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Community and Q&A

Boiler upgrade recommendation?

jpavia1 | Posted in Mechanicals on

I’m currently looking to upgrade my old boiler as it developed a small crack and is leaking. My current boiler is a P-468U-wt (151,000 btu/hr) Weil-McClain oil-fired boiler with a 40-gallon hot water holding tank.

I’m getting two different suggestions from two of my plumbers and would love some input on their recommendations. One recommends replacing it with an Energy Kinetics System 2000 and the other one is suggesting a Navien Combi-boiler.

I do have propane piped into my house so that wouldn’t be an extra cost. I also like the fact that the Navien is sealed combustion as my house is pretty tight (3ACH50).

Also, I believe my current boiler is oversized and am planning on getting a heat-loss calculation done but would like to know if anybody can recommend software that would enable me to do a basic load calculation. I’ve listed details about my house below but if I’m missing any pertinent information, please let me know and I’ll be happy to post it. Any input would be greatly appreciated. Thanks!

Location: Norwalk, CT (Zone 5)
Ranch style house

Currently there are two heating zones:
100ft of baseboard on main floor and 30ft of baseboard in the basement.

MAIN FLOOR – 1325sqft
Windows (double pane):
North – 44sqft
South – 32sqft
East – 90

East – 21sqft
South – 21sqft

BASEMENT – 748sqft:
Windows (double pane)
North – 8sqft
South – 8sqft
West – 16sqft

r13 fiberglass in walls on main floor
r38 open cell on attic roof
r10 Closed cell foam on walls and rim joist in basement
r28 Open cell foam in garage ceiling (750sqft of the main floor).

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  1. GBA Editor
    Martin Holladay | | #1

    I wouldn't trust your plumbers. If they are suggesting new boilers without first performing a heat-loss calculation, they are incompetent.

    Get the heat loss calculation done. There are several free online programs, but I hesitate to recommend any because there is quite a bit of judgment required when entering inputs. Most methods result in oversized boilers and furnaces, for reasons explained in this series of blogs: How to Perform a Heat-Loss Calculation.

    Your existing 151,000 Btu/h boiler is grossly oversized. It wouldn't surprise me if your home has a design heat loss of 30,000 Btu/h or less. Dana Dorsett is likely to come along shortly and provide more detailed advice.

  2. user-2890856 | | #2

    Joe ,

    throw any plumber or HVAC technician that comes into the house and makes any suggestion without first performing a room by room heat loss calc right out the door , he is hot your guy .

    As far as the first 2 recommendations . System 2000 from Energy kinetics is an oil boiler that is customizable as to size , has thermal purge technology to lessen stack losses and uses that energy toward DHW production , it also is supposed to draw it's makeup air from the outdoors , although many don't do this piping . It is quite probably the best oil fired system available , the Marran family was really thinking about the end goal when this unit was designed as part of a system that can be adaptable . Oil and gas are usually similarly priced but Lp or propane only contains about 93K worth of calorin value while oil contains about 140k +- .

    there is very little wasted energy with the system 2000 where as I cannot say the samre about the Navien stuff as it will likely short cycle if you have more than 2 zones and has not yet proven to be reliable .
    The other factor would be your installed baseboard . The installed baseboard will allow one to know whether or not ther exists an opportunity for lowered water temps . Without lower water temp capability the High fallutin mod con equipment may give you no more than 90% AFUE . They get their efficiency from lower return water temps of less than 130* which is the condensing point of Ng / LP . Oil on the other hand has a condensing point of 140* and does not rely on condensing to reach it's efficiency. The only concern there is boiler protection and returning higher temp water than 140* . Your baseboard may very well not play nice with a mod con and you will have spent too much money on a boiler that is not capable of giving you the AFUE number you purchased it for .
    Martin is quite right in what he stated about what your needs may possibly be in terms of BTUh . this is something you must know to even begin this process and hope for success .

  3. Tim C | | #3

    Richard - you've got your condensing points backwards. Oil flue gas has a dew point about 15f lower than natural gas. That means you can get a bit more energy out of the flue gas without unintentionally straying into condensing territory (which is why they system 2000 can hit 87% rather than stopping at 85%). The main reason condensing oil boilers aren't common is because oil doesn't burn as clean, so the condensing accelerates soot buildup (and the condensate is even more acidic).

