Water heater 35k btu and boiler 52k btu.

[Cpeters]

With a total of about 90,000 btu's what size double wall flue pipe would be code. The boiler is 4" and the
HWH is 3" The vertical run is 15-20 feet. I'm thinking 5 or 6 inch. Thanks

 

[Dana]

See charts 4A & 4B beginning on p16.

 

[Sponsor]

 

[Cpeters]

See charts 4A & 4B beginning on p16.

I'm thinking of a Navien combi boiler I can hang off the wall. Then I can use PVC out the wall and get
rid of my 40 gal. HWH. It's a 480 sf garage apt and was thinking about 35 linear feet of slantfin baseboard.
Will I need 2 expansion tanks for both the Heat loop and HWH? Thanks. I'll search for a piping diagram.

 

[Dana]

Combi boilers are best suited for high heating loads and modest to moderate hot water needs. An apartment over a garage has modest loads for both. There has to be sufficient heat emitter to keep them from short-cycling at minimum fire.

The NCB 150 can support ONE shower in mid-winter (not much else at the same time), and has a minfire output of ~11,500 BTU/hr, and would balance with no cycling at condensing temperature if there's over 50' of baseboard and only one zone.

The NCB 180 has a lot more margin on the hot water front, and a min-fire output of about 13,300 BTU/hr at condensing temps, and would need something like 60-65' of baseboard. Even if you have to add some baseboard or some other heat emitters to get there it's probably the better choice. Going any bigger than that would be overkill, and just create more design problems.

The heat load of the apartment might be less than the minimum fire output and won't really modulate in space heating mode. With enough baseboard it can just run at fixed temp 120-130F out on the heating side- no need to set up the outdoor reset curve. As long as there's enough baseboard to keep it from short-cycling it'll still deliver great efficiency without becoming a maintenance nightmare.

It's a better choice overall than a tiny (but still way oversized) cast-iron boiler + standalone water heater.

Yes, it will need two expansion tanks. With only 35' of Slantfin the NCB 150 would have to run at 145-150F out of the boiler to keep from cycling at all, and would short cycle likc crazy at the ~125F it would need to come even close to it's AFUE numbers. The NCB 180 would need to run at 155-160F to keep from cycling.

In the grand scheme of things baseboard is pretty cheap stuff, and in this case more is better. Adding another $300-500 in extra baseboard now saves more than that in fuel & maintenance headache later.

 

[Cpeters]

Thanks Dana. I'll put some extra fins in the garage below to give it some heat. In the summer I have an outside shower that gets used quite a
bit, but is shut off during the heating season. I always thought 180f was the temperature needed for baseboard but I guess the new boilers operate
a lower temps.

 

[Dana]

At temps below 120F low-rise baseboard output gets a bit non-linear, harder to predict/design-for. For condensing boilers designers tend to target 140F as the temperature needed at the 99% outside design temperature. At 140F out with 125F return temperature (average water temp between 130-135F) most condensing boilers will still be delivering 88-90% efficiency, but it takes an average water temp (AWT) of less than 120F to start climbing into the mid-90:

^^^ this is entering water temperature, not average water temperature^^^

180F is a typical design temperature for non-condensing residential system to hit an AWT of 170F (180F out, 160F back, 170F average water temperature). From the curve above it's obvious that return water of 130F isn't dramatically more efficient than return water of 160F. So an AWT of 140F isn't a lot more efficient than an AWT of 170F. It takes more baseboard to deliver the same amount of heat a the lower temperature, so designing for 170F AWT saves money on baseboard, and if they screwed up the math there's still a bit more room on cast iron boilers to run them at a higher temperature. It's also easier to do the math in your head- at 170F AWT baseboard puts out about 500 BTU/hr per running foot, so if the load calculations say it needs 23,000 BTU/hr at design temp, most people can divide 23K by 500 to come up with 46 feet of baseboard without straining their brain.

With condensing boilers it's similarly easy to divide the heat load number or minimum boiler output number by 200 BTU/hr to come up with about how much baseboard what it takes for an AWT of 120F, to deliver deliver 90%+ efficiency. For 140F AWT its ~300 BTU/hr per running foot- most people can't divide by threes as easily in their head, but most people still aren't reaching for a calculator.

If it's an open floor plan a 3/4 ton cold climate mini-split might be a better heating solution, since it's also a great air conditioner. Depending on local utility rates the operational cost might be slightly more or slightly less than heating with condensing gas, but if the space is going to be air conditioned with window units the mini-split might still have lower annual operating costs. In my area a 3/4 ton Fujitsu -9RLS3 or -9RLS3H runs about $3K all-in to have professionally installed, and can still delivers 15,000 BTU/hr @ +5F, (13,500 BTU/hr @ -5F). Even if that apartment is pretty leaky it's heating load won't be more than 12K @ 0F. The modulation range on that mini-split is pretty good- it can throttle down to 3K @ +47F. During the summer it can also deliver 12K of cooling when it's 95F outside if it needs to, even if it's AHRI "rated" output (the modulation level at which it's SEER efficiency was tested) is only 9K. The 1% design load of an apartment that size would usually be well under 9K, but probably not under 6K, which would make it a good fit.

 

[Cpeters]

Dana, thanks for the feedback.
I got a quote for a 15,000 Mitsubishi Hyper heat pump. And I think this makes the best sense. Easier install, one blower and will take
care of heating and cooling. I currently don't have heat and use an 8,000 btu ac unit to cool the place. Just need to crack the door to circulate the
air. I'll have my natural gas HWH in the basement/garage hopefully to keep it from freezing. Maybe add a piece of electric baseboard for the
extreme cold. But I do appreciate your feedback.

 

[Dana]

A 1.25 ton Mitsubishi is probably sub-optimally oversized for the heating & cooling loads. Use coolcalc.com &/or loadcalc.net with the most aggressive assumptions that make any sense (eg: if it's a 2x4 wall, assume it's R13 not R11, even if you're not sure, and assume the place is air-tight, not leaky, etc.) to come up with the range of loads. Those tools both tend to oversize a bit compared to a proper Manual-J using a pro tool, but with aggressive assumptions it won't be 2x, more like 1.25x reality.

The -FH15 Mitsubishi is good for 18,000 BTU/hr @ +5F, which is probably getting on to twice your actual load at that temperature, but only modulates down to 5000 BTU/hr @ +47F, which is probably more than the heat load at that temp. That means it won't modulate very much, but will cycle on/off a lot, which reduces it's as-used efficiency (and reduces comfort.) To get the most efficiency out of them they have to be size in a range where they'll modulate most of the heating season. The (cheaper and less efficient) -GL15 or -HM15 aren't any better on that score. You're probably looking at an FH09, FH12 tops.

If the FH09 makes it for the peak loads that would be ideal, since it's minimum modulation of 1600 BTU/hr @ +17F is one of the lowest in the industry, meaning that it will modulate well at super-efficiency all season long. It delivers ~12K @ +17F, ~11K @ +5F. The FH12 wouldn't be bad either- it's comparable in many ways to the Fujistu 9RLS3 in low temp capacity and modulation range.