Navien NCB-240 - Indoor Temp Won't Rise??

[Marco I]

Howdy folks,

Desperate here...help needed please.

Recently, a NCB-240 was installed in my home having replaced a CH-240 (whistling noise issue).

I live in Toronto, Canada and it - like most of eastern Canada-US - has faced some extreme cold days as of late. My home has cast iron baseboard radiators. Single zone setup.

For some reason, I canno't get the heating system to increase the room temperature. The rads get very hot but no improvement in room temperature. For example, today it is about 12F outdoor. I've set the thermostat to 73F and the unit keeps running steady but canno't get to the desired indoor temperature. Never had this issue with the old conventional boiler or previous CH model.

My technician put in a bigger pump. No effect. Called Navien, they had me adjust the high temperature dip switch but again no improvement.

My readings are supply=177, return=148.

My mind is officially boggled as I am trying to trouble shoot this. My technician is equally perplexed.

Any insight from this community is much appreciated from your friend in the North.

Marco

 

[Reach4]

You might consider blowing a portable fan at the radiator intakes to increase air flow during this extra cold period.

 

[Sponsor]

 

[Dana]

The max output temp of that unit is 180F, so you're basically achieving that. The delta-T is somewhat on the high side, which may be an indication that something is screwy with the near-boiler plumbing or there is something restricting flow on either the primary or secondary loop. With your input/output temperatures it looks like you're only getting an average water temp (AWT) of about 160F. If you can bump up the flow to enough get a return temp of ~160F, for an AWT of 170F you'll get about 10% more heat out of the radiation , which may get you there.

 

[BadgerBoilerMN]

The accepted ambient design temperature of N. America is 70°F. If you are there, there you are.

If you are uncomfortable at 70°F I would look to the envelope and Dana's incomparable grasp of building systems.

The other factor which is often overlooked with the new Navien combi is the limited output while space heating. You do not have 199mbtuh boiler, you have a 199mbuth input water heater that is capable of limited space heating entirely dependent on factory limited flow factors, i.e. 4.5 gpm at 180°F - 160°F gives you at output of 88mbtuh.

While heating DHW the Navien works with the Delta T of ground water vs. design shower water temperature, say 85° delta. This allows the relatively small HX to exchange more heat than with the higher, closer, Delta T you are attempting in a relatively high temperature emitter such as your cast iron baseboard.

This is why we still use real condensing boilers in most combi systems, more especially in high temperature hydronic retrofits such as yours.

The Navien is a fine tankless water heater and we use them, but as with all combi-boiler systems careful sizing and relevant factory training are a must.

 

[Dana]

Actually, the IRC only requires 20C/68F (not 70F) in each room @ the 99th percentile outdoor temperature bin. If it's colder than that outside and it's losing ground, it could still be a code-legal heating system. In recent weeks I've seen overnight lows that were fully 13F/7C below my 99% outside design temp, and I'm sure that has been the case in Toronto too.

Without knowing the condition of the house it's hard to start throwing out suggestions on the potential for building upgrades, but there is usually some low-hanging fruit somewhere. It's pretty rare to find radiation systems in older houses that were originally designed for 180F AWT that wouldn't keep up with 160F AWT if the house has had any air-sealing/insulation/window upgrades since the original heating system was installed. Many will still make it at 140F AWT.

 

[BadgerBoilerMN]

I humbly stand corrected.

Cold up der eh?

 

[Dana]

Last Sunday it got down to -13F/-25C in Toronto, which is a hefty 14F/8C below the 99% outside design temp for Toronto. The projection for Monday night is -9F/-23C. Yes, it's been a bit cold-ish up der!

 

[Marco I]

Thanks everyone for the kind response. I am definitely better understanding the situation.

I actually called Navien and was able to boost the max. supply temperatute to 194F. This has enabled the house to achieve and maintain a comfortable indoor temperature.

What still puzzles me is the return temperature peaks at 150F.

It just seems like the boiler is working super hard all the time. I believe it is cycling (unit runs for a while, stops, and then starts up again). I am worried I am not maximizing the efficiency of the system.

?

 

[Dana]

The short-cycling is a symptom of something, but it's not clear exactly what. As long as the burn times are at least 3 minutes and there are fewer than 5 burns/hour it's not an efficiency or short-cycling disaster. If it's 1 minute, on, 3 minutes off, 5 minutes on, 1 minute off etc. with 10+ burns/hr it's a problem worth solving. If the house were broken into a gazillion tiny zones of low mass radiation that sort of behavior might be expected, but not a single zone moderate mass system like yours.

At 150F return water temps the thing is getting about 85-86% efficiency. When it's no so freaking cold out and you can still heat the place with the return water at 125F or cooler you'll hit the 90s.

BTW: I've never pondered the manual for this beast, but what does it take to get 194F water out of it?

 

[jeffsnavien240ncb]

when I called Navien they also suggested to increase my temp to 194. The book said 180 was max, so I shied away from the 194, increase.
do you think your system is ok with the 194 setting?

 

[Dana]

As long as you aren't hearing a bit of sizzle or banging from the boiler at the higher temp the system is fine. Those are symptoms of steam-boil on the heat exchanger becoming macroscopic, which interferes with the heat transfer efficiency. Even if you're hearing some sizzle, raising the system pressure another 3-5 psi will usually take care of it.

Some PEX tubing is only rated for 180F operation, but in fact even if rated at 180F max most will handle temps up to 200F (which is where a lot of newer PEX is now rated.)