    Joe - if I had to pick between atmospheric vented oil and sealed combustion propane, I'd pick the propane every time, although it could cost 20% more to run. Although at New England fuel costs, I'd want to use a heat pump for as much heating as possible.

  4. Expert Member
    Dana Dorsett | | #4

    If the windows are clear glass rather than low-E your heat load could come in as high as 15BTU/ft^2 x 1325' = ~21,000 BTU/hr @ Norwalks' +9F 99% outside design temp, maybe 28K if the basement has NO insulation and leaks as much air as a flyswatter. With the foundation insulation, if all the windows are low-E, and it's fairly air tight you could be looking at ~16,000 BTU/hr or less.

    EVERY oil fired boiler is at least 2x oversized for your actual loads as are 99% of the propane-burners out there, and you may be able to heat the place with a single modulating 1.5 ton mini-ducted mini-split mounted in the basement ceiling above where boiler once stood for less upfront money than a boiler swap, and an operating cost about half that of heating with oil. (And you'd get air conditioning with that.) If it's a fairly open floor plan you might be able to do it with 1-2 wall-coil type mini-splits. Dropping an HTP Versa propane burner or a System 2K oil burner in there would make it without short-cycling it into low efficiency, but it's almost never the "right" solution to pipsqueak loads like yours.

    If you have an oil use history it's dead-easy to come up with an upper-bound on the true heat load, using the old boiler as the measuring instrument. If you're on a regular fill-up service that stamps a "K-factor" on the bill, find a mid to late winter bill and tell us the K-factor. Otherwise, a couple of fill-up quantities with the exact dates between fill-ups would be just as good. And, if you used overnight setbacks, what were the thermostat settings & times?

    Is the basement finished and fully conditioned?

    What is the foundation type, and how much of it is above grade?

    If uninsulated the basement could be literally half your heat loss, even if you turn that zone off. With an oil boiler behind it the 30' stick o' baseboard could easily dump 20,000 BTU/hr into that zone if it needs to, but your actual basement losses would be under 5000 BTU/hr if insulated. And it would cost less than the difference between a ducted mini-split and a replacement boiler to insulate the basement if it's DIY and on the cheap. (We could show you how to do that without creating moisture problems if it comes to that.)

  5. Expert Member
    Dana Dorsett | | #5

    In-re condensing temps: Tim has it right- the stack temps can be run lower for non-condensing oil than for non-condensing gas. Richard's 140F is correct for minimum return water temps for limiting both flue & heat exchanger condensation for oil burners, important to pay attention to since the condensate is so corrosive.

    I've seen houses in MA 2x this size at comparable insulation levels heated with as few as 3 ductless mini-split heads, at outside design temps lower than Norwalk's. A System 2000 or a condensing propane burner solution would be north of $10K, installed. If there's easy room for running ducts in the basement a 1.5 ton Fujitsu would be maybe $6-7K. An office mate of mine just installed 3 separate mini-splits in his ~1700'-1800' house in suburban Boston last week for about $12K (before taking state rebate money), mothballing his ridiculously oversized oil-burner, which may come out after he's gone through a few winters and has gained some trust in low-temp capacity and the heating distribution in the doored-off spaces.

    Hydronic heating is nice, but it's not everything, and it doesn't air condition. The boiler fuel options here are expensive, and not likely to come down any time soon.

  6. user-2890856 | | #6

    Joe ,
    Do you also depend on oil for your hot water or do you utilize 2 appliances ? You could gain about 10% in AFUE by using a modulating condensing propane boiler , HTP also has mod cons that will fire down as low as 12,000 BTUh . They also manufacture the Pioneer boiler that has 55 gallons of mass which enables one to program the units parameters to insure that it does not short cycle . The Versa Flame can also produce DHW through a FPHX at reasonable rates for a 2 bath home with low flow shower heads . You can achieve this without redesigning your home's entire heating system . The System 2000 does come with a hefty price tag and many times places itself outside most folks budget . HTp also has a new unit ready for release (after Beta) that does not require P/S piping and has a 10:1 turndown , link is below . The HTP stuff mentioned could quite possibly be installed at right around the number Dana mentioned dependent upon the installers knowledge and integrity .

