Low Limit Setting to Avoid Flue Gas Condensation?

MichaelG

I have a 20 year old oil-fired Burnham V7 boiler with a tankless coil. It's controlled by a Honeywell 7224U. The system works well and the settings are 170/20/190/10 (LoL,Ldiff,HiL,Hdiff). The emitters are all standard baseboard.

My question: I'm going to abandon the tankless coil and convert DHW in my house to a hybrid electric HW heater. With this change, can I reduce the aquastat low limit?

The Burnham V7 manual specifies the following settings for the aquastat:
-boilers equipped with a tankless coil: 190/20/210
-boilers not equipped with a tankless coil: 210, but can be changed "as needs require" (no low limit)

Basically I don't want to waste a lot of fuel keeping the boiler hot for DHW when we aren't using it for that, especially in the shoulder seasons. But I don't want to drop the settings to the point where I end up corroding the boiler from condensation and costing myself a lot more money than I would save.

To those considering suggesting I add an outdoor reset, I looked into the cost of the module and consider it to be prohibitively expensive.

Jamie Hall

In my humble opinion, there is a low limit. If the boiler is not designed and built to be condensing, I don't think that anything much below 160 is wise... others may differ.

SeymourCates

@MichaelG

Without a tankless coil, it really makes no sense to keep the boiler at 160F 24/7/365 for the sole purpose of heating the building.

If you are diligent regarding annual cleanings, there is no reason to utilize a LL whatsoever. Let the boiler cool completely and when there is a call for heat, the boiler will climb to the HL setpoint. A typical oil boiler will climb past the condensing range (below 128F) fairly rapidly and there is no concern for excessive flue gas condensation provided the boiler can run for a period of 10 minutes or more after it climbs above 128F.

The L7224U can be utilized without LL as follows:

To use the L7224U in a cold start boiler application, disable the Low Limit function by pressing the
UP
arrow button,
DOWN
arrow button and
I
buttons simultaneously for three
seconds. Then push the
I
button until
LL
is displayed. Then
press the down arrow button until
OFF
is displayed.

EBEBRATT-Ed

I agree, disable the low limit.

Especially since you have multiple zones the temperature will climb quickly. About 130 deg is minimum for continuous operation

SuperTech

Get the outdoor reset module that connects to the L7224U. Set the design temperature at what the coldest temperature you get in your area. I live in NY and set mine at 0. High limit 180. My you can also add in a boiler protection mode with the ODR module to prevent flue gas condensation. Don't use a setback thermostat afterwards, let the reset module save you energy.

captainco

At what temperature does steam change to water? 128 degrees?
170 degrees? The first condensing furnaces in the early 80's had flue temperatures as high a 220 degrees and condensed. Also it is not the just the temperature coming out of the equipment but what is the temperature as it exits the cap. As long as it is above 230 degrees condensation is not The temperature of flue gas usually causes condensation not the temperature of the water. On an induced draft boiler the flue temperature should be 170 degrees higher than the water temperature and on a standard natural draft boiler the flue temperature should be 270 degrees higher than the water temperature. Condensation is usually caused by underfiring, improper venting, improper combustion air and/or cooling of flue gases with too much dilution air from a drafthood.

the_donut

Condensation temp of flue gas depends on combustion fuel, air ratio, and pressures of gases.

Zman

captainco said:

At what temperature does steam change to water? 128 degrees?
170 degrees? The first condensing furnaces in the early 80's had flue temperatures as high a 220 degrees and condensed. Also it is not the just the temperature coming out of the equipment but what is the temperature as it exits the cap. As long as it is above 230 degrees condensation is not The temperature of flue gas usually causes condensation not the temperature of the water. On an induced draft boiler the flue temperature should be 170 degrees higher than the water temperature and on a standard natural draft boiler the flue temperature should be 270 degrees higher than the water temperature. Condensation is usually caused by underfiring, improper venting, improper combustion air and/or cooling of flue gases with too much dilution air from a drafthood.

