Boiler advise for new shop, considering Navien NCB-E 150

[Brandon H Hicks]

Hi guys,

I need some help choosing a boiler/DHW system for a new wood shop that we’re wrapping up framing soon on. My plumber and I were considering a Navien NCB-E 150 but I’ve come across a few Navien horror stories on these forums and it has me second guessing that decision.

I'll share a few details about the building...

The shop is about 800 SF. Radiant heat is via a 6" concrete slab with 4 equal loops of 1/2" hePEX on 9" centers - about 1,000 ft total. Walls are insulated to R21 and the 14’ cathedral ceilings are to R49 and we are in climate zone 6. There are some fairly large windows but they are Argon filled double-paned Pellas so the building is pretty tight. It will have a small kitchenette as well as a bathroom where we did go ahead and plumb for a shower as well. Fuel source will be propane and I am also installing a wood stove since we’re on a large wood lot with an endless supply of free firewood.

The shop will be mostly used on weekends until I retire and because the thermal lag on radiant heat is so long, the plan is to basically use the boiler for domestic hot water (not much) and freeze prevention to keep the building around 50 degrees or so and then use the wood stove to warm it up when in use above that. I’ll probably go ahead and add a supplemental electric heater to the bathroom as well since I’m pulling all the electric myself and hey, what’s another 20ft of 10/2?

Anyway, is this Navien unit overkill for this use? Even the smallest one isn’t cheap and I’m concerned that even that may short-cycle with the amount of thermal mass we have and how tight the building is but I don't really know how to calculate all that.

Thanks for any advise.

 

[Dana]

The min-fire output of the -150e at condensing temperatures is about 11,500 BTU/hr. Depending on air leakage and the total amount of window area the heat load at 70F indoors/0F outdoors could be that low or lower. At 50F indoors/0F outdoors it's likely to be well under 11,500 BTU/hr With 1000' of half-inch PEX there is about 75lbs-90lbs of water in the system, which is probably enough to suppress short-cycling when combined with the amount of concrete thermal mass that would be engaged (a much lower number than the full mass of the concrete).

To get to an optimal solution it's worth running an I=B=R type heat load calculation at the 99% outside design temperature and the targeted indoor temperature.

Given the volatility of the price of propane, a better choice for freeze-protection might be a 3/4 ton or 1-ton cold-climate ductless heat pump (mini-split) and use a small electric water heater for the minimal water needs. The Fujitsu AOU/ARU-9RLS3H can deliver 11,500 BTU/hr at 70F indoors, -15F outdoors, and even more at lower indoor temps, This series has a "minimum heat" mode that sets the indoor setpoint to 50F (in normal heating mode the lowest is 60F.) In my neighborhood in competitive bidding the all-in installed price would be under $4K, often closer to $3K. With 14' ceilings it's useful to spring for a wired or wireless wall remote which could add a couple hundred. It needs to be programmed to use the temperature sensor in the remote rather than the sensor in the head for it to behave similarly to a wall thermostat. For a bit more there are Wi-Fi versions to be able to check on it and adjust it remotely.

A 3/4 ton to 1-ton mini-split would also be able to cool a building that size as long as there isn't a lot of west facing window taking huge gulps of solar gain in the afternoon & evening.

 

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[Brandon H Hicks]

Dana,
Thank you for such a thorough response. I apologize I wasn’t able to check back earlier and say thank you.

I’m going to read through a few times tonight after work and run these calculations and will report back with my findings.

thanks!