    Tim ,
    If you are returning water to your oil boilers of the regular garden variety at less than 140* you should learn about boiler protection devices , unless of course you are using very expensive oil equipment . That is for another discussion however . I have yet to see an oil fired appliance that has has an approved venting material other than metal . I wonder why that is ?

  7. Expert Member
    Dana Dorsett | | #7

    Richard: A mod con that only fires down to ~8k (like the UFT-80W ) won't modulate much in a house with a probable heat load of about 16K.

    Even it it doesn't short-cycle on the radiation, so what? Propane is ridiculously expensive in CT compared to air source heat pump technologies that don't demand as much system design or installation expertise.

    The Fujitsu -18RLFCD mini-duct unit can modulate down to about 3000 BTU/hr out, which would be just fine for a house with a 15-20K design load. Air velocites would be so low that you wouldn't hear it over the refrigerator except on design day. It's not as cushy as radiant floors or panel radiators, but I'd put it a healthy notch above fin-tube baseboard on the comfort scale. The biggest problem in Norwalk may be finding somebody to install it.

    Let's see where the real heat load numbers lie (using the fuel use against heating degree-day numbers as the sanity-checker) before pushing too hard on any given solution. If the load at +9F isn't already under 20K, it certainly CAN be taken that low, without breaking the bank.

  8. user-2890856 | | #8

    Not much system design or expertise required here as there is already baseboard radiation installed , this does not require primary secondary . More gain can be had by varying fluid velocities to match load at all times if the proper survey is done of radiation to heat loss room by room .
    While fuel usage is used by some I don't agree with it unless the system has been logged . I'll explain , for others . If you used 100 gallons of oil but the unit because of any number of factors only delivered 65% to the house you still run the risk of oversizing , No ? Too much of a guessing game for my liking .
    If one goes with propane he must also be able to accept bulk delivery to hedge against mid winter shenanigans also .
    Besides all that I was just attempting to answer his original question . Much too busy to be a salesman for electric equipment manufacturers .
    . His installed baseboard taking away the magical 15% HEF is capable of delivering 64,090 BTUh , I'd say there is quite an opportunity to utilize 130* SWT at design and achieve >mid 90% efficiency throughout the entire operating seasons , shoulder through design . that is not bad numbers for garbage baseboard like Slant Fin . Those numbers were based on Fine line # 30 and the flow required would be somewhere around 1 gpm which could be done using about 75 Kwh based on a 3000 hour on time using a single <200.00 circ which maintains 20* fluid Delta .
    But , as you stated , finding someone qualified to even install the basics is getting difficult .

  9. jpavia1 | | #9

    First off, thanks for all of your replies.

    Dana, The windows are Low-E and the basement foundation is block. It is finished, conditioned and insulated with 2” of closed cell spray foam. I almost never need to turn on the basement zone even on the coldest day. Also, the house including the basement is pretty tight (3ACH50). I don’t see the K-factor on the bill but I’m sure I can get that from the oil company. When I do I’ll post it. If you think going with a heat pump is a good option, I do have central AC serving the main floor but not the basement. It’s a Trane XL14i (2.5 Tons – 14S.E.E.R.) condensing unit with a two-stage air handler located in the conditioned attic. Is it worth looking into switching that over?

    Richard, yes, I do depend on oil for my hot water.

    So far I have a cost of $9500 for the system 2000 installed. I don’t have the installed cost for the Navien Combi yet but I know I can get the unit for $2K after a $700 rebate. I’m sure I can get it installed for half the price of the system 2000. As everybody stated, my biggest hurdle is getting a qualified person to install whatever system I end up going with so I’d like to keep it simple.

  10. user-2890856 | | #10

    You must be careful with combi boilers , they are not all the same . Many have a 3 way diverting valve that switches the unit from Central heating to Domestic hot water , these valves are notorious for having issues and most are buried deep inside the unit . You also want to try to avoid the remaining units that do not have an internal smallish buffer for the hot water , these small buffers eliminate the cold water sandwich that many tankless and combis suffer . HTP's EFTC140-F is a very nice unit that does not require that diverting valve and has an 11 gallon buffer , does more hot water with less BTU input and is manufactured in Mass , if that makes a difference . Cost after rebate of 700.00 that you mention would be about 300.00 more than the Navien but does not have nearly as many potential problems . Parts would not be an issue as HTP is based in Mass .
    I would also make sure that the installed price includes new ECM circulators and near boiler piping so you don't suffer issues later . I can easily send you piping arrangement diagrams or even spec it for you (n/c) . You can have potential contractors bid that spec to avoid confusion . There is an offer not often seen .
    Remember that a boiler (combi) is only one part of a system and as such is not magic , the system should be addressed as a whole .
    Can you post pictures of the existing boiler and near boiler piping ?

