"Navien Combi Boiler: PSI, Gas Consumption, Condensation, and Baseboard Concerns – Seeking Advice!

joseluisheating · January 3

Greetings to All,

I recently installed a homemade Navien combi boiler with a manifold and baseboards. While I have no prior HVAC experience, I followed a combination of YouTube videos, books, and advice from online forums to set up the system. It’s been running well overall, heating the second floor effectively using an outdoor reset temperature sensor. During the recent cold snaps in Newark, NJ, where temperatures dropped to 10°F and below, the boiler performed as expected, running at 160°F water supply temperature. On warmer days, it also adjusts well. I keep the thermostat set at 74°F to maintain a comfortable indoor temperature.

However, I have a few concerns and hope the community can help:

1. PSI and Expansion Tank Issues

My expansion tank always feels empty, even though I’ve ensured the valve is open to the system. The tank is set to 18 psi, and the boiler autofill is set to 19 psi. However, the pressure gauge shows 22 psi when idle and rises to 30 psi when the system is running.
The connection to the expansion tank does not get hot, which suggests no water is flowing into the tank.
Is this a sign of a blocked or improperly installed expansion tank? Could it also be an issue with the size of the expansion tank or trapped air in the system?

2. Gas Consumption Concerns

The boiler is temporarily running only the heating system (DHW is disabled), and I’ve noticed higher gas consumption than expected. The supply temperature is usually set at 150°F, but during extremely cold days, it climbs to 160°F.
With 7 loops on the manifold, each running at 1–1.4 GPM, and 120 feet of 2000 series fine-tube baseboard in total, my Delta T is around 9–12°F. The apartment is about 1100 sqft, built in 1911, with no insulation and plaster-lath walls. It’s located between an attic and a poorly heated first floor, both of which contribute to heat loss.
Could the high gas consumption be due to insufficient baseboard heating for the space, or is this primarily an insulation issue? Does keeping the thermostat at 74°F significantly impact the gas usage for this setup?

3. Condensation Issues

The boiler produces significant condensation—about a bucket of water every three days. This seems excessive and is inconvenient to manage.
Additionally, the exhaust vent sometimes sounds like it’s drawing in water. I suspect this could be because the exhaust vent was not installed with the proper incline. Could this also affect the boiler’s performance or safety?

4. Baseboard Heating Concerns

I installed 120 feet of 2000 series fine-tube baseboards. Based on their output (500–600 BTU per linear foot at 150°F water temperature), I estimate this provides 60,000–72,000 BTUs of heating capacity.
The apartment’s heating load, based on rough calculations, is around 38,500–55,000 BTU (assuming 35–50 BTU per sqft for an uninsulated space in Newark, NJ).
While the system seems to meet the theoretical heating demand, I’m concerned it may still feel underpowered due to significant heat loss through the attic, first floor, and old plaster walls. Should I consider adding more baseboards, or is insulation/sealing the better long-term solution?

Summary

I’d appreciate any insights into the following:

Expansion tank operation and system pressure concerns.
High gas consumption and efficiency troubleshooting, especially with the thermostat set at 74°F.
Condensation management and exhaust vent incline requirements.
Whether my baseboard setup is adequate or if I should focus on insulation improvements instead.

Looking forward to your advice!

joseluisheating · January 3

In case image appear blur, The system has a primary secondary loop. It has 2 weeks and 2 days working ( as of today: 01/02/25). The operation time has is around 120 hours. Also, Gas consumption has 115thm. Adding, that during the second week, I add the check valve to the pump, first week didn't have it.

EBEBRATT-Ed · January 3

Expansion Tank

As far as the expansion tank goes you have to remove it from the boiler system to check the air pressure or have the boiler system drained. In other words, you should have no water pressure on the bladder when checking the air pressure.

If the boiler is on the same level as the heating system 12-15 psi is normal. If the boiler is in the basement 15 psi is enough for a two story house. The circulator should be pumping away from the expansion tank.

The expansion tank air pressure and the PRV (fill pressure) should be connected to the system in the same location and should both be set to the same pressure.

When the boiler is hot it is normal for the pressure to rise however it should not go over 25ish psi. Your expansion tank may not be large enough if the other things are correct.

Gas Consumption

If the boiler heats the house there is no need for additional baseboard although more baseboards will allow the system SWT to be reduced which can save fuel. Not sure if that is your problem. I would suggest doing an accurate heat loss.

Condensation

It's a condensing boiler it is supposed to condense water. That's what makes it high efficiency.

Install a condensate pump or pipe the drain to a proper drain. Include a neutralizer. Look in the boiler manual.

Install the air intake and flue as shown in the manual

Insulation and sealing is more cost effective and will save fuel

Was this installation commissioned with a combustion test?

joseluisheating · January 3

Thanks for your comment @EBEBRATT-Ed I appreciate

Expansion tank:

The pump is pumping away from the expansion tank, the air eliminator comes with an expansion tank, valve and fill pressure. However I disable the fill pressure that is connected and instead use the auto boiler feeder because it is more accurate. The boiler is on the basement and has to deliver to a second floor.

Gas consumption:

is 120thm good for 2 weeks of working and around 120 of operation ?

