Replacing old cast iron boiler with - Bosch SSB, Navien, Lochinvar?

[Leonard Bayard]

Ripped out the old Weil Mclain 210K cast iron boiler and old radiators in my house and wanting to replace it with one of these listed above. I'm in the Hudson Valley and the house is circa 1900, 2 floors, 3 bedrooms. No insulation at all just lathe and plaster. Coldest it gets here is about -9ºF but that only lasts a few days, average is about 10ºF at night

It's 2400 sq ft. and had 12 old school large cast iron radiators in it (7 downstairs, 5 upstairs). Coming from the boiler was 3" cast iron pipe splitting off to 1.5" cast iron going into the radiators.

Attached is the heat loss calc that I did (per floor). I'm leaning towards the bosch ssb 160K boiler and am planning to install hudson reed traditional cast iron radiators with it. Would very much appreciate any input and any opinions at all! Thanks

 

[Leonard Bayard]

Also, previous owner was using about 300 CCF worth of gas per month in December and January (said keeping the house at 70ºF). The floor joist and the attic is not insulated and am planning to do that before winter.

 

[Sponsor]

 

[Dana]

Ripped out the old Weil Mclain 210K cast iron boiler and old radiators in my house and wanting to replace it with one of these listed above. I'm in the Hudson Valley and the house is circa 1900, 2 floors, 3 bedrooms. No insulation at all just lathe and plaster. Coldest it gets here is about -9ºF but that only lasts a few days, average is about 10ºF at night

It's 2400 sq ft. and had 12 old school large cast iron radiators in it (7 downstairs, 5 upstairs). Coming from the boiler was 3" cast iron pipe splitting off to 1.5" cast iron going into the radiators.

Attached is the heat loss calc that I did. I'm leaning towards the bosch ssb 160K boiler and am planning to install hudson reed traditional cast iron radiators with it. Would very much appreciate any input and any opinions at all! Thanks

Which load tool was that? Some online tools are better than others. Try running it with loadcalc.net as a sanity check. Be aggressive rather than conservative on air tightness issue or it's going to oversize it on you, and use the "after upgrades" condition of the house if you're going to air seal & insulate.

If you have a heating history on the place you can do one better: Run a fuel use based load calculation, (wintertime use only, for best accuracy) which uses the old boiler to MEASURE the heat load. The 99% outside design temp for Kingston NY is +2F, so use that for coming up with the load number, or use 0F, which is fine.

Per ASHRAE ideal no-modulating cast iron boiler would have an output capacity no more than 1.4x the load at the 99% outside design temp. With modulating boilers you can cut that to 1.2x and still have enough to cover your Polar Vortex disturbance lows.

Assuming that online load calculator you used is correct (doubtful, but maybe) at 0F outdoors, 70F indoors you have a 70F temperature difference and a design heat load of 65,230.16 (love the "xxx.16"! ) BTU/hr. With a 65K load biggest cast iron boiler you'd be looking at would have an output of 91K, which would make the SSB 160K boiler insanely oversized, and you really DON'T want to oversize it. Any 100,000-110,000 BTU/hr modulating condensing boiler would have you covered, but it might be worth spending something on air sealing and insulation to bring the load down a bit. A reasonably air tight but uninsulated 2x4 house that size with storm windows and some fluff in the attic would come in around 50K @ 0F, with an insulated foundation & band joist it would be under 50K, maybe even under 40K, and and an 80K condensing boiler would get you there.

Even though the SSB boilers have a 5:1 turn down ratio and the SSB160 would modulate down to your peak output load requirements the SSB120 would be a better choice and would modulate with continuous burns at your average wintertime temp of ~25F. But a 100-120K fire tube boiler with a 10:1 turn down like Lochinvar's KHB 110 or HTP's UFT 100W or -120W would be even better and would modulate a lot even during the shoulder seasons. Sizing a modulating condensing boiler is about adjusting the outdoor reset curve for the lowest water temps/maximum efficiency, getting it down to where you "set and forget" the thermostat and use less fuel than an overnight setback strategy due to the higher condensing efficiencies gained. With overnight setback you need higher water temps/lower efficiency, and with higher-mass radiation like yours there is a long recovery ramp unless you really blast it.

With an oversized cast iron boiler deep temperature setbacks make sense, but it's just a waste with a mod con. With high mass radiation it won't short-cycle even at fairly ridiculous oversizing factors, but that oversizing is buying you nothing but higher cost up front, and more burn cycles putting wear & tear on the boiler.