  11. gusfhb | | #11

    the only fuel more expensive than oil is propane.

    that said, the boiler may be so much cheaper that it would be years before you pay the difference

    I have a Buderus gb125be condensing oil boiler and am happy with it.[5 years]

    Cleaning requires a trained tech however

    I would suggest you do your research and tell the installer what they will be installing and get a price

    All of the quotes I attempted to get wanted to count feet of baseboard[sigh]

  12. Dana1 | | #12

    The min-fire output of a Navien Combi is about 17K- could be higher than your design heat load. That's also over 500 BTU/hr per foot of baseboard on your smaller zone, which emits less than 200 BTU/hr at condensing temps. The 100' zone would be fine at condensing temps from a short-cycling point of view, but it would cycle, not modulate.

    A quick & dirty I=B=R heat load calc looks something like this. We'll assume the following U-factors, some of which may be higher than reality:

    Low-E windows: U-0.34
    2x4 walls: U0.10
    Basement walls w/2" cc foam: U 0.07
    Doors: U0.50
    Ceiling: 0.03

    You don't mention the gross wall height or wall lengths, let's call it 10' high, and a simple 53 x 25' rectangle for the first floor, a 156' perimeter, for 1560' of gross wall area, and 30 x 25' for the basement (110' perimeter) with 4' of the foundation being above grade for 440' of relevant gross wall area.

    Your 99% outside design temp is +9F, let's call your indoor design temp 69F, for a 60F delta-T.

    So on the first floor you have:

    Ceiling losses:

    U0.03 x 1325' x 60F= 2385 BTU/hr

    Window area adds up to 166' for window losses of:

    U0.34 x 166' x 60F= 3386 BTU/hr

    You have 42' of U0.5 door for door losses of :

    U0.5 x 42' x 60F= 1260 BTU/hr

    Gross wall area is 1560', less 166' of window and 42' of door comes to 1352' of wall, for wall losses of:

    U0.1 x 1352 x 60F= 8112 BTU/hr.

    Add up the first floor losses and you're at 15,143 BTU/hr.

    Basement losses:

    32' of U0.34 window comes to:

    U0.34 x 32' x 60F= 653 BTU/hr.

    440' of gross wall area less 32' of window is 408' of U0.07 wall comes to:

    U0.07 x 408' x 60F= 1714 BTU/hr

    Total basement losses: 2367 BTU/hr.

    Total losses including both floors: 17,510 BTU/hr.

    This doesn't factor in the floor losses to the garage or the floor losses, or infiltration losses, but it also over-estimates (by a lot) the basement wall losses on garage side. (With the garage door closed it's going to be WAY above +9F when it's that cold outside.) It also completely ignores the constant electric plug loads and the output of human bodies. Those factors make a difference, but are relative unknowns, but 17.5K is the right ball park, and may an overestimate. Reality could be as high as 20K, or as low as 14K, depending on the particulars.

    If you build a spreadsheet using the real dimensions we can fine tune it for you, but the fuel use against heating degree-days will probably be more accurate. Your wall U-factors could be U0.08, not 0.1, and your windows could all be U0.28, who knows? But any Manual-J calculation that comes in over 20K needs to be scrutinized for errors. (Default values for infiltration is a classic gross over-estimate of reality.) If you have your oil bills, give the fill up dates and quantities for all deliveries after October.

    A 2.5 ton air conditioner is overkill for a 1325' house in CT, which probably comes in at between 1-1.5 tons of cooling load, but there's no point in sketching out that calc too.

    At 92,000 BTU/gallon, a propane boiler at 95% efficiency delivers (92K x 0.95=) 87,400 BTU of heat to the house per gallon or 11.44 gallons/MMBTU. At $3/gallon that's $34/MMBTU.

