goes wrong.

$18 is a pretty big number for a simple boiler swap, but it's not clear exactly how "simple" this one will be. The 6 plate G215 runs about $3.5K at internet pricing. At 256,000 BTU/hr out (DOE) it's ridiculously oversized for a fuel-use calculated 58K load, of course. (You probably have enough thermal mass in the system that it won't short cycle, but... ) A more appropriately sized 3 or 4 plate G115WS runs about a grand cheaper than a 6 plate G215.

I'd be surprised if the clay liner isn't oversized for this boiler, but if if the chimney is surrounded by conditioned space (not on an external wall, with an external face the condensation risk is a bit lower.


A Taco Viridian 00e is a decent pump, about $250 at internet stores for the smaller ones. On a high volume system they may be opting for one of the much bigger (and more expensive) in that series, not the VR2218 or VR1816.
 
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