 

[Leonard Bayard]

Thanks for your reply! I used this load calculator https://www.usboiler.net/heat-loss-calculator.html. The band joist will be insulated this winter but probably no way the foundation, it's stone and just don't have the budget for it.

The link you sent me for the heat load is coming out about the same?

But yeah, seems like the 120k would do the job. Because I'm going to be installing new cast iron radiators would that affect the sizing? thanks!

 

[Leonard Bayard]

Any thoughts on those three brands?

Also, the min input rate difference between the 120K and 160K SSB is only 8K btu

 

[Reach4]

The floor joist and the attic is not insulated and am planning to do that before winter.

Working in the attic in winter is usually much more pleasant than doing that in the summer.

 

[DIYorBust]

I did a Navien, and it's been fine. Was quite a bit cheaper. I think some of the fancy ones have more built in components to make installation easier, but this also means more internal components that could require service. I've had a few issues, the igniter has needed to be replaced a couple times, due distortion of the gap, but its a ten dollar part and easy to change. Had to replace the gas valve after 2 years. So it's been a little more maintenance, but gets the job done and saved me 10k up front. Parts are easier to get and faster an cheaper than European boilers. That said it's definitely annoying when something goes wrong. The Navien has 15:1 turndown, so it's good for keeping the system temp low which reduces fuel usage when the load is low. Another recommendation is to oversize the radiators. The more radiators you have the lower temp you can run the system, and if you have too little radiation somewhere on a -9F day, you're probably already running at 180 degrees, so you can't increase the delta T by increasing the water temp. Plus this will save energy on non design temp days.

Sizing the boiler is tricky, and heat loss calculations aren't really perfect so you will either be a little over sized, or risk having a few days a year when the temp drops below 72. If that happens you can improve insulation, make a fire, or put on a sweater. When it comes to insulation I'm sure it will help. But also, drafts are often a real culprit, so I'd check that all the windows and doors seal well and are caulked, door gaskets or sweeps are present where needed, etc. Zero insulation is a problem for sure, but going from say R-8 to R-20 isn't going to do much if cold air is getting in somewhere.

If you want something hands off, I've heard good things about Lochinvar. Veissman was the one I heard endless praise for, but I didn't think it was worth the money.

One often overlooked area in these systems is electrical efficiency. Consider using Viridian VT2218 ECM pumps for the circulators. Saves a lot of electricity. The viridian can also adjust the circulator speed depending on system delta T. A dirtmag is a good idea too, especially if you go with the ECM. Whatever system you get, make sure they put good TRVs on all the new rads to help you balance the system.

At the end of the day, I love having it vs the old cast iron. Gas bill is lower, and the heat comes up faster. Energy savings is more than expected by looking at AFUE ratings, and the air quality in my basement is noticeably better.

 

[Leonard Bayard]

I did a Navien, and it's been fine. Was quite a bit cheaper. I think some of the fancy ones have more built in components to make installation easier, but this also means more internal components that could require service. I've had a few issues, the igniter has needed to be replaced a couple times, due distortion of the gap, but its a ten dollar part and easy to change. Had to replace the gas valve after 2 years. So it's been a little more maintenance, but gets the job done and saved me 10k up front. Parts are easier to get and faster an cheaper than European boilers. That said it's definitely annoying when something goes wrong. The Navien has 15:1 turndown, so it's good for keeping the system temp low which reduces fuel usage when the load is low. Another recommendation is to oversize the radiators. The more radiators you have the lower temp you can run the system, and if you have too little radiation somewhere on a -9F day, you're probably already running at 180 degrees, so you can't increase the delta T by increasing the water temp. Plus this will save energy on non design temp days.

Sizing the boiler is tricky, and heat loss calculations aren't really perfect so you will either be a little over sized, or risk having a few days a year when the temp drops below 72. If that happens you can improve insulation, make a fire, or put on a sweater. When it comes to insulation I'm sure it will help. But also, drafts are often a real culprit, so I'd check that all the windows and doors seal well and are caulked, door gaskets or sweeps are present where needed, etc. Zero insulation is a problem for sure, but going from say R-8 to R-20 isn't going to do much if cold air is getting in somewhere.

If you want something hands off, I've heard good things about Lochinvar. Veissman was the one I heard endless praise for, but I didn't think it was worth the money.

One often overlooked area in these systems is electrical efficiency. Consider using Viridian VT2218 ECM pumps for the circulators. Saves a lot of electricity. The viridian can also adjust the circulator speed depending on system delta T. A dirtmag is a good idea too, especially if you go with the ECM. Whatever system you get, make sure they put good TRVs on all the new rads to help you balance the system.