    A better-class mini-split heat pump sized correctly for your heating load will deliver an average COP of about 3, which means it delivers (3 x 3412 BTU/kwh=) 10,236 BTU/kwh. That's 98 kwh/MMBTU. At 20 cents/kwh thats $19.50/MMBTU. At 25 cents (some people paid that much this winter) that's $24.50/MMBTU, still a big discount from $3 propane. While it's sometimes possible to buy propane in this area at $2/gallon over the summer, the recent 5 year average in CT has been over $3/gallon.

    Depending on your exact air handler equipment you may be able to retrofit a heat pump unit into that Trane cooling system. Odds are you won't, and even if you can, the seasonal COP will likely be about 2-2.5, and it may not have 17K of capacity at +9F. It still may be worth investigating that once we've closed in on the actual loads more accurately.

  13. Expert Member
    Dana Dorsett | | #13

    For sanity-checking on propane & electricity price averages in CT, see:

    Look like electricity has bumped up to about 22 cents in CT, so assuming you get a decent min-split the operational cost will be about $21.50/MMBTU

    Propane pricing has dropped to about $2.80 (for now), so with a condensing boiler you'd be at about $32/MMBTU, 50% higher than heating with mini-splits.

    Looking out over the lifespan of the equipment, propane & oil are diminishing resource commodities with high price volatility, but with nowhere to go but up as the easy oil & gas gets depleted. Only the high price of oil & propane can finance continued development in the shale fracking biz. The price of oil & propane is ultimately set by the rate of increase in driving in the developing economies (primarily China & India) relative to the cost of extracting oil at a rate that can match demand. The tight-shale oil & propane drilling rates have slacked off considerably with the fall in oil prices. If oil prices don't rise soon, propane prices will, since half the propane in the US comes from natural gas production, which is currently experiencing a glut due to the rapid development of shale fields. It takes about 3 years on average to deplete a shale well, so as drilling falls off, the glut will dry up.

    Electricity, on the other hand is set up for future downward price pressure, as more zero-marginal-cost renewable come on line. The lifecycle cost of new fossil burner generators and new wind & solar are comparable now, but unlike fossil burners, wind & solar are manufactured goods, which have a substantial "learning curve" where the price falls by some significant percentage every time the production volumes double. Now that they have approached (and in some cases beaten) "grid parity" on cost, the ramp up in production volumes will take the production doubling time down from the current 22-24 months for solar, down to sub-year intervals well within the lifespan of a heat pump or boiler. The recent price spikes in electricity have been related to over-reliance on natural gas for peak power in New England with insufficient storage & pipeline infrastructure that led to 5000% (that's not a typo!) spikes in the price of gas to some power generators during wintertime peaks last year, which is a temporary condition.

    Between the gas infrastructure improvements and the impending fast ramp up of renewables, there will be deflationary price pressures on New England electricity beginning in under a decade. The energy portion of the bill with some of the larger New England utilities right now is significantly higher than what I'm paying on a 3 year contract from 100% wind-power retail suppliers, all due to one bad winter week of wholesale natural gas prices to power generators in 2014. Those Polar Vortex price spikes did not recur in 2015, and prices should be moderating soon, as gas fired power generators recover from the financial shock, even before new wind & solar cut very much into peak grid demand from fossil burners.

    Bottom line: A good heat pump is currently 1/4-1/3 cheaper to run than best-in-class propane or oil right now in CT, and the spread in operating cost is likely to grow in the next 10-20 years. And as the ISO-NE grid greens-up , so will your heating solution.

    It can be fun (for energy nerds only) to track the 5-minute grid contributions & spot pricing etc on the ISO-NE grid operator site:

    The renewables portion indicated only tracks utility-scale power that is monitored & paid for, which is only about half to 2/3 the PV power going onto the grid. All the small scale production on the customer's side of meter only shows up as reduced grid load, not as solar production. it's a tiny sliver right now, but it won't be in July 2025.

  14. jpavia1 | | #14

    Richard, Thanks for the nice offer. The HTP's EFTC140-F sound like a nice unit. I’ll definitely look into it more. I’ve attached a picture of my existing boiler. It’s a small room so this is as far back as I can get for the picture.

