Help Calibrating a Navien NCB 210E - It has efficiency problems

[Teresa G]

Hi all,

I was sold a Navien NCB 210E combi boiler in August with the golden promises of its efficiency and effectiveness. The background: It was officially operational Sept 1. Since then, it has failed to keep up with the heat demand when the temperature hit -10C and the propane consumption has been ridiculous. The installer said it would take 200-400 lbs of propane to run it until next summer. In three months it has eaten 220 lbs of propane and there are more than a few months until the snow goes. The technician has been out multiple times, increasing temperature, adding a sensor, turning on a K-curve etc. Nothing seems to help, and its usually about 10 days between visits so I want to see if we can just fix it ourselves (with some help from you wonderful online folks)

We have a very small house. two floors and a half loft with all three floors providing 1700 sq ft of floor space. It previously was heated with a giant wood burning stove I suspect was installed in the stone age. The installer put in 3 zones, about 20 ft of fin baseboard in the basement (zone1), 25 ft on the main floor(zone2) and 16 in the loft (zone3). As mentioned its an NCB 210E that also supplies hot water on demand. There is one sink in the kitchen, one in the bathroom and a shower (very little in the way of plumbing in the house, and not much demand for multiple hot water taps running at one time). I see that I can program in a return temperature according to the manual but I'm not sure how to make that part happen, and based on the outflow and inflow temps I currently have I think it is impossible to get any kind of condensate happening.

As of last friday the tech put it on a new curve with a new outdoor sensor and since then it has gone through about 55L of propane (about 11L per day) . I had it down to 7-8L a day (which is still above what was promised) by turning off all the curves, lowering the domestic hot water temp and setting a max outflow temperature of 60C. There has to be a better way to get this to something approaching efficient.

Anybody have any ideas?

 

[Teresa G]

Just a follow up...I was reading a bunch of the other posts and did some tests timing firing and rest periods. On the technicians settings I was getting 10-11 burns/hr which explains why it was going through so much propane. The thing is, there was always exactly 3 minutes of down time between firings. so i adjusted the burn suppression time to 15 minutes between burns. Now there are 2-3 burns/hr, with the burns lasting 9-16 minutes and we kept temperature all night. It was only this morning that the main floor is occasionally lagging about half a degree behind the thermostat. It is currently about -11C (12F). I will keep tracking the burn/rest cycles and the thermostats in the house.

It seems like a) we don't have nearly enough baseboard per zone
b)there is not enough time for the water to cool to take advantage of the condensate.

BUT

with the 15 minute intervals off forced shutoff between burns the house is still easily maintaining its temperature (with the exception of the half degree back and forth on the main floor) so why was it cycling on and off so frequently on the tech's settings?

 

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[Dana]

The amount of propane you are using has very little to do with the short-cycling or lack of condensing.

A pound of propane has about 21,500 BTU/lb source fuel energy, so the installers high end estimate of 400 lbs would be about 8.6 million BTU (MMBTU) , or 86 therms. Unless you only shower 5 minute per week and don't heat the house to more than 2C for freeze protection you might make it until summer on 400lbs of propane.

Most houses with 2-3 people who shower daily and wash hands & clothes regularly would use 8.6 MMBTU in 6 months just for the hot water at B.C. type incoming water temperatures. If you're heating the house with it, count on using at least 100 MMBTU for 9 months of heating season, and it could easily be 2-3x that much, depending on the thermal efficiency of the house.

That's at least an order of magnitude higher than 400 lbs.

The short cycling is due to too little radiation for the boiler's full output to be emitted at the output temperature you are running. Simply limiting the number of burns with a time-out is going to eventually limit it's ability to heat the place when it's cold out. What needs to happen is to increase the space heating output temperature to where you get at least 3 minutes (5 minutes would be even better) of burn time during a continuous call for heat, and stop the time-outs.

With very short burns you won't be getting much condensing efficiency due to the amount of heat blown out during every flue purge a the beginning & end of a burn cycle. But at higher temperatures the temperature won't be cool enough to condense. You're going to have to settle for 87% efficiency (non-condensing) until & unless you add enough radiation to run it at condensing temperatures without short-cycling the thing into even lower efficiency and an early grave.

Condensing efficiency has nothing to do with the amount of cooling time between burns, and everything to do with the return water temperatures during burns.

