Is my Navien Combi install salvageable?

[Dana]

The manual-j we did last year has the whole house heating load at 72,130. The problematic small basement zone is at 13,000 by itself.

A design load north of 72K for a 3000' house built in the 1980s that has glass in the windows and insulated walls & ceilings/attics isn't very credible. If the house is THAT crazy-leaky the air leakage would (and should) be mostly fixable.

It's a bit hard to fully verify the load numbers if using multiple heating fuels (propane + mini-split + wood stove, etc), but a fuel-use based heat load calculation is a measurement of the heat load rather than model based on WAGs on the construction entered into a Manual-J.

With zero foundation insulation a 3000 foot 2x6/R19 type house with clear glass (no low-E) windows should still come in under 50,000 BTU/hr @ 0F outdoors, even if it's leaking a fair amount. With a reasonable non-obsessive amount of air sealing and some insulation on the basement walls it should be around or under 40,000 BTU/hr @ 0F.

For reference- my 2x4 framed 2400' (fully conditioned, fully above grade) + 1600' of (mostly below grade) insulated basement 1920s antique with clear storms over the original wood sash double-hungs comes in at about 40,000 BTU/hr @ 0F. Without directly heating the basement it hangs around 65F year-round down there from the heat radiated from the subfloor, and the distribution losses off heating system pipes & ducts.

House has a total of 90,350 BTUH radiation @ 180 AWT. Basement has 20' of fin-tube, so 12,000 BTUH @180.

At condensing temps the individual zone loads are more problematic. Basement goes down to 5400 BTUH @130 AWT. About half the 11,000 min firing range.

So at the moment I'm considering a couple options:

1) Stick with mod con NKC110 and upgrade the radiation in basement zone to 20' of Runtal UF-4 which should get to 11,000 BTUH @ 140 AWT. The remaining concern is the 2.6 GPM DWH. We are ok with running only 1 shower at a time, but I'm wondering if a 77 degree rise is going to be enough in cold New England winters to get water up to 110-120 degrees. Today measured the tap at 49degrees cold water from the well.

2) Forget the combi and do an equivalent condensing boiler + indirect tank. (WHB110) Still needs a basement radiation upgrade as well, but at least the DWH won't be an issue.

As mentioned above we do also have some mini splits that serve heat but because of the room layout we can't rely on these 100%. Lots of cold spots since we don't have one in each room.

Any other options I'm missing or overlooking?

HTP's UFTC-140 combi boiler throttles down to as low as 13K-outm and will support a single full-flow shower. It's 199KBTU/hr bigger brother will drop back to about 18K-out. It's comparable to (but cheaper than) the NKC-150, either which would be a better choice than the NKC-110 if you have more than one bathroom.

One advantage these fire-tube combis have that your Navien didn't have is the thermal mass of the water volume in the heat exchanger. Navien now has fire tube versions too, the smallest of which is the NFC-175, which would be another option. It can throttle back to about 16.5K-out.

Of those three I believe the Navien has the most thermal mass (water volume) to work with at ~38lbs of water, which helps quite a bit for suppressing short-cycling. That's three times what the NFC-110 has on board, more than twice the NFC-150, and about 10% more than the UFTC-140, (28% more than the UFTC 199, which has less water volume than it's smaller brother).

Once you have the outdoor reset dialed in the stubby basement zone's calls for heat should still overlap with other zone calls, assuming a total design heat load of ~50K.

If it were my choice I'd be on the fence between the UFTC 140/199 vs, the Navien NFC-175. The much lower thermal mass of the Lochinvars is more likely to force you to upsize the radiation to suppress short-cycling. The UFTC or NFC can probably get it under reasonable control with programming tweaks alone.

But seriously, at the price of propane if the heat load of your house is truly north of 70K it's well worth spending some money on blower door + IR-imaging directed air sealing plus some insulation upgrades. The tighter house will be more comfortable no matter what you're heating it with.

If you're going to keep the place long term it might be worth doing both the air sealing & insulation, installing heat pump water heater and a right-sized (for the new improved lower load) ducted air source heat pump, and abandoning the hydronic system. Clearly a quickie swap out with a different combi boiler is the cheapest up front, but the operating cost of a propane combi is insane compared to what you'd be paying to do it it all with heat pumps. With some tightening up a 4 ton cold climate heat pump (ducted, ductless or a mix) should be able to cover the heat load with margin. With even more tightening up a 3 ton or 3.5 ton is even likely.

A "ductless head in every room" approach is usually dead RONG, 9 times out of 10, leading to gross oversizing of the multi-split compressors, and consequently lower comfort & lower efficiency. That's great for the contractor's abilty to make their boat payments, but bad for you. Consolidating a handful rooms into a single zone that can be served by right-sized mini-duct cassettes is usually a better approach for areas of the house with a lot of doored-off rooms, with ductless in larger more open spaces.

But fully ducted solutions are also viable, if a bigger PITA to retrofit.

 

[Mike Bernardo]

Thanks Dana. There is a *lot* of glass in this place and no attic. Blower door was 50Pa/1780 cfm. Design temps are 2deg outdoor, 68deg indoor. Two of the biggest open-space rooms are 22K by themselves. Not sure if credible or not. <shrug>

Yeah I'm not going to be able to do that kind of retrofit under these emergency conditions. Seems like my only option is to stick a modcon back in for now and suck it up.

You wouldn't be too concerned with the amount of radiation in the basement zone by itself, assuming we can get away with other overlapping zones? (We didn't have a outdoor reset with the now-dead Navien.)

If you *had* to put back in boiler like I do in this situation, would you bother looking at moving to an indirect DHW tank, or just go back to mid-sized combi?

 

[Sponsor]

 

[Dana]

1780 cfm/50 is reasonably tight for a 3000' house, so it's not the air leakage.

If the other zones are reasonably balance with the radiation to where the outdoor reset gets dialed to near perfection the basement zone will only short cycle occasionally (when not overlapping with other zone calls) and isn't a big deal.

At your very low electricity rates I'd still go for a heat pump water heater and go with an ~80K boiler with a 10:1 turn down. Rheem's current crop of heat pump /hybrid water heaters are ultra-quiet (your refrigerator might be louder) and pretty reliable compared to what was available a decade ago. Unless you have a big soaker tub to fill a 50 gallon is fine. HTP's UFT-80 fire tube boiler is cheap and effective, comparable in most respect to Lochinvar's WHB (or KHB)-085 (which has more bells & whistles to program, but the basics are similar). If the boiler room is inside of conditioned space those are enough, but if not the ~100KBTU/hr-in versions would still work. There are others, but Navien's smallest fire tube boiler is way too big for your application.