I have no idea what he is trying to say.
For simple, easy to understand advise. Keep your return water above 140, your boiler cycles less than 6 per hour and keep as much of the vent as possible within the building envelope.

And of course be sure the boiler is installed and setup according to the manufactures instructions.

HomerJSmith

Steam in a closed sys is nothing more than compressed water vapour. It depends on altitude. You can boil water at 32 degs. Steam is invisible. Wet Steam is what is visible when vapour is condensing into a liquid. At sea level steam (water vapour) condenses at 212 deg. At 8000 ft it condenses at about 206 deg (a guess).

captainco says, "...furnaces in the early 80's had flue temperatures as high a 220 degrees and condensed." Not possible at sea level. The temperature along the stack decreases and the difference between 220 and 212 deg is not much so the condensation that you refer to happens further up the stack when temperature of the gasses fall below 212 deg which is why flue gas temperature is recommend to be about 350+ deg so that the gasses can exit the flue before condensation take place.

Mechanized induced draft flues moves the gasses up the flue faster than natural convection, therefore, a lower flue gas temperature doesn't condense before the gasses exit the flue.

If the temperature of flue gas is less than the vapour point of water, the vapour will condense. The vapour point of water is dependent on pressure (altitude).

"The temperature of flue gas usually causes condensation..." No, it can result in condensation, it is a consequence or outcome of the temperature drop. There is a difference. The cause is the properties of water.

The temperature of the water in a cast iron boiler, if low enough, can lower the temperature of the flue gasses as they pass thru the heat exchanger, so, the temperature of the water does impact flue gas temperature, therefore, condensation.

Condensation is the result of anything that lowers flue gas temperature below the vapour point of water.

captainco

Is the condensate in flue gas just water? It has acidic properties and is not just H2O.

Not sure what that was dripping out of the furnaces in the 80's but it sure looked like water to me.

If something is heated to 2000 degrees it contains moisture. When it is cooled is not the due point much higher than something that was just heated to 230 degrees and then cooled down? I am not smart enough to figure this out but I have asked engineer friends of mine to try. In the mean time I just watch that liquid come out, whatever it is.

HomerJSmith

"Is the condensate in flue gas just water? It has acidic properties..." No, it is water and other products of combustion. Air is comprised of 70% Nitrogen and most boilers operate with excess air. I suspect that the acidic properties you mention is from Nitric Acid. I'm not a chemist, but from Wikipedia--"Nitric oxide is then reacted with oxygen in air to form nitrogen dioxide.
2 NO (g) + O2 (g) → 2 NO2 (g) (ΔH = −114 kJ/mol)
This is subsequently absorbed in water to form nitric acid and nitric oxide.
3 NO2 (g) + H2O (l) → 2 HNO3 (aq) + NO (g) (ΔH = −117 kJ/mol)

"Not sure what that was dripping out of the furnaces in the 80's but it sure looked like water to me." It was water and combustion products. It is what happens with a cold heat exchanger. You see this quite often when starting up a water heater with cold water in it. So much water pours out of the chimney on to the floor, you think you have a leak in the tank.

"If something is heated to 2000 degrees it contains moisture. When it is cooled is not the due point much higher than something that was just heated to 230 degrees and then cooled down?" Not necessarily! First, something heated to 2000 deg doesn't contain any measurable water vapour. We are talking about air containing water vapour, here. The dew point can rise and fall depending on humidity and temperature. The dew point is, from Wikipedia, "The dew point is the temperature to which air must be cooled to become saturated with water vapor." Also, "When air cools to its dew point through contact with a surface that is colder than the air, water will condense on the surface." Warm air has more capacity to hold water vapour than cold air. In the winter the humidity is much lower, perhaps, 10% and in the summer as high as 90%. But...the air(specifically combustion air) does contain water vapour which will remain as vapour in the combustion process until the temperature of the combustion gasses drops to the dew point.

captainco

I told you I wasn't smart enough to know the science formulas. But that leaves the question, "Can there be a condensing boiler if the water temperature is above 130 degrees, which should make the flue temperature at least the same?"

