Replacing everything with Navien combi unit

Discussion in 'HVAC Heating & Cooling' started by Faderus, May 11, 2016.

  1. Faderus

    Faderus New Member

    Joined:
    Feb 9, 2015
    Location:
    Poughkeepsie, New York
    Hi all, thanks in advance for the input.

    I live in Poughkeepsie, NY and am looking for some input. I have a 1960 Split Level 1800 square ft house which is getting a complete HVAC system replacement. We have spray foamed the roof deck and the envelope around the garage (6-8" open cell foam) and replaced the windows with Okna 5500 series units. There is old fiberglass insulation in 4" stud walls.

    I use natural gas, and have a traditional NG hot water heater and boiler driving hotwater baseboards. A/C is provided by through-wall ac units, one of which is over 30 years old. The boiler is rated at 175k BTU and is 20+ years old and massively oversized. The slantfin gave me a heatloss of about 40k if I did it correctly.

    We are going to be ripping everything out and converting to a Navien combi unit for heat and hot water and installing a 3 ton Goodman 16 SEER central air unit and variable speed air handler.

    I have an oversized solar array and over-produce by approximately 3500KWH annually. We live in a net metering market so I can sell back some of this overage at retail, but that could change in the future. There is a maximum buy back amount, and by my math I gave away about 1500 KWH last year.

    I have a couple of questions:

    1) Given the low price of natural gas, and the essentially free 1500 KWH of electricity would it make sense to install a heat pump unit (not a mini-split) that is essentially a $1500 upgrade to the A/C only system? We are looking at a GSZ16 (16 seer 9 HSPF). The Navien combi unit would provide supplemental heat via the existing baseboards as needed.

    2) Would an Ecobee thermostat be able to control this type of system? My installer isn't familiar with these units but it seems to do many more things than the Honeywell units he typically uses. He is confident he can get it to work but said it might cost a little more due to the additional labor needed.

    3) I also have the option of upgrading to an Amana unit for $500 more than the similar specced Goodman units, whether I choose a heatpump or A/C. Is their "replace the entire compressor" warranty worth this premium?

    Thanks for any input.
     
  2. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    First, the Slant-Fin tool shoots reliably high, typically by 25%, so your real heat load is likely to be in the low 30s, maybe even less than 30K. That's a pretty lousy fit for most combi-boilers due to the minimum fire output being more than half the design heat load.

    Then, three tons of AC is ridiculously oversized for an 1800 house in the Hudson Valley. Your real cooling load is probably about half that, unless you have a massive expanse of "sunset view" west facing window.

    Before you do anything, save yourself a lot of time & money hiring a competent engineer or HERS rater to run a real Manual-J load calc on the place. THEN you'll be able to sort out what makes sense and what doesn't. Do NOT leave it up to an HVAC contractor to run the numbers- some can do it competently, most not, and some who claim to be able to do it have both conscious and sometimes unconscious thumbs on the scale.

    How many feet of baseboard (per zone, if broken up into zones)?

    If the ratio of the min-fire output of the boiler or combi to any one zone is more than 200 BTU/ft it won't be able to modulate in condensing mode on that zone. If it's much more than that it'll cycle, and if you go deeper into the condensing zone it will short-cycle, taking a toll on both efficiency and longevity. To hit the mid-90s for average combustion efficiency you'd ideally want it to be under 150 BTU/ft.

    There are a few fire-tube boilers (but no combis) that can modulate down to 7000-7500 BTU/hr out that might be appropriate for both your load and your radiation, but let's have the radiation length numbers and we can work from there.

    The up-charge for a heat pump instead of straight AC is likely going to be worth it, once you figure out what the true heating & cooling loads are. The modulation range of something like a 2-ton GreenSpeed is enough that it won't be ridiculously oversized for your likely 1.4-1.6-ish ton cooling load, and it would be able to cover your heating load down into the mid-20s, if not the whole shebang.
     
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  4. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    It occurs to me that you probably have a heating history on this place(?). With a couple of wintertime (pre-April) gas bills and the EXACT meter reading dates and your ZIP code (for weather data), with the nameplate efficiency of the beastie boiler it's possible to derive a firm upper bound on your space heating load. The method is spelled out in detail in this bit o' bloggery, but I'd walk you through it on this forum if you like.