    Dana, Lots of great info. Thanks! A mini-split heat pump sounds like a great option but my house isn’t an open floor plan. It’s a four bedroom ranch. I already have my current AC ducted to all of the rooms on the main floor and can probably get to the basement through a closet. How about retrofitting a heat pump unit into my current Trane cooling system? The model # of my air handler is 4TEE3F31A1000AA. I’m missing some of my oil history so I’m going to call my oil company on Monday and get the info to you then.

  15. Expert Member
    Dana Dorsett | | #15

    I was recommending a mini-duct mini-split (specifically the Fujitsu 18RLFCD ) to deal with heat distribution issues, provided that your true heat load is under 20,000 BTU/hr, as anticipated. The thing has a rated HSPF of 11.4 (=seasonal average COP of 3.34) in zone 4 climate. Norwalk is at the warm edge of zone 5, so you'll beat an average COP of 3 in your climate, but not by a lot, which is why I sketched out the operating cost based on no more than 3.

    With a pair of 3/4 ton Mitsubishi FH09NA or Fujitsu 9RLS3 wall-blob units you'd be averaging a COP north of 3.5. but you'd need an open floor plan to make those work. The mini-ducted Fujitsu is the only model even close to working well in your climate on both capacity & efficiency. (It has a rated output down to -5F.) I don't have access to he extended temperature capacity tables for the 18RLFCD, but it's really pretty good. A Fujitsu installer should be able to tell you what it puts out at +10F, +5F, 0F, and -5F. I don't think it's good for the full "rated" 21.6K output @ -5F (probably isn't) but it's good for that much at +17F, if that's the number they use in the HSPF submittal, and should be good for at least 17K @ +9F, your 99% outside design temp.

    BTW: The reason why you almost never have to run the basement zone is that the standby losses on the boiler is higher than the load except during the coldest hours of the year. (I'll bet the boiler room is the warmest spot in the house, most of the time.) When the boiler is mothballed or pulled out, that heat source will go away. Be sure to calculate the real load for the basement, and give it the proper amount of air flow if going to a heat pump.

    Also, an electric hot water heater is cheap to install, but it may still be worth installing a heat pump water heater, depending on your actual hot water use. What you DON'T want is an electric tankless, all of which are under-powered for your incoming water temps, and exacts a toll on the local distribution grid infrastructure (which you may have to pay for, one way or the other, depending on how electric rate structures are calculated now or in the future.)

  16. jpavia1 | | #16

    Dana, below is my oil use history. Hopefully this helps. Let me know if you need anything else. Thanks!

    Sept 9th – 166.5 gal. K-factor 6.5
    Dec 24th – 147.1 gal. K-factor 11.3

    Feb 19th – 172.6 gal. K-factor 12.4
    May 13th – 156.9 gal. K-factor 12.2

  17. Expert Member
    Dana Dorsett | | #17

    Looks like your wintertime K-factor is a bit over12 HDD/gallon. Times 24 hours in a day that's 288 degree-hours per gallon.

    At 138,000 BTU /gallon source fuel, in an 85% burner (not likely to still be that high but...) that delivers 117,300 BTU/gallon.

    So, 117,300BTU for heating 288 degree-hours is 117,300/288= 407 BTU per heating degree-hour.

    Since the presumptive heating/cooling balance point base is 65F (common for 2x4 construction) at your +9F outside design temp the load is 65F-9F= 56 heating degrees.

    56 heating degrees x 407 BTU/degree-hour is 22,792 BTU per hour.

    That's not a super-precise number- it's an upper bound. Some of that heat was for hot water use, and a bunch of it was standby loss oveheating the boiler room (the walls of which appear to be uninsulated in the picture.) Since the thing is about 7-8 oversized for the load it's unlikely that the average efficiency is truly as high as 85%, even if it's steady state efficiency is that high. With that level over oversizing you'd probably be lucky to be averaging 75% efficiency, which would imply a heat load of just about 20K.

    Based on fuel use alone it's not a slam-dunk to do the whole thing with a single -18RLFCD, unless you can identify ~2-3K of further load reduction. But if you backed it up with a 7-8kw electric boiler (a ~$1K piece of hardware) using the same radiation as the old boiler, the mini-split would cover you for all but the coldest hours of the coldest days, and you'd have sufficient output in the electric boiler to meet code requirements for heating, and to cover you for those rare sub-zero days.