At the end of the day, I love having it vs the old cast iron. Gas bill is lower, and the heat comes up faster. Energy savings is more than expected by looking at AFUE ratings, and the air quality in my basement is noticeably better.

Thanks so much for this info! It's really helpful. I had a lochinvar in a house in Reno and it worked great but found it pretty noisy. And yes about the heat calcs. My Reno house was 1000 sq ft and with a 80k boiler the lowest it would modulate was 35%, so not sure that a 160k would be that far over but going to definitely take a look at a 120k.

The Viridian looks great! Will look into that. Here in the Hudson valley it seems everyone has a navien too. Appreciate the feedback.

 

[Dana]

Thanks for your reply! I used this load calculator https://www.usboiler.net/heat-loss-calculator.html. The band joist will be insulated this winter but probably no way the foundation, it's stone and just don't have the budget for it.

The link you sent me for the heat load is coming out about the same?

But yeah, seems like the 120k would do the job. Because I'm going to be installing new cast iron radiators would that affect the sizing? thanks!

You're really going at this wrong, erring to the ultra-conservative side on R-values & U-factors and air leakage when you need to be going hard in the other direction to get it right. The US Boiler tool is a fairly simple crude I=B=R type load tool almost guaranteed to oversize by 20-30% even when you punch in the right assumptions, but it appears you didn't. If it's coming out about the same using the LoadCalc tool it means you're not being aggressive enough on the air tightness and other assumptions.

In the US boiler tool you entered the maximum leakage rate 1.5 ACH , which is a mistake. If your house were that leaky you'd be miserable as hell from all the drafts and would/should do a round of air sealing to bring that under control. Even cutting it down to 0.75 ACH is probably overstating reality if you're air sealing the attic floor and sealing & insulating the band joist & foundation sill, but assuming 0.75ACH would knock 7390 BTU/hr off the 65,230 BTU/hr load, bringing it down to 57,840 BTU/hr @ 0F.

Then you assumed a scant 3" of fluff in the ceiling, whereas you'd be crazy to only install 3". There's almost certainly going to be enough room for 9", which would reduce the 5100 BTU/hr ceiling losses down to 3060 BTU/hr, a reduction of 2040 BTU/hr for a design heat load of 55,800 BTU/hr.

The window assumptions used was insulated single panes/ no storms. (Really- no storm windows? ) Even if you're not replacing the windows, storm windows will "pay off" in both comfort and fuel savings, and low-E storm windows would pay off even quicker (despite the higher upfront cost of the glass.) Changing the US boiler tool to storm windows over single panes takes your windows & doors losses from 22,140 BTU/hr down to 18,375 BTU/hr, a savings of 3675 BTU/hr, for a design load of 52,035 BTU/hr.

That's a credible number for a house with wood sash single panes + storms, no wall insulation, no foundation insulation and 8-10" of cellulose in the attic, but it's probably lower than that (or could be with some air sealing.) With no wall insulation or foundation insulation it probably won't be below 40K, but reality could be 45K.

At ASHRAE's 1.4x multiplier that would call for an output of 52,035 x 1.4= 72,849 BTU/hr. That makes even a 100K boiler oversized, let alone a 120K boiler. Remember, with a mod-con even 1.2x is plenty if you're not using deep overnight setbacks, that would call for 52,035 x 1.2= 62,442 BTU/hr

You're really looking at an 80-85K boiler, not 100K, and SERIOUSLY not a 120K boiler, which would be insane. Even at non-condensing water temperatures an 80,000 BTU/hr boiler delivers more than than 62,442 BTU/hr, and at condensing temperatures delivers over 75,000 BTU/hr.

You would only need/want a 120K boiler if you really thought it was going to hit -70F outdoors sometime in the next 20 years. The climate may be changing, but that's colder than the coldest temps Fairbanks Alaska sees in a typical year.

The comparatively inexpensive HTP UFT-080W (or the Westinghouse branded version of the same thing, the WBRUNG-080W) don't have all the setup bells & whistles that come with the KHB-085 is easier to install plumbing-wise and easier to program, but either is reasonably sized to your load. Product support matters- find out who is installing what in your neighborhood- there are several other 10:1 turn down mod-cons in that size range that would work.

