Making an oversized Navien NCB Combi Unit work ? (seasonal cabin)

[skylonda]

Trying to install staple up wood floor hydronic combined (maybe add a radiator or two) for a seasonal cabin about 1100sf. will have approx. 1100ft 1/2" pex with Al emitter plates. There is r19 batt in cathedral ceiling, most of walls have no insulation other than 3/4" interior and exterior solid wood siding, will add r19 underfloor , later insulate walls with foam. Half windows are newer double pane, others are single. Lots of air leaks 1950s cabin. At Lake Tahoe, approx 6800degree days, would use heat in months where high/low are approx. 15/40. Wondering if I can get a Navien NCB-240 Combi to run the hydronic heat without short cycling, I think you can reduce max. output to 50% and I plan to a run lower temp to the staple up, might add runtal type radiators if more extraction of heat is needed ( all on a single zone/ tstat). I realize the occupants of the small building will only need less output than this can produce but wanted to try to make it work mainly for snow season short term usage- glycol also). This is unit is near new, was removed from a SF bay area house due to short cycling problems, I had nothing to do with that and do not know what was attempted to make it work.

 

[Dana]

The short-cycling risk is all about the minimum firing rate, not the max, and you can't program it to go any lower than it's minimum firing rate of ~18,000 BTU/hr in, ~17,000 BTU/hr out.

The total amount of water mass you have in the PEX , boiler & manifold is roughly 100 lbs (without correcting for the glycol), which isn't a whole lot. When the floor is emitting 11,000 BTU/hr (10 BTU per square foot of radiant floor) that means you have 6000 BTU/hr (100 BTU/minute) of "extra" heat going into the water, and the temperature of the water will be rising at 1F/minute. With a differential of 5F that would be a 5 minute burn cycle, which isn't terrible. But under lighter loading that cycle time gets a lot shorter- it'll probably need more radiation to keep the minimum burn times above 3 minutes, but it's do-able.

So, what is the heat load of the house at 15F, and what is the load at 40F (the outdoor temperatures at which you intend to use it), both before and after the walls get insulated? Any idea at all? (Are you going to calculate it, or just hack away on the system until it works?)

Sheet metal heat spreaders, or extruded aluminum for the radiant floor?

How thick is the subfloor? Is there finish flooring above the subfloor?

 

[Sponsor]

 

[skylonda]

The short-cycling risk is all about the minimum firing rate, not the max, and you can't program it to go any lower than it's minimum firing rate of ~18,000 BTU/hr in, ~17,000 BTU/hr out.

The total amount of water mass you have in the PEX , boiler & manifold is roughly 100 lbs (without correcting for the glycol), which isn't a whole lot. When the floor is emitting 11,000 BTU/hr (10 BTU per square foot of radiant floor) that means you have 6000 BTU/hr (100 BTU/minute) of "extra" heat going into the water, and the temperature of the water will be rising at 1F/minute. With a differential of 5F that would be a 5 minute burn cycle, which isn't terrible. But under lighter loading that cycle time gets a lot shorter- it'll probably need more radiation to keep the minimum burn times above 3 minutes, but it's do-able.

So, what is the heat load of the house at 15F, and what is the load at 40F (the outdoor temperatures at which you intend to use it), both before and after the walls get insulated? Any idea at all? (Are you going to calculate it, or just hack away on the system until it works?)

Sheet metal heat spreaders, or extruded aluminum for the radiant floor?

How thick is the subfloor? Is there finish flooring above the subfloor?

Yes- I plan to do get a calculation before the hacking- going to do the measuring this weekend. Was looking at if it might work at all as it's time for new HWH and I have the NCB and thought I'd go ahead and install it with a dummy loop w air elim. and exp. tank. I think floors are 3/4" solid pine subfloor with 3/4" oak or fir on that, thin vinyl plank over 3/4" ply in bath. I plan to use the extruded type Viega plates V15210, or similar ( I prefer the ones that are open on the side rather than bottom, easier to snap PEX in by hand while on your back-don't remember who makes them).
I thought I could add a radiator in the main room and a small one in the bath to add emittance, would complicate the single loop/ temp. concept.
Thank you for the wisdom, Jon

 

[skylonda]

Or, could this be a small enough demand to use buffer tank and plate Xchanger on a tankless ? ( I also have an "orphan RInnai R75LS) (http://www.greenbuildingadvisor.com.../stuff-i-learned-joe-lstiburek-s-house-part-1),
or just a small dedicated tankless for heat separate from DHW, like the TKJr? We have a NOS one of the original "Flash" version that was deemed to small for the DHW. ( that still has a minimum input of 19.5K ), that would be two units but simplifies cold weather shutdown and still opens up floor space.

 

[Dana]

All sorts of solutions are possible. Spending the money on a buffer tank vs. more radiation without doing the basic math would be a mistake. Even if it's cost neutral, more radiation has higher benefits. But how much radiation you need to even cover the current (pre improvements) load and whether the contemplated radiant floor can even heat the place with out additional radiation are currently unknown.

All solutions require doing at least some of the math.

Start with a Manual-J heat load calculation (of both the before & after conditions of planned upgrades) before choosing a path. I'm sure the combi boiler can heat the place. I'm not convinced that the radiant floor can on it's own, at least prior to air sealing and insulation upgrades.

 

[Jadnashua]

Staple up radiation trying to heat through what may be 1.5" of wood usually requires a fairly high water temperature. What is the range of setting for outlet temp on that system?

 

[skylonda]

Staple up radiation trying to heat through what may be 1.5" of wood usually requires a fairly high water temperature. What is the range of setting for outlet temp on that system?

The Navien NCB claims a boiler output set point of up to 194deg. , I would probably add a panel radiator loop to two locations on lower level to add to output capacity. Got the building measured and will now see what the calcs. look like.