(Yet another) Boiler Sizing Question

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tropostudio

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This is a great resource and a great forum. I'm a couple weeks away from starting into a major renovation on our 1915 bungalow in St. Paul, MN. It's currently a 1-1/2 story and we have opted to top the house and add a full second story. Drawings and spec are done, and we have a contractor on board to do the framing and structural stuff. I'm doing the mechanical, plumbing, electrical, HVAC. Not a stranger to most of it, as I have reno'ed a 1904 church into a studio/workshop.

House is currently hot water heat with cast iron radiators and a 70's vintage gravity boiler system. It's going away, along with the old gas water heater and the chimney. I've done 2 heating and cooling calcs using 2 different Manual J programs for the whole house. Heat loss is 57-59kBtuH for a -10 F outdoor temp and 70 F indoor temp. I assumed an "Average' sealed house. We may get a bit better than Average, but I'm not banking on it. We did fine with a family of 3 using a 40 gallon water heater with a 38kBtuH burner. But 50 gallons might be nice.

I'm looking at installing a WM Ultra 80 or Ultra 105 boiler with an WM Aqua Plus 45 indirect tank for DHW. I'm not locked into that set-up yet, but I figure it's going to be a 93-95% efficient condensing modcon with a 4:1 or 5:1 modulation ratio. If I want to bump up to a 50 gallon indirect DHW tank, should I look at going with a 100 kBtuH boiler, or would I be OK with something in the 75-80 kBtuH range? My thinking is with such a large modulation ratio, the bigger unit wouldn't be oversized like it might for a single stage boiler. FWIW, I'll be using new panel radiators for both floors in the reno, and will probably oversize them to keep water temps down. No in-floor loops going into this job.

Thanks,

Chris Krumm
 

Dana

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It's almost certain that the 2.0 story version of the house will have a lower heat load the previous 1.5 story, due to enhanced air tighness and improved R-values on he upper floor. Since you have a heating history on the place, it's worth sanity-checking the Manual-Js against a fuel use derived heat load analysis.

The boiler should be sized for the space heating load, and the indirect sized for the domestic hot water. Unless you plan to spend literally two continuous hours in the shower when it's in negative double digits outside you'd never lose ground on the space heating. The recovery time on a 45 gallon indirect with a 75-80K boiler (even at 87% efficiency at full fire at a non-condensing temperature= more than 65,000 BTU/hr going into the hot water tank) is literally half that of what you have gotten used to with the 40 gallon 38K, ~80% combustion efficiency standalone (= ~30KBTU/hr of heat going into tht tank.) As long as the indirect can manage the biggest tub you have to fill, there is no real point to upsizing, even if you went with a slightly smaller boiler.

If the Manual-J calculations prove to be true, an 80K modulation boiler is still right-sized per ASHRAE 1.4x maximum oversize factor, but the Ultra-105 would be. If there is more low-hanging fruit on the air sealing or insulation elsewhere, even smaller boilers become attractive options. You may be able to drop to a 50-60,000 BTU/boiler with fairly modest building envelope upgrades, which would be in improvement in both modulation range and comfort. If the foundation is not yet insulated (common for houses of that vintage) the foundation losses could easily be 20% or more of your design heat load. It's worth insulating the foundation, even if it is never destined to become living space.

Newer stainless steel fire-tube heat exchangers are good for 7:1 or even 10:1 or higher modulation ranges, and should be considered for any complete do-over of the heating system, especially if you are breaking it up into zones, and using lower thermal mass radiation and plumbing. The napkin-math reasons are laid out in this bit o' bloggery. The minimum firing rate of the Ultra-80 is 16K-in (~15K out), whereas the min-fire output of a NTI Tx81 is 10.25K (the TX51 can drop 7.12K, and up to 57K at max fire, which is almost enough per your Manual-J), and the HTP UFT-80W can drop to 80K. There are others. Stainless heat exchangers are also less sensitive to system water chemistry than the aluminum heat exchangers in the WM Ultra series (not that you can't avoid those issues with proper maintenance.)
 

tropostudio

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Thanks for the reply, Dana. The link to the Green Advisor blog is great. One of the Manual J calcs I did is for the entire house. The other is for each level - basement, existing 1'st floor, new 2nd floor. I was able to accurately specify wall, floor, ceiling systems and insulation levels in both programs. Door and window types (with specific U and SGHC values) , overhangs, etc. were also accurately specified.

