Navien NCB-240E - supply heating water temp?

Dsisson

Hello all,

I'm renovating a historic carriage house that's heated with a Navien NCB-240E.

Right now, this building has one circuit with a couple of cast iron radiators, and another circuit with fin tube base board. It never got very warm, because it was essentially an uninsulated garage.

I'm working on remodeling and insulating it, so it can be a guest house.

Total approximately 1925 square feet of heated space. About 625 of that is attic space (sloped ceilings).

I intend to have 3 heating circuits - the two existing circuits and another one for the attic.

I've done some heat loss calcs (which I intend to verify with a HVAC engineer friend). I'm hoping to use more cast iron radiators in the future, OR maybe modern aluminum radiators.

Regardless, I see that the BTU output of a radiator heavily depends on the supply water temp from the boiler.

What's the expected / desired SWT from this Navien boiler? I know the return temp needs to be below 130 deg F to condense. So should I expect 140? 150? out of the Navien? I have another property with a gas conventional boiler that runs about 180 deg, and would have no hope of heating the house at 150 deg, so I want to make sure I install enough radiator.

Jamie Hall

If you want it to condense, the output temperature needs to be less than 150 so you get the optimum 20 degree drop through the radiation — which should be sized to the average temperature (140), not the input.

However, you can set that Navien to go higher if need be.

HVACNUT

I believe you can program it to sense SWT or RWT, so you have quite a bit of play.

hot_rod

the best first step would be a room by room load calculation. Next select the type of heat emitters, then size the emitters based on a 150 supply. If in fact the goal is to always run in condensing mode. It will take more radiation than a system running 180, of course.

Steel panel radiators are another option. Those design round a 30-35 degree temperature drop.

You need some solid numbers to design around

Outdoor reset should be involved also to maximize efficiency and provide long run cycles

This journal has good info on designing for lowest supply temperatures, and derate formulas for most heat emitters.

https://www.caleffi.com/sites/default/files/media/external-file/Idronics_25_NA_Lowering%20water%20temperature%20in%20existing%20hydronic%20heating%20systems.pdf