    Most 2x4 framed houses with upgrades like yours come in at or under 15 BTU/hr per square foot of conditioned space @ 0F, give or take. Poughkeepsie's 99% outside design temp is +6F, so a design heat load under 25K isn't out of the question here, which could be within range of a 2 ton Carrier Greenspeed @ +6F, among a few other ducted heat pumps, and very likely within the output of the 3-tonner. The turn down ratio on theses isn't great- the, low speed output is 40% of the max, so you won't get a lot of modulation out of the 3-ton unit at your anticipated true cooling load, but it could be the total heating solution, skip the baseboards.

    If you're zoning the AC with zone dampers it gets messier and less efficient, so the zone loads for both heating & cooling need to be considered too. That's another reason to hire somebody whose primary business is the accuracy of their calculations, and not an HVAC contractor. The latter has a vested interest in selling you more equipment than you might need, and an anxiety about undersizing. An engineer or HERS rater doesn't have those pressures. Expect to pay a few hundred (maybe even a grand in gold-plated neighborhoods) for that service, but it'll save more than that on equipment, and with right-sized equipment your comfort factors rise.
     
  5. Faderus

    Faderus New Member

    Joined:
    Feb 9, 2015
    Location:
    Poughkeepsie, New York
    Hi Dana, thanks for the help.

    I'll verify the length of baseboard, but it is a cookie cutter raised ranch, 2 zones (main floor and basement), roughly 1200 square main floor and 600 basement.

    I do have a heating history, and will do the math on that.

    The concept of relying on a heat pump without supplemental heat in this region seems to contradict common wisdom, but if it works it sounds like a good option. I'd rather have a unit that worked every time, even if it cost somewhat more to run. Are the Greenspeed units known to be trouble free, or do they tend to be finicky? I've seen a couple of posts about them having issues with humidity control...is this likely a sizing issue?

    I haven't gotten any quotes yet but would you expect the all-in price for a heatpump only system and a NG powered on demand water heater to be similar, higher, or lower to the initial package I described?

    Thanks again.
     
  6. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    The Carrier GreenSpeeds are known to be a bit pricey up front, but reliability is good when installed correctly. I saw a quote from a gold-plated upscale contractor on Martha's Vineyard last summer for a 4 ton GreenSpeed set up with 2 zone dampers PLUS a 1-ton Daikin mini-split for about $40K, all-in, to serve a ~3200' house in a location with 99% outside design temp of +12F (somewhat warmer than yours.) That was way oversized and ridiculously overpriced, but if that much was actually needed something like $25K might have still been profitable, but under $20 not likely.

    After an aggressive Manual-J and some back & forth with some other contractors the homeowner opted for a (still oversized) pair of 2 ton 3 head Fujitsu multi-splits to be able to micro-zone it with 6 separate zones. (One of those zones should have been a mini-duct cassette split between two adjacent rooms, but the contractor threw a hissy-fit, and it was already late October.) The multi-split solution came in at ~$15K all-in, before state & utility rebate incentives dropped it to about $10K post-rebate. The homeowner is happy with that solution, I thought it was overkill, but didn't want to turn it into a case of the perfect being the enemy of the pretty-good.

    Oversizing AC does indeed reduce the latent cooling capability. If the off-cycles are long, and run times short, some of the moisture on the coils re-evaporates and gets re-introduced into house at the beginning of the next cycle. This isn't any worse with a GreenSpeed than any other heat pump. Even though you don't want to consider them, the wider turn-down ratios of mini-splits makes them marginally better at latent cooling. (The Daikin Quaternity series can even dehumidify without sensible cooling, operating to a programmable dehumidistat setpoint, which is useful for extremely low sensible-load houses.) There are cold-climate ducted mini-splits that would cover your space heating load and still modulate over wider ranges than a GreenSpeed. A typical 1.5 ton mini-split (even ducted mini-splits) will have a 5:1 turn down ratio, some more than others. The Greenspeed is only good for a 2.5:1, and it's one of the better versions. With a typical ducted 2-stager the low setting is ~ 60% of full-on capacity.