No, the installation was made by me, no experience what so ever. I didn't perform a gas pressure with a manometer, neither a combustion test.

Do you think it needs to be perform ?

Condesation and Vent :

I'm doubts free.

SuperTech · January 3

Performing a combustion analysis with a digital combustion analyzer during minimum and maximum firing rates is a critical step in the start up procedure of any condensing boiler. This is in the installation instructions and is not supposed to be ignored.

The expansion tank air pressure needs to be charged to the static pressure of the fill valve before installing the tank. If you actually did this and you still have pressure increasing near 30 PSI while the boiler is operating the tank is too small. Sizing information is available from the expansion tank manufacturers.

Install a condensate pump and neutralizer.

Hot_water_fan · January 3

Run time is too low. See about limiting max fire %.
Lower supply temps
Condensation is the goal. Did you read the manual?

hot_rod · January 3

there have been cases of the diaphragm being stuck in expansion tanks. Stuck against the nipple of the connection. Increasing fill pressure may pop it off. But remove and check pre charge first.

The more condensate you get the higher the boiler efficiency. I’m surprised you see that much with a fin tube system.

If it is piped with copper? It should not have a lot of water capacity, a #30 tank should be adequate.

Tune the boiler as others mentioned. A heat load calc would tell you if it is an energy hog structure.

The control should have other functions to dial it in, ramp delay, anti short cycle, burnrr limit, boost function? It varies from brand to brand.

Yes the 74 degree indoor will cost more than degree.

joseluisheating · January 4

Thanks for @SuperTech, @Hot_water_fan, and @hot_rod comments.

@SuperTech I will request a technician to perform combustion analysis while being on low and high load. But why it is so important to perform that ? (i'm a amateur).

@Hot_water_fan do you think it will be more efficient if it runs more than 120 hours biweekly ? I just disable outside reset temp and set the water temp to 135f. The return is coming @ 126f. I will take a lot at the consumption tomorrow. For context, the outdoor reset temp was always @ 150f.

@hot_rod can you tell me a good way to determine a head loss ? because technician didn't want to perform only the manual j for me. The functions you mentions like "ramp delay", "anti short cycle", "burnrr limit" etc. need to be adjusted in order to have highest efficiency ?

hot_rod · January 4

here is the output chart for that fin tube. You will need more like 180 supply to get 500 btu/ ft

The boiler manual should tell you all the functions and how to adjust them

Yes, a manual J should or should have been performed. The installer, you, or a third party would do the heat loss calc

With a building like yours, a guess isn’t good enough

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Hot_water_fan · January 4

do you think it will be more efficient if it runs more than 120 hours biweekly ? I just disable outside reset temp and set the water temp to 135f. The return is coming @ 126f. I will take a lot at the consumption tomorrow. For context, the outdoor reset temp was always @ 150f.

yes - that boiler can modulate so during especially cold weather it should run near nonstop. 150F was too high, 135F is a good temp to try. Lower temps at higher outdoor temps is the key.

psb75 · January 4

It sounds like the choice of a mod/con boiler with baseboard heating is a questionable choice for your location and building type. The "lowest hanging fruit" for you to "grab for" at this point is building envelope improvement i.e. air sealing first, then improve insulation. Then, you can focus on improving the operation of your boiler by 1. reading the manual 2. having a combustion analysis done, 3.installing the outdoor sensor and dialing the boiler in to all of the appropriate settings. This is the best you can do right now with what you have. You could install some more baseboard in the future. In the end, your fuel bill will inform you HOW WELL you did!

joseluisheating · January 7

copy @psb75

@Hot_water_fan, @hot_rod, @SuperTech and @EBEBRATT-Ed

I have a doubt. If lower return temp enhaces condensation. The more condensation, the more efficient. Would a series loop be more efficient than a manifold ? because series loop would lower the return by a lot and that would promote condensation. However, with a manifold, you can't lower the temp that easily unless you put long baseboards or other emitters.

Hot_water_fan · January 7

stick with the manifold and work on dialing in the ODR curve

SuperTech · January 7

The combustion test in high and low fire is to make sure the gas valve is functioning within the manufacturer specifications. The boiler is not good to go out of the box and every installation is different. This is a pretty critical step in the start up procedure.

We read posts on here all the time about the problems caused by this critical step not being performed.

GroundUp · January 7

Turn the temp down until it can't keep up anymore, then back up a few degrees. That'll be your most efficient point. Keep the manifold setup- a series loop would have the first rooms warmer than the last rooms and throw your balance all out of whack. Why are you draining condensate into a bucket rather than an actual drain? Regarding the pressure, if you're relying on the digital one in the boiler itself, that's a Navien thing. Even if the expansion tank and everything is perfect, they always spike during operation. I have one locally here that'll read 37-38 PSI on the screen but won't blow a 30 PSI relief so I can only assume the sensor is inaccurate (I don;t install Navien and have never seen a Navien system with a secondary pressure gauge).

hot_rod · January 7

https://forum.heatinghelp.com/discussion/comment/1832536#Comment_1832536

basically running a high delta system. It is commonly done with panel radiator systems, 30-35 delta.

The end of the loop would need more or larger heat emitters. So a room by room load calc would be critical.

This baseboard simulator makes it easy to design or troubleshoot series fin tube.

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