HomerJSmith

Give yourself more credit, you're smart enough to ask questions.

The question is: Can there be condensing in a boiler if the return water temperature is above 130 degrees? Yes, but it is very short duration and at a specific point in the burn cycle. The flue temperature would more than likely be higher than 130 deg in a non condensing boiler. In a mod/con boiler the latent heat in the exhaust is extracted by the incoming cold return water which lowers the temperature of the flue gasses below the dew point resulting in condensation.

lchmb

my only concern would be that boiler has been warm for 20 years.. I dont think I'd take it cold..but if you do have it checked first to include making sure your expansion tank is properly charged...

MichaelG

lchmb said:

my only concern would be that boiler has been warm for 20 years.. I dont think I'd take it cold..but if you do have it checked first to include making sure your expansion tank is properly charged...

Forgive my ignorance, is the expansion tank equipped with something like a schrader valve on a tire? Can I check it with a dial gauge? What would a proper setting be? The tank is pretty standard in appearance, a little bigger than a basketball.

HomerJSmith

You can't check the expansion tank pressure when it is in the system and get an accurate reading.

hot_rod

The dew point of the fuel based on the % of CO2 in the exhaust stream.

It's the inlet or return temperature to the boiler that you need to look at for condensing mode operation.

A few slides from a combustion webinar Caleffi presented a few years back.


https://www.youtube.com/watch?v=N5qpYSqkbLE

HomerJSmith

'hot rod', I've watched this before and look forward to watching it again. You're my hero, I've learned so much from your videos and writings. Caleffi and you have often led me to musing.

I have one question that keeps me awake at night. You say, "It's the inlet or return temperature to the boiler that you need to look at for condensing mode operation." in a non-condensing boiler, true. I must ask, in a mod/con boiler, Catagory IV, the combustion air is pulled from outside. If that air is 10 deg, what part does that play in condensation. Would it be a greater contributor than the condensation from the inlet or return temperature?

Squeezing as much heat energy from the flue gasses is certainly a cost savings, but I contend the real cost savings with a mod/con boiler is the TDR & ODR.

hot_rod

Well thanks for tuning in! I merely pass along what I learn and observe. Then part two of that webinar that Jody did gets nine to how to leverage mod cons. On the combustion side something like 100,000 BTU of natural gas produces around 1.15 gallon of water. Using that vapor could add 10,100 BTU. So a big win is to condense and return the lowest possible temperature.

No doubt the reset and adjustable firing adds to the mix. That too I suppose can be calculated and documented.

In this webinar Jody looks back at some of that early Trethewey/ Viessmann data to show how a typical weather in a city like Boston would additionally leverage ODR and reduced cycling to add to the efficiency mix. If you noticed the credits in the slides of both thee webinars a lot of industry folks contributed to the material presented.

The key to a great system design is to look at all the variables, use the immense data base of information and knowledge here and throughout the industry and blend it all together.

I'd say we are all still learning, and more technology will further our quest for the best system design and installations.

Being a master in any trade, to me, is a right to keep on learning comparing, and sharing, not an end game title or label. Maybe we all have a bit of Elon Musk or Tesla in us? They both believed it is only impossible until it isn't

https://www.youtube.com/watch?v=6mNECDHgDrg

the_donut

Under high enough vacuum condensation occurs at sub 100 temps. Some power plants will use near vacuum conditions at feed extraction pumps to get the maximum amount of energy out before discharging.

captainco

The percent of Hydrogen in Nat Gas is 10%, 8% in LP and 6% in Oil. According to the actual combustion formula the amount of H2O in natural gas combustion is 18% at 11.7% CO2 not 10%. This percent is reduced as the CO2 goes down or the O2 goes up.

Many combustion books state that you can't have efficiencies above 84% to 86% without condensation.

I can't work the formula but my combustion engineer friend explained to to me.