If you haven't already, run the napkin math on your zone radiation compared to the minimum fire output of a NCB 210E which is 17,000 BTU/hr in condensing mode, 15,500 BTU/hr when too warm to condense. At 180F output temperature it would take about 30 feet of baseboard to balance perfectly with the output BTUs at minimum fire (non-condensing). At 130F (condensing) it would take about 65-70 feet of baseboard to balance perfectly at minimum-fire, due to the lower heat emittance of the baseboard at the lower temperature. There isn't much thermal mass in fin tube baseboard, so you can't really get away with having only half as much radiation as the minimum-fire output of the boiler. Even zones as long as as 50 feet might be pushing it with an NCB 210E at condensing temperatures, and anything under 20 feet would be a short-cycling problem even at high temperature operation.

You may have to combine the zones, or add a buffer tank for thermal mass to tame this puppy if you've micro-zoned the place with a bunch of stubby baseboard zones.

 

[Teresa G]

Thank you very much for your helpful advice, it is truly appreciated. We were thinking of trying to combine the whole house into one zone. Is there any issue with just putting all the floors on one zone and running them off the main floor thermostat? We would end up with 66 feet total baseboard I believe if that is the case.

 

[Reach4]

he installer said it would take 200-400 lbs of propane to run it until next summer.

Did he maybe say 200-400 gallons of propane? I think home heating propane is usually measured in gallons.

 

[Teresa G]

No he said pounds, I remember because I was asking him what size tank was better, the 250, 350 or 500 and he said we would be fine with the 250. Sure wish I'd gone with the 500 now though...

 

[Dana]

If there is only 66 feet of baseboard it's a reasonable assumption that your actual design heat load is less than 40,000 BTU/hr, assuming it was able to keep up during cold weather with the prior boiler. So the minimum fire output of the NCB is on the order of half the peak heat load, which means if properly set up it could modulate for most of the heating season, and would still deliver reasonably long burns during the shoulder seasons if combined as a single zone.

There may be room-to-room or floor-to-floor temperature differences to address if combining it into a single zone if the baseboard isn't proportional to the zone loads. That may be tweakable with ball valves on each of the zone loops to balance flows a bit (or not, depending). But simply running all three zones off a single thermostat would be a good test, and may be (if you're lucky) all that's needed. The room temperature differences will be lower during the shoulder seasons when the water temp requirements are lower, which would buy some condensing efficiency.

Without knowing how the system zoning is currently plumbed and controlled it's hard to make more specific suggestions. Is it three pumps, or is it one radiation pump and three zone valves? Something else?

 

[Reach4]

No he said pounds, I remember because I was asking him what size tank was better, the 250, 350 or 500 and he said we would be fine with the 250.

This would be a 100 gallon (420 pound) propane tank.

 

[Dana]

This would be a 100 gallon (420 pound) propane tank.

That would be 100 US gallons, not Imperial gallons. (Prince George is still in Canada, after all...)

In Canada if it's being sold by volume it's usually denominated in litres in Canada, not gallons (US or Imperial), and I'm not sure how many places it's being sold in units of kg or lbs.

A liter of LPG weighs 1.12 lbs or 0.51 kg, thus a 25o lb tank would be about 225 litres , or about 60 US gallons.

 

[Teresa G]

its very odd here, they measure things in BTU's, sell the tanks in pound sizes and then charge you per liters of fill and provide no really useable info on how the numbers are convertible. Just to jelp us build character I guess I know the last fill of our 250 lb tank was 758 liters so I've no idea how or why they call the tank sizes the way they do.

It's one pump with three zone valves as far as I'm aware.

 

[Dana]

With 1-pump and three zone valves you could manually open the valves to a pair of zones, and turn off the thermostats to those manually opened zones. Then whenever the active thermostat is calling for heat, all three zones will get flow. The details of how to manually open zone valves is manufacturer and sometimes model specific. If you don't have printed copies you can probably find enough documentation for yours online to figure it out.

At 1.12lbs/liter putting 758 liters into a 250lb tank reads a bit like packing 3lbs of poop into a 1lb bag. But a 250 US gallon tank has a full capacity of about 950 litres, for which a 758 litre fill-up would be both possible and reasonable.

Canada has had it's fair share of characters (Rob Ford, anyone?), but I didn't realize this was how they were built?!