    Taking the raw 22,792 BTU/hr @ +9F and dividing by 1325 square feet comes to ~17 BTU/hr per square foot of conditioned space, which is on the high side for a fully insulated 2x4 house with an insulated foundation, but not ridiculously high, and this IS an upper bound. Once you seal up the flue your infiltration drives will come down a bit, and if the chimney is on an exterior wall insulating over that section would also be worth it.

    The load is probably already under 20K, but we can't prove that absolutely with simple arithmetic on the raw fuel use numbers. As a sanity check, take a peek at this tool developed by the BrookHaven National Labs for estimating heat loads & operating efficiency with fuel use on oversized boilers: (Download and install the FSA Calculator.)

    Selecting Bridgeport CT for the location and entering a K-factor of 12 , then clicking on "Adjust Heat Load" it comes up with a heat load of 16,197 BTU/hr @ +7F. Selecting Hartford and adjusting the load it estimates 19,250 BTU/hr @ -1F. (They're using the 99.7th percentile temperature bins, not the 99th, but that's OK.)

    You're PROBABLY good to go with just the -18RLFCD, but it could be close. The ~17K @ +7F number out of the FSA tool is totally credible, and I also believe the 18RLFCD is good for more than that. Consult a Fujitsu installer for the max capacity at +5F and +10F.

    A few other options to consider:

    Looking at Carrier's capacity tool for the GreenSpeed series, it looks like a modulating 2-ton GreenSpeed (model 25VNA024A ) would be able to handle your load with any of their air handler units.

    Click on the "Heating Capacities" tab, set your max design load to 22,000 BTU/hr and look at the green curve's capacity at +12F.

    It's a pretty pricey unit but it has comparable or better efficiency (HSPF 11-13, depending on the combination) to the 1.5 ton Fujitsu slim-duct mini-split.

    If there's a 2-3 ton Trane heat pump that works with your existing air handler it could be quite a bit cheaper to install than a Carrier GreenSpeed, but most equipment combinations have an HSPF less than 10, but if it's over 9 it's probably still worth considering:

    You may be able swap-in a 2 ton Mitsubishi MVZ -A24AA4 air handler onto the existing ducts married to a 2.5-ton MXZ 3C30NAHZ Hyper Heating multi-split compressor, it would deliver an HSPF in the high 9s and would capacity to spare. (The 1.5 ton MVZ has less capacity than the 1.5 ton Fujitsu, and lower efficiency to boot.)

    (I couldn't find efficiency test submittals on this air handler series, which may be pending, since it was only released a couple of months ago in the US.)

    The fact that it would be slightly oversized means you'd probably meet or beat the HSPF numbers, since it would be modulating at part load & higher efficiency even at your 99% outside design temp.

  18. Expert Member
    Dana Dorsett | | #18

    BTW: In the FSA tool if you click on the "boiler" box in the lower left, and select " Purge Control 87 AFUE-Highly Insulated tank" and calculate, the result is the performance you'd get out of a brand new System 2000, which comes in at only 75% "as-used" AFUE even when you hand enter an output BTU to 80,000BTU/hr (which the smallest they make.)

    That's just how bad your level of oversizing is for oil boiler. Even the best in class with heat purge control and buffering thermal mass can't hit anywhere near their AFUE test numbers when 4-5x oversized, and your existing boiler is more like 8x oversized.

  19. Expert Member
    Dana Dorsett | | #19

    Though it's not a current model, the Trane XL14i already IS a heat pump, not just an air conditioner, with an HSPF in the 9s!

    I'm trying to find capacity information on it with your air handler combination. Apparently it came in both R22 and R410A refrigerant versions.

    According to p26 (PDF pagination) of this manual, with another air handler the 2TWX4030B1 (2.5 ton XL14i) compressor unit was good for 16.1 MBH (=16,100 BTU/hr) @ +7F, which isn't too far from our estimated heat load:

    There is apparently an accessory sensor that allows you to lock out the resistance heating coils to a programmed temperature of your choice (down to 10 below zero.)

    If it has the capacity (it might), the right thing is probably to re-commission the thing to make use of it's heat pump capacity and spend any upgrade money on lowering the heat load, not buying new equipment. If you have (or can still get) the outdoor sensor to lock out any resistance heating strips you can find out just how cold it gets outside before it actually loses ground, and adjust accordingly.

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