The HTP/Westinghouse UFT/WBRUNG boilers are designed and manufactured by Kiturami, a first tier Korean company (like Navien) that competes head to head with Navien in their home market, but took an OEM strategy for the North American market, leaving the product support to the importers. Laars and a few others are also selling related Kiturami boilers, and the feedback on all of them seems to be pretty good. HTP's headquarters is an easy day's round trip from Kingston NY if you ever felt the urge to heave the thing through the front window of the home office, but I've yet to hear of any disasters with that product line. Installers like them- especially if setting it up for domestic hot water with an indirect water heater, since it has a secondary port and controls for supporting the indirect built in, reducing the amount of additional plumbing and design work for that.

Navien's NHB-80 would also fill the bill. Unlike the fire tube heat exhanger Lochinvar KHB or HTP UFT it uses water tube heat exchangers with a higher pumping head, and MUST be plumbed primary/secondary, with separate pumps for the boiler and radiation. Lochinvar insists on primary/secondary too, but will relent if you show them the math and allow it to be pumped direct. HTP advertises the UFT as "no primary / secondary needed" and in 19 out of 20 systems that's really true.

 

[Leonard Bayard]

Hi Dana,

Thanks for your reply, especially at the end where you write about the boiler options.

I just wanted to say that when I wrote there is no insulation in the house, and when I say that I mean:

There.. is.. no.. insulation..

None

At

All.

Yes, single pane windows, no storm windows, you read that right. In the attic it's so drafty that when it's windy I can see dust balls blow across the floor (with the windows closed).

No offense but I think everyone is getting a bit too focused on calculators and what SHOULD be the right number. The real world is different. I had a house in Nevada with a 80k boiler and it never modulated below 35%, and it was only 1000 sq ft. It had insulation in the attic but that was it.

I really appreciate your reply but when you find someone on the boards here with an uninsulated victorian 2 story house that's around 2,400 sq ft and has a 80k boiler and they're happy, I'd love to hear from them.

 

[Dana]

I understood you correctly the first time (really!)

What I also read was:

"The floor joist and the attic is not insulated and am planning to do that before winter."

"The band joist will be insulated this winter but probably no way the foundation, it's stone and just don't have the budget for it."

Even an old uninsulated Victorian can be air sealed to well under 0.75ACH without breaking the bank. (Been there, done that.) Air sealing the attic floor prior to insulating is an important prerequisite that most professional insulators understand (they hate crawling around in moldy wet insulation when they screw up.) Air sealing the band joist is huge. The combination lowers the stack effect infiltration dramatically, and makes all the leakage in between less important too. That alone should get it to under 0.75ACH unless you sleep with the windows open.

So, OK, if you're not going to install storm windows or code-min replacement windows before winter, add the 3675 BTU/hr back in, call it
55,800 BTU/hr after your already planned upgrades. (And that's without factoring in any of the flab margin built into that not-so-great load tool.)

1.4 x 55,800 BTU/hr = 78,120 BTU/hr

1.2 x 55,800 BTU/hr =66,960 BTU/hr

That's still KHB-085 / UFT-080 territory. The KBH puts out 80,750 BTU/hr at high fire. The UFT-080 might be marginal at only ~70K out but only during the most extreme Polar Vortex disturbance events- even at non-condensing temperatures it still fully covers the load down to about -17F or so.

That means if it dropped to -20F outside it might hit 67F inside, but so what? How many hours do you really expect it to hit -20F in the next decade or two? Turning on a 1500 watt space heater in the rooms where you need it more than covers the shortfall.

When the budget allows, it's worth adding tight low-E storm windows. It's much cheaper than replacement windows at about the same performance, and will protect the antiques. The Larson low-E storms sold through box stores aren't bad but Harvey's storm windows are the tightest in the biz, and have a low-E glazing option.

 

[Dana]

"I really appreciate your reply but when you find someone on the boards here with an uninsulated victorian 2 story house that's around 2,400 sq ft and has a 80k boiler and they're happy, I'd love to hear from them."

Not exactly the same, but a comparable shaggy dog story...

Here in the real world, when I first moved into my current 2400' 1.5 story 1920s bungalow there was about 6" of sagging rock wool between the joists in the vented attic spaces behind the kneewalls, R7 rock wool batts in the 2x3 kneewall framing, and air would sail through the handy drawers built in to the kneewalls with every gust.

No wall or foundation insulation, single pane windows w/clear glass storms, minimal insulation, huge air leakage, and a house footprint & shape with far more exterior exposed surface area than a typical 2-story Victorian with the same amount of floor space.

The fuel-use based load numbers were in the ~50KBTU/hr @ 0F range. An 80K boiler would have done just fine. My current boiler can deliver more than that, but it never hits that mark. I scrapped the existing 120K cast iron boiler and never looked back, but that was concurrent with my beginning to tighten up the place. I'm a happy guy.