The basement does have 2x4 R13 energy walls on the interior and 1.5" EPS board from grade to footing, which was done 20+ years ago when I hand dug around the foundation to tuckpoint, waterproof, and install a drain tile system. I may have the 'energy' to pull out the firestops and clear the rubble fill at the rim and spray foam that - we'll see. Our basement is very usable all winter with the existing gravity system, because of all the big black iron pipes and an inefficient boiler and DHW heater. It will get new double pane, low E sliders and 1 egress window. The manual J calcs give me roughly 14.5kBtuH loss at the basement (4.5k infiltration, 10k surfaces and glazing). I plan to install radiators down there, with the new system going in.

The 1'st floor has blown cellulose which we did 25 years ago. Cavities are tight - I've seen that when I had to open a couple bays. Windows are double pane inserts I installed years ago, with full storms. We will keep the windows and install new low-E storms. Not much more to do on 1'st floor other than be sure everything is caulked well when we re-side and re-paint interior. Calculated losses here are 17.5k (3.5k infiltration, 14k surfaces).

New 2'nd floor will be R21 stud walls with R50 at trusses, and foamed energy heels with a light grey metal roof. Marvin Low E Ultrex/wood windows. Calculated loss is 18.3 k (4 k infiltration, 14.3 k surfaces).

Add in 8k ventilation losses, and I ended up with ~58kBtuH total heat loss for a -10F exterior and 70F interior. Actually, we keep it around 68F when we are around, and down to 62F at night or when we are gone. 58K x 1.2 oversizing puts me at 69k, which is right where most of the 80k modcon DOE values sit. We are creating a very open plan in the reno, blowing out most of the interior walls on the main floor and installing a central, open staircase from basemnt to 2'nd level. Basement will have a door to isolate it, but 1'st and 2'nd level will be pretty contiguous from an air movement standpoint. I'm not sure how applicable an fuel-use derived analysis will be with all the changes we will be making, but I'll certainly check it out. I don't plan to get into Manual J calcs by room. It seems pretty obvious how to size radiators as a percentage of floor space for bedrooms with doors.

The 10:1 modulating boilers are interesting. HTP Elite 5:1 are readily available for DIY, and comparable in cost to the WM Ultra80. I don't see the HTP Leite Premier with 10:1 turndown being readily available. My guess is it'll cost an extra $1000 if I can find one. As a really new product, I wonder if it'll ever pay itself back and what it will cost to maintain?

Since the basement can be isolated from the other 2 levels, I plan to make that one zone with a wall-mounted thermostat. It is basically a single craft/hobby space. We may build it out in the future, Probably not. Thinking is another zone for 1'st level, with a wall mount thermostat and a thermostatic valve only on the radiator in the one bedroom/study that would allow turning the room temp down in there if desired. Second floor would be a 3rd zone, again with thermostatic valves at the 2 bedrooms to allow individuals to cool their sleeping space more than the wall mounted thermostat. With the 1'st level and 2'nd level so contiguous, would it make sense to just put them on one zone and then install thermostatic turn-down valves for the bedrooms? DHW via the indirect tank would be its own zone, of course.
 

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The fuel use analysis would put a firm upper bound on the heat load. The load of new-improved house is clearly going to be lower.

Even with convective air flows between levels it's almost always to zone the first and second floors separately.

The 8K in ventilation losses can be cut substantially using heat recovery ventilation.

Your current foundation insulation (R6 on the outside, 2x4/R13 on the interior) wouldn't meet current MN code, which is fairly particular about having most of the R on the exterior of the foundation. But if the foundation hasn't frost heaved in the past 20 years, it's a good indication that it won't. From a total R point of view the performance meets IRC 2015 levels (see Table N1102.1.2, about half way down the page. MN is zone 6 in the south, zone 7 in the north.)

Mind you, ~R6 on the exerior isn't sufficient wintertime dew point control for R13 cavity fill in your climate, which may not be an issue if the exterior foam stops at grade(?) and the above grade foundation can readily dry toward the exterior? The performance of the 2x4 wall alone for the above grade section without exteiror foam makes it lossier- you may be able to do something on the exterior with highly vapor permeable rock wool, but it may or may not be "worth it" depending on the amount of exposure. You might be able to improve it with sufficient dew point control using 2-2.5" of EPS protected by a cementicious EIFS, again, may not be worth it.