    The cooling load of even a walk-out basement is well under a half ton, unless it's a bunch of glass sliders facing west. The heating load of the basement might be as high as 6000 BTU/hr for that zone, but not likely, unless the foundation walls have literally no insulation.

    The cooling load of the 1200' upper floors might come in at about a ton or even a bit more, depending on window size & orientation. If tightened up as-described the +6F heating load is probably on the order of 15,000 BTU/hr. If any of the upper floor is cantilevered out over the foundation, there may be some air-sealing and insulation remediation to be done on the cantilevers, a common air & heat leak in raised-ranch designs. If the cantilever is leaking a lot of air that's sometimes a significant latent cooling load too.
     
  7. Faderus

    Faderus New Member

    Joined:
    Feb 9, 2015
    Location:
    Poughkeepsie, New York
    Window size is a little above average (not much), and the upstairs is cantilevered on the southern exposure. The half that is over the garage (the bedrooms) has been filled with foam. Basement is not a walk-out.

    How does a ducted mini split differ from a regular heat pump?
     
  8. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    Ducted mini-split air handlers or "mini-duct cassettes" are fairly diminutive. To get an idea of the scale see this. They have lower overall cfm but a much larger modulation range from minimum to maximum output than a typical modulating or 2- stage heat pump. There are limitations on duct impedance that have to be taken into account with the duct design, but the ducts are typically smaller and installed in shorter runs than what you might be used to with 3 ton heat pumps or gas fired hot air furnaces.

    eg: The 1.5 ton Fujitsu 18RLFCD will modulate with load between 3100 BTU/hr (1/4 ton) and 21,100 BTU/hr in cooling mode, and between 3100 BTU/hr and 25,600 BTU/hr in heating mode at 47F, and will still deliver 21,600 BTU/hr @ +17F (at max speed at that temp). As temps drop, so does capacity and efficiency. But the efficiency SOARS when modulating at part load, and the best power-use strategy with mini-splits is "set and forget", letting the mini-split sense and ramp up & down with load running nearly continuously, rather than using large overnight set backs. It would have to run at max speed/lower efficiency during the recovery period if a large set back is used. That unit still has a rated output a -4F (I don't have the extended temp capacity tables in front of me to tell you what that is), but it keeps on chugging away putting out some un-specified but significant amount of heat at temps well below -4F.

    Mitsubishi has some bigger deal ducted mini-splits with somewhat larger air handlers that have rated outputs down to -13F, but with similarly large modulation ranges, but you probably don't have that much load. I'd be surprised if the 18RLFCD wasn't a pretty good fit for the upstairs zone. Since you already have baseboard installed, using a condensing water heater as a backup or auxilliary heat might be reasonable too, if the Fujitsu didn't cover the upper floor heating load. Since you have a sealed insulated attic it's probably pretty easy to drop one of those in the attic to cover the upstairs, and do something else in the basement.

    I'm a bit partial to Fujitsu for the 1.5 ton & smaller mini-ducted units, due to the better air handler characteristics than the competition. At 2 tons & up with bigger air handlers there are others that can work, but you probably don't need or want to up-size from there. We'll know more after you do some load analysis.

    If the basement has a fairly open floor plan a 3/4 ton mini-split with single ceiling-cassette or wall-blob/floor unit is going to cover it. If it's all doored-off there's probably a 3/4 ton mini-duct cassette that can work for both heating & cooling. But since the cooling load is near zero, you may just want to heat it with an isolated loop of the water heater. The sensible cooling loads of basements in this region are almost never high enough to handle the latent load in the basement even with a half-ton Mitsubishi FH06NA that modulates all the way down to 1700 BTU/hr in cooling mode. If you have needed a basement dehumidifier to keep the musty basement smell under control in the past, you'd still need one even if cooling with a mini-split, unless you opt for a 3/4 ton Daikin Quaternity . It appears they now have an extended range down to -4F (the lowest rung of the capacity table used to be at about +13F), so that COULD work. The Quaternity can be set to both temperature and relative humidity, and can dehumidify without sensible cooling, unlike the competition. They only make a wall-mount version though.
     
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