With air sealing and insulation I've gotten the load down to about 38K @ 0F, and at the water temps I'm running I'm radiation limited to about 45K of heat emitted. It sails through -10F temperatures without losing ground except for the overglazed family room addition with the radiant floor, where it sometimes hits about 60F when it's -10F outside. I could crank up the boiler temps to force it to keep up there too, but it doesn't get that cold often enough to matter, though I've consider adding a second stage of panel rads under the big windows to quell the cold convection during colder weather, which would do it too.

Even with all zones calling for heat and the kid is taking an endless shower the boiler never modulates over ~60K out. There isn't priority zoning for the water heater- it's a "reverse indirect" buffering the heating system to allow for micro-zoning without short-cycling the boiler (it's minimum output is about 18K) but some of the heat for domestic hot water is being recovered with a drainwater heat exchanger. Without the heat exchanger it might need an 80K, or set it up for priority zoning using a different water heater.

There are still sections of cathedralized ceiling that aren't insulated, and a few stud bays that aren't completely filled with cellulose due to blocking framing, but it's pretty comfortable overall, and some of those flaws will eventually be corrected.

If you're going to tighten up the place don't bother with anything bigger than 80-85K. If you miss the mark and that's not quite enough to keep up with your first round of tightening it'll still be fine most of the time, and more than cover your average wintertime load.

Taking a stab at the fuel use derived numbers, without knowing the exact meter reading dates and amounts it's hard to get super accurate, 300 CCF/month for December/January doesn't sound crazy high- it sounds a bit low for the house pre-upgrades, which means it's probably already under 0.75 ACH. Using weather data from weather station KPOU in Poughkeepsie for that time period (warmer than Kingston, which will exaggerate the load) there were 2109 heating degree-days.

So assuming 600 CCF and 100KBTU/CCF that's 28,450 BTU/degree-day or (/24=) 1185 BTU per degree hour.

Assuming the old boiler is 80% efficiency (it's probably lower than that, so this too is going to exaggerate it) that would be 0.8 x 1185 BTU/F-hr= 948 BTU /F-hr.

With a presumed 65F balance point that would be 65F heating degrees @ 0F, for an implied heat load of 65F x 948 BTU /F-hr = 61,620 BTU/hr.

@ -10F you'd have 75F heating degrees and an implied load of 75F x 948 BTU /F-hr= 71,100 BTU/hr.

OK, not super accurate, with the known errors mostly to the high side, but still TOTALLY within the range of a KHB-085 or UFT-080, and that's BEFORE sealing & insulating the attic & band joist.

I don't think this one is even close. An antique oversized boiler is probably only 70-75% as-used efficiency and there were a few more heating degree days in Kingston than in Poughkeepsie.

Every way we've looked at it comes in WAY below the output of a 100K boiler, and even below the output of an 80K boiler.

Oversizing isn't your friend. It's not buying you more comfort efficiency, it's only buying you some margin against outdoor temperatures not seen in Kingston since the last ice age. Right sizing means fewer burn cycles, longer run times, more stable room temps.

 

[Leonard Bayard]

I certainly don't want to sound ungrateful and do appreciate everyones responses here, I know that it comes truly from a desire to help.

But I have to say and then I'll leave it at that, I've noticed that through reading most of the responses here that it seems there is an overall lack of understanding and perhaps general knowledge of turndown ratios.

Again, very much appreciate everyones time and opinions.

 

[Dana]

But I have to say and then I'll leave it at that, I've noticed that through reading most of the responses here that it seems there is an overall lack of understanding and perhaps general knowledge of turndown ratios.

I'm not sure how to parse that statement.

Within the industry there is a bias amongst installers to oversize modulating boilers, with the presumption that the modulation will take care of it and it'll find it's balance point. But modulation isn't infinite. Oversizing to the point where it rarely modulates wastes the capability of the equipment leading to sub-optimal performance. It isn't opinion- it's arithmetic. Real hydronic designers do the math.