If there is current fiber insulation at the rim joists and it's not moldy, there's not whole lot to be gained by spray foaming it unless it's demonstrably leaking air there under blower door testing.

A 2x6/R21 wall 16" o.c. isn't a very high performance wall, and wouldn't quite make IRC code min for your climate zone, even if it meets current MN code. The "whole-wall-R" after factoring in the thermal bridging of the framing, will come in at about R16, which is roughly what you have for basement insulation. The IRC 2015 prescribes 2x6/R20 + R5 continuous, or 2x4/R13 + R10 performance, either of which comes in at about R20 -R21 after thermal bridging, a U-factor in the U0.045-0.050 range, compared to U0.60 or higher for a basic 2x6/R21 wall.

From a total resilience point of view, the 2x4/R13 + R10 is far better, since it has sufficient exterior R for dew point control at the sheathing to be able to use standard latex paint as the interior vapor retarder See TABLE 702.7.2 CLASS III VAPOR RETARDERS (about 1/4 of the way down the page), and it's not substantially thicker than a 2x6/R21 wall. If you go that route, 2" of foil faced polyiso is probably the easiest, even though the polyiso would have to be derated for temperature in this application for your climate. If instead you went with 2.5" of EPS it would be a half-inch thicker, but would outperform it's labeled R value, giving you greater dew point margin. Using 2" XPS would get you there from a labeled point of view, but it's performance falls to that of EPS over a few decades, and would have to be 2.5" to be good enough over the long haul.

The HTP UFT series have 10:1 turn down, are tolerant of high delta-Ts and have fairly low pumping head, which means they can (almost) always be pumped direct rather than primary/secondary, making them pretty easy for a DIY, and they're comparatively cheap. The Westinghouse WBRUNG-080W the exact same boiler as the UFT-080W, available online for about $1.7K. The UFT-080W could be priced higher or lower than that through local distribution, or direct from HTP.

Fellow Minnesotan Morgan Audetat (posts here as BadgerBoilerMN) is either installing soon, or has recently installed a UFT series boiler in one of his own houses. You might ping him for details.
 

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Sizing a Modulating Condensing Boiler and Out With The Old, In With The New. Two invaluable articles for my upcoming hydronic home heating project. I'll be doing a fuel use history check this weekend to put a cap on my max heat loss, as a check against my Manual J calculations. With the info from these articles I'll be confident both in my boiler and radiator sizing. Thanks, Dana!

Perhaps I should have title this thread '(Yet Another) System Sizing Question.' I'm hijacking my own thread, but now I have to ask about zoning and system set-up options. After going through the wants list for temperature control with my partner, we've arrived at this scenario:
  • Basement (~800 sq ft) is one zone: a single hobby/work room typically kept 3-5 degrees cooler *say 62-65 F) than the rest of the house even when we are using it. Turned back another 5 degrees at night.
  • 1'st floor (~900 sq ft) is comprised of 2 zones: one zone is an open-plan living/dining/kitchen space (~720 sq ft). This area will also have a sealed combustion NG insert installed into an existing masonry fireplace. The second zone is a guest room and 3/4 bath (~180 sq ft) at the NW corner of the 1'st level, separated by doors from the open-plan area. These areas will typically be kept at 68-70 F when occupied, and turned down 5 -8 F when unoccupied or at night.
  • 2'nd floor addition (~950 sq ft) is one zone: an open family room + small side study + large central stair opening (~420 sq ft); full bath with door (~90 sq ft); master bedroom with door (~270 sq ft., no bathroom); another bedroom with door (~170 sq ft.). These areas will typically be kept at 68-70 F when occupied, and turned down 5 -8 F when unoccupied or at night.
  • The plan is to remove all the existing cast iron radiators on the main floor, and to install Runtal panel and baseboard units at the 1'st and 2'nd floors, and less expensive panel radiators (Ecostyle or Pensotti) in the basement. The cast iron has high thermal mass, but they take up too much floor space.
Does this sound reasonable? I'm certain a careful reading of Dana's articles will have me right-sizing radiators to have the system working largely in condensing mode. I had an initial concern with the small zone for the guest room/bath on the 1'st level, but it sound like a properly sized modcon with 10:1 (or even 5:1) turndown, outdoor rest, and correct radiator sizing should prevent short cycling and running outside condensing mode too often.