 

[DIYorBust]

Maybe we can see it both ways. On paper, you can model the heat loss for a building and size the boiler with minimal overhead and this is probably more efficient. Even considering turndown, the bigger boiler is probably less efficient at the lower firing rate, and it's modulation ability is somewhat more limited. However if you're going from a 250 boiler to a 120, is worth the risk to go to 80? If the building is a brand new passive house, it will probably perform in line with specs. But try to keep an old house sealed up, you'll be chasing drafts all winter. What about wind? Will the heat loss be the same on a windy day? What if there's not enough radiators, or the distribution is not perfectly balanced? What if the temperature drops, will it take 1 hour or 12 hours to bring it up to temp? If it takes 12 hours, how much electricity is used to operate the pumps? If you're pumping 50% more frequently, are you saving money? Every minute the the system is on, the fan is running, the pumps are running, and flue losses are occurring. I've seen people tear out these systems at go back to a cast iron because the results just couldn't keep up with the heating load. Another advantage of the 120 boiler is that it will give you faster hot water recovering if you ever decide to use an indirect. If you wanted to do a smaller system, it's probably possible, but there's less room for error. If you try to get an engineer to specify it for you, do you think they'll guarantee the performance on a subzero day? If you ask the installer to guarantee the performance, and upsize the boiler at his expense if it's too small will they do it? Is it worth messing around to save a hundred bucks a year on fuel when you're already making a huge upgrade in efficiency?

Also, keep in mind these things probably last around 10-15 years. So if you insulate everything by then, you can install the 80k on the next round. But you'll probably have to re-pipe everything for a wind powered boiler at that point anyway.

 

[Dana]

I'm not sure where to begin with all of the mis-conceptions/anxieties presented here. (Try line breaks/paragraph breaks- it's easier to read.)

In general mod con boilers are more efficient at lower firing rates than at higher rates, but more efficient when modulating than when cycling on/off.

When all of the available data (including online load tools that usually oversize by 25% or more) is pointing to a 99% heat load well under 70K even when using max-leakage assumptions, there is NO risk to going with an 80K boiler, no matter how ludicrously oversized the original boiler was. Even the fuel use estimate came up with only a 61K, even when using unrealisitcally high boiler efficiency on the ancient beast.

A "typical" 2400' uninsulated house + 1200' basement with only single panes would still only run in ~45-55K range, and a 60K boiler would be the "right" boiler. With a bit of air sealing and some fluff in the attic those houses usually duck under 50K.

Manual-J also includes wind factors, which do increase the infiltration rate.

If the house was kept at 70F last December and January it has enough radiation, but even if there weren't enough radiators a bigger boiler won't fix it. If the room radiation is un-balanced with the room loads, oversizing the boiler isn't going to fix that either.

It won't take 12 hours to recover from a 20F drop in indoor temperature even if the boiler is EXACTLY sized for the load at -10F and it's actually -10F outside. The recovery rate is a function of the entire thermal mass of everything in the house and the amount of radiation, not just the burner size. I was out of power for two full days early this winter (again, in a house of comparable size to yours) and most of the house hit 45-50F indoors, and even at ~45,000 BTU/hr (radiation-limited at the water temps I'm running) the house got up to 70F in less than three hours (I think it was less than two, but I wasn't trying to time it.)

Any modulating system should use only high efficiency ECM drive pumps, since the goal is to be pumping at pretty much a 100% duty cycle during most of the heating season. It doesn't take any more pumping power to deliver the heat from a modulating 150K boiler than an 80K boiler- the pumping rates are the same.

An 80K boiler delivers heat at twice the rate of a typical 50 gallon standalone water heater, and can keep up with a continuous shower. By the time you've dried off and dressed the indirect will have recovered. A 120K boiler only cuts that already short recovery time by 1/3 relative to an 80K boiler, so instead of 6 minutes it's only 4. Do you really care about those 2 minutes?

The flue losses are accounted for in the steady state efficiency of the boiler. A 95% efficiency 250K boiler has the same fractional flue losses as 95% efficiency 50K boiler for any given amount of heat delivered.

I've never seen anybody tear out a right sized properly installed mod-con to go back to a cast iron beast, but you have? If you want a cast iron beast you're still better off sizing it to no more than 1.4x the design heat load, so you're looking at something no bigger than 100K-in for even your exaggerated worst-case pre-insulation upgrades scenario, 90K if you follow through with the upgrades.

The type of air sealing I'm referring to isn't something that would require maintenance or "...chasing drafts all winter..." . When you foam-seal the band joist or properly seal the attic floor prior to insulating they don't regain any appreciable leakage for decades. Sealing the top of the house and the bottom of the house have the greatest effects on your overall infiltration load, since that's what defines the stack effect pressures. The band joist and foundation sill usually adds up to more than all window & door crackage combined.

Most professional engineers specify the equipment to the ASHRAE 1.4x multiplier, and if running a careful Manual-J would/could guarantee the performance on a sub-zero day, but maybe not on a -30F day unless you insisted on that ahead of time.

With high mass radiation like high volume cast iron radiators going with a right sized boiler isn't going to save you 100 bucks a year since even a 120K boiler isn't going to short-cycle, but the smaller boiler is likely to improve comfort and the longevity of the equipment due to fewer (but longer) burn cycles, etc. Installed & sized correctly a stainless steel mod-con should be good for at least 20 years (not 10-15), even though the warranty may only be for 10.

Even using crappy rules-of-thumb sizing the "analysis-challenged" hacks in my area (which has comparable design temps to yours) use for sizing boilers they tend to go with 35 BTU per square foot of conditioned space for uninsulated houses like yours, which would put you in a 100K-in cast iron boiler, not a 120K condensing boiler. The same guys use 25 BTU per foot if it has storm windows and some fluff in the attic, which would would be a 70K-in cast iron boiler.

You took the time to run the math using US Boiler's simplified (and well padded) load tool using conservative rather than aggressive assumptions as if it were the crummiest house in the Hudson Valley (which it probably isn't). So why don't you believe the math?

Even with the errors all skewed to the high side?

The tool-derived 74KBTU/hr load at -10F (the 80F temperature difference) is still only an 80K condensing boiler.

And -10F is 12F colder than your actual 99% design temp.

The first 10-15K of load is pretty cheap and easy to peel off on the first round when weatherizing a house like yours. An 80K boiler is arguably oversized for the load of the "after" picture if you're eventually taking it all the way with wall insulation, foundation insulation, better windows, etc, and a 50K boiler would probably probably more appropriate than an 80K boiler on the "...the next round...".

In 20 years there may indeed be reversible hydronic chillers capable of handling the new-improved lower load using the same radiation for heating using wind (grid) power, but I'm not placing any bets on that just yet. CO2-refrigerant compressors can deliver 140F water pretty efficiently from air coils at -10F outdoor temps, but they require bigger delta-Ts on the water loop than you're likely to get. R410A heat pumps can do it too but at lower efficiency. R410A is probably going away by then but there are other refrigerants, other approaches. Depending on where your "after all upgrades" load comes out you could probably get there with a 3-4 ton ground source heat pump with the existing radiation. Dandelion hasn't released the specs on their hydronic output version (currently under development), but they're marketing their ducted air lineup pretty aggressively in your neighborhood.

 

[Dana]

BTW: Did you measure up the EDR' (square feet Equivalent Direct Radiation) of the old radiators you yarded out, room by room? How are you specifying the sizes of the Hudson Reeds? Any plans for breaking it up into zones? (Zoning by floor usually works best in a 2-story.) You'll be spending a lot more for radiation than you will for the boiler if replacing it all from scratch.

A room by room Manual-J or I=B=R load calculation would be needed to get the sizing & balancing right. For a condensing boiler it's fine to size the rads to be able to deliver the room load with 140F water at the 99% outside design temp, though a lower temp (= bigger radiator) is also fine. It doesn't have to be perfect, but it's important for the rads on a given zone to be in roughly proportional, with the last few on the flow path slightly oversized or at least not undersized for the average load-BTU/EDR' average on the zone.

 

[Leonard Bayard]

BTW: Did you measure up the EDR' (square feet Equivalent Direct Radiation) of the old radiators you yarded out, room by room? How are you specifying the sizes of the Hudson Reeds? Any plans for breaking it up into zones? (Zoning by floor usually works best in a 2-story.) You'll be spending a lot more for radiation than you will for the boiler if replacing it all from scratch.

A room by room Manual-J or I=B=R load calculation would be needed to get the sizing & balancing right. For a condensing boiler it's fine to size the rads to be able to deliver the room load with 140F water at the 99% outside design temp, though a lower temp (= bigger radiator) is also fine. It doesn't have to be perfect, but it's important for the rads on a given zone to be in roughly proportional, with the last few on the flow path slightly oversized or at least not undersized for the average load-BTU/EDR' average on the zone.

That EDR was exactly the one I used to calculate what I needed room by room. And yes, will be spending probably 50% more on the new radiators than the boiler. Will be adding a zone upstairs and the Hudson Reeds of course can't match the old radiators as they are smaller but going to run longer and vertical where space permits. The Hudson Reeds are about 300-550btu short each on the old radiators by the EDR chart, but am hoping with the attic floor insulated and rim joist that will cover the shortfall.

There was a good recommendation earlier for a Viridian taco pump which I'm going to do a bit of looking in to. I really can't overstate how much floor space the old radiators took up but the pipes going into them were 2" to give you some idea.