Any suggestions re direct pumping vs primary/secondary? Zone valves vs. circulators? I plan to pressure drops in all circuits - I can do the math. I'm also a fan of keeping complicated systems simple (or at least forgiving). How's that for an oxymoron?

I make interactive exhibits for museums , and have built water exhibits with variable speed pumps and automated chemtrol systems. I used to control systems with PLCs, but more typically now control them with an Arduino and hand-spun code. But in the interest of initial cost savings and user-friendliness, I'm as apt to pick a single speed pump at peak efficiency pumping to a header tank with overflow than to using pressure transducers and variable speed pumps. Bells and whistles are cool, but stupid-simple and obvious can really help in the long run.
 

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Direct pumping would work for most systems, but you have to be careful about checking the minimum flow numbers and delta-Ts with just one zone calling for heat to make sure the boiler stays happy. These details are best left up to experienced designers with the software tools in front of them, even if the implementation ends up being a DIY.

Some of the fire tube boilers with big turn down ratios have fairly low pumping head and are, more tolerant of high delta-Ts than most other types of mod-con heat exchanger designs, making them a bit harder to abuse when pumped direct rather than some others. Morgan is a fan of designing the system to always be pumped direct, even with manufacturers that insist on primary/secondary with their boilers. But specifying the pump correctly isn't always super simple. Like some others, the UFT series fire tube boilers are super tolerant of pumping direct, and they've even turned that a marketing feature "No Primary/Secondary Piping Needed" ( "Look ma- I can drive with just one pump!" :) ) For a DIYer design it doesn't come much better than that, and if you're insisting on doing the whole design yourself, that's probably your best bet.

Rather than breaking up the first floor into a big zone and a smaller one it probably makes more sense to run it as a single zone, using thermostatic radiator valves for tweaking sleeping temp setbacks. If you dial in the outdoor reset curve to perfection you won't gain any efficiency by setbacks, the right sized boiler would run almost continuously at the maximum efficiency achievable with the radiation it has to work with.

Single speed pumps don't make a lot of sense with modulating boilers, since the optimal flow varies with the water temperature and heat load. With continuously variable ECM drive it can save quite a bit on power use if it's set up right. A couple hundred or so for a single smart pump vs how much for a header tank to optimize dumb single-speed performance? The pressure drop (and pumping power required) changes pretty dramatically with changes in flow, and if the system is set up to modulate the firing rate with load, a fixed speed pump is sub-optimal.

You can probably save more than the cost of the consult if you gave Morgan a call, emailed him your room-by-room load numbers along with your preferred radiation type and just let him specify it all. There is a strong penchant amongst newbies (particularly enginerds like you and me :) ) to over-design stuff, and it ends up being more expensive, and not always as efficient.

BTW: Myson has some pretty cheap flat panel radiators that might save some beer-money on the basement zone where you don't care as much.
 

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Thanks for the info, Dana. And the tip to contact Morgan.

My thinking behind two zones for the 1'st level was more that with the large open plan room and a gas fireplace, the NW guest room/study would be cold if the door were closed with the fireplace running and only one zone. A thermostatic valve on the radiator in the guest room wouldn't help if there is no call for heat, right? If I mount the 1'st level thermostat in that guest room, with the door closed and the fireplace running, there likely WOULD be a call for heat with the fireplace running and the big room would overheat. Or maybe not, because the fireplace thermostat would just ramp the flame down. But then someone won't be happy the flames disappeared ;)

I wasn't actually advocating for big single speed pump and open header tank in the house. Was more a response to pictures I've see of some systems with umpteen circulation pumps in a home-run manifold set-ups that look like an oil refinery. BTW, I'm more than likely to use a variable speed pump with a flow or pressure transducer in water exhibits now. I must say though, that single speed pump and PVC header manifold with overflow I built for my first big water exhibit - a 96' long model canal system back in 2004 - has worked without a hitch since first installed. That's a good thing, because they aren't performing routine maintenance on stuff like the pH, O3, and ORP sensors that really NEED to be watched!

Oh, I am looking at installing an HRV system. My guess is it would probably drop my Manual J ventilation loss from 8k to 4k, with extra cost for electricity. Still don't think I'll get down to a 50k boiler. Thanks again.
 
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