I will concede the argument to you that a 160k is almost certainly overkill but if you look at Veissman has the same min btu for both the 94K and the 125K. (attached).

I just really have a hard time getting behind a 80k boiler for this house. I had a 1000 sq foot house near Lake Tahoe, similar construction (no insulation) and it doesn't get as cold as here. I had an 80k Lochinvar and the modulation never got down below 38%.

I do take your point however, it's just a few of the comments about barns that have a 32k heat loss just does my head in...

 

[Dana]

So, what BTU/EDR' ratio did you design to?

And the total EDR' for the whole house is...?

The 94K Viessmann with the 5:1 turn down is still overkill, and the 68K Viessmann isn't, and still enough for Polar Vortex events if you commit to a decent round of air sealing. I'm not sure if NYSDRDA still subsidizes blower-door & IR guided air sealing, but on a house like that fixing all the obvious big leaks and following up with a serious round of blower-door/IR air sealing prior to insulating would be a good investment.

It's likely that the comfort issues in your house in Tahoe were fixable with air sealing and spot insulation, and there's no point in trying to figure out why the 80K Lochinvar couldn't hit it's min-fire output- it's more likely to have been a system design problem than a total load problem.

Seriously, a brand new code-min 2400' 2-story house over a 1200' code-insulated (but not actively heated) basement and heat recovery ventilation can easily come in with a load of 32K @ 0F (with MANY existence proofs out there), but with retrofit insulation & air sealing treatment of a 2x4 framed Victorian you'd be lucky to get it much under 40K, maybe 35K if you can get it tight enough. Getting it under 60K is dead-easy to get to, and typically 45- 50K isn't hard.

It might already be there- with better precision on fuel use/HDD data we could say that for sure- the 61.6K @ 0F derived with overly optimistic assumptions on the boiler's efficiency is an upper bound, but we don't know if the owner rounded down or rounded up to the 300 CCF/month numbers, or the exact meter reading dates- the real "before upgrades" load could easily 10K higher or 10K lower than that, but it's prior to the already planned insulation & air sealing upgrades, and it's in the same ballpark as the 65.2K number spit out by the US Boiler load calculator that has a lot of built-in padding, so it's more likely to be under 55K than over 70K, and will be dramatically lower after air sealing. (14.8K of infiltration at a 60F temperature difference is like leaving a couple of windows open.)

Air sealing is a prerequisite to insulating, and the cheapest thermal performance you can buy. Air sealing will avoid common moisture problems later by not letting humid indoor air into the attic to accumulate in the structural wood and insulation over the winter.

 

[DIYorBust]

Hey Dana!

Sorry didn't mean to create a bunch of mis-conceptions and anxieties! I just wanted to share insights from the projects I have done. One article I thought you might find interesting is posted here: http://www.burnhamcommercial.com/technical/High Turndowns/Discussion.pdf

It's an experiment where a 10:1 boiler turndown ratio with continuous operation is compared with a 4:1 cycling operation. The engineers found "contrary to conventional wisdom-there is not necessarily an efficiency advantage to operating with high turndown."(p. 4). Also, "This is due to the longer operating time(60 minutes vs. 24 minutes) at high excess-air levels with hot gases continually exiting the boiler."(p. 3).

I'm not saying turndown is bad, but just that under certain circumstances, and also factoring in flue losses and electrical losses, you may not achieve the efficiency advantages you think you will, and could in fact be less efficient. I think boiler manufacturers don't really want to publicize this in their product literature for obvious reasons, and that is the main source of information dispensed to the trade.

I also don't necessarily agree that you can get 74k out of an 80k condensing boiler when the outside temp is -9 degrees. That' 92% efficient, but you won't be able to run at condensing temps, plus there will be jacket loss to the basement that doesn't necessarily contribute to warming the home. Meanwhile -9 degree air will be running straight through the boiler and out the exhaust. What external air temp is AFUE calculated at?

So now, what's the price difference between an NHB 80 and an NHB 110? Like 200 bucks or something? The 110 can turn down to 10k, whereas the 80 bottoms out at 8k, seems unlikely to make a big difference. Maybe a tiny bit more mass in the boiler? Not significant. Reduced gas demand for piping purposes? But he's replacing a 200k+ boiler. Could maybe be an issue if installing a lot of other new gas appliances in the near future, but even then, really? Bragging rights that you installed the smallest possible boiler and got away with it? Alright fine, that would be pretty cool, but still.