Questions on boiler temp and condensation

[Joe88]

Hi everyone,
Just recently bought a new house and it has an oil boiler. My last house had gas so oil is new to me. The house is 1700 so ft, the boiler is a Columbia model csfh4150wtl, and has a Honeywell 8124 aquastat. I get hot water from the boiler, I believe it’s coil based. There are 2 zones in the house.

I noticed that we were burning through oil very quickly and saw that the aqua stat settings were 195 hi, 170 low, and 10 dif. I decided to try turning it down a little to see if I could get the boiler to run less and still have adequate heat. I changed it to 170 hi, 150 low, and 10 dif. I’ve been running this for a few days now and I’m happy with the results so far, boiler is running a lot less and house is still warming up well and hot water is still hot.

I’ve read about possible condensation issues with the boiler/flue from boilers running too low temps and I’m hoping to get some feedback to make sure I’m avoiding that . My understanding is that the low temp should be at least 140 to prevent any condensation buildup. Do my current aqua stat settings seem adequate to avoid condensation buildup? As of now the burner kicks on at 140 when there is no heating call. Also, I watched while both zones were calling and the boiler temperature never went below 140 while the burner was heating. The only way I could see the boiler getting below 140 is if both zones were calling and someone hopped in the shower, but that would probably only be for a few minutes until a zone stopped calling. If anyone has any opinion on my current settings I would appreciate it. Thanks!

 

[Dana]

There is no harm to the boiler dropping it all the way to 140F for a low limit, as long as the domestic hot water service is adequate.

If the chimney is on an exterior wall and doesn't have a stainless steel liner you may run into flue condensation issues if the duty cycle on the boiler is too low. The CSFH4150 is a pretty hefty 175,000 BTU/hr- out boiler if it's still using the 1.5 gph nozzle it was shipped with (there are derating kits for that boiler for downfiring to something more reasonable.) That would be about 5x oversized for the heat load at my house, and probably yours too, which is TERRIBLE for efficiency unless using a heat-purging economizer type control. The oversizing being so large (=low duty cycle even when it's cold out) aggravates flue condensation issues. With a stainless liner it's fine, but with a terra cotta lining flue condensation slowly destroys the mortar of a cold brick chimney.

If the chimney runs through the middle of the house with heated conditioned space it's much less of an issue, but the efficiency problem due to gross oversizing is still an issue. If you're keeping the boiler a retrofit smart heat purging boiler controller such as the Intellicon HW+ or HydroStat 3250 Plus would probably pay for itself in under one heating season with a DIY install. Those types of controls come as standard equipment on many/most new oil fired boilers.

A typical 2 x 4 framed 1700' with some fluff in the walls and attic with clear glass storm windows and full basement with NO foundation insulation would typically come in under 35,000 BTU/hr @ 0F, and your 99% outside design temperature is in positive numbers (in all CT locations.)

It's unlikely that you have enough radiation to emit the full output of that boiler, but for the record, how much (and what type of) radiation do you have on each zone?

 

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[Joe88]

Hi Dana,
Thank you for your response and sorry for not replying sooner, crazy week. Let me respond to some of your points:

-The house has copper baseboard as radiation. The first zone has roughly 110 feet of baseboard, and 60 feet on the 2nd zone.

-The chimney is on an exterior wall, and I believe it is a standard clay liner

-The boiler does have a Becket Heat Manager attached. I have not seen the heat manager go into economize mode once, and it wants to keep the boiler sitting around 200 degrees. I've turned it off as I don't seem to be getting any benefit from it. I have not yet spent much time looking into why it doesnt go into economize mode.

So at this point I've burned through roughly 150 gallons in 5 weeks or 30 gallons a week in a pretty cold November and December. Does this sound like a reasonable amount to burn?

I'm thinking the cheapest option to see some savings would be to separate the domestic hot water with an electric hot water tank. That way the boiler could sit a reduced temperature and not have to run as much.

Regarding the low duty cycle, what is considered a normal cycle? 10 minutes?

 

[Dana]

At an average water temp of 170F (180F out of the boiler, 160F back) typical ~8" tall fin-tube baseboard puts out about 500 BTU/hr per foot of length, so with a total of 170' you'd be looking at about 85,000 BTU/hr, only about half the output of that boiler, making it 2x oversized even for the radiation. The 60' zone is only capable of emitting 30,000 BTU/hr, so the boiler is about 6x oversized for just that zone, and might be prone to short-cycling if just one zone is calling for heat.

Try checking the program settings on the Heat Manager- if the low-limit is set to something pretty high it may never do any economizing. Also check that it was wired up correctly (fallible humans installed the thing.) The Beckett Heat Manager is/was manufactured by Intellidyne, and is a somewhat simpler version of the Intellicon HW+. That series purges heat from the boiler down to the programmed low-limit on a new call for heat. The high limit is typically set quite high to be able to utilize the thermal mass of the boiler & system water to maximum benefit by slewing through a much larger temperature range than most simple aquastat controls allow.

It's also possible that it has failed. If you determine that it's defective, replacing it with an Intellicon HW+ should be pretty straightforward, since they are for all intents and purposes the same thing, different paint. Or, Beckett burners have been married to AquaSmart 7600 series boiler controllers on recent model boilers. They're pretty similar in function- I'm not sure if Intellidyne is under the hood on those or not, but the programming setup has a different look & feel.

Regarding burn times and duty-cycle, those terms mean different things. Any burn cycle longer than 5 minutes is fine from an efficiency and longevity point of view for most cast iron boilers, as long as the entering water temp was above 140F for most of the burn. The duty-cycle is the ratio of burner time to total time. If it's burning only 6 minutes out of an hour, that's a 10% duty cycle. A burner that big would deliver about 17,500 BTU in that 6 minute burn, so at a 10% duty-cycle it's delivering 17,500 BTU/hr. At a mere 20% duty cycle it would be delivering 35,000 BTU/hr, which is about what my house needs when it's +5F outside, 70F inside. With your 170' of baseboard emitting only 85,000 BTU/hr the most you'll ever get out of it is a ~50% duty cycle, but the odds are it never even reaches those levels if it's an insulated house with glass in the windows and doors that shut.

The downside to operating at a low duty cycle is that it's spending most of it's time in standby, with high idling losses. Even at 3x oversizing for the heat load the idling losses are taking a double-digit percentage efficiency hit on the "as used" AFUE. With a fully functional Heat Manager or similar set up properly, that efficiency hit can be reduced to single-digit percentage.

What is considered "reasonable" oil use depends on the size and insulation levels of the house, and the average outdoor temperature during the period of use. 150 gallons over 5 weeks could be really great or really terrible.

At typical CT electricity rates a plain old electric water heater isn't likely to save a lot of money during the heating season, but would save a decent amount during the shoulder seasons and summer. A better bet would be a heat pump water heater, which uses only about 1/3 the amount of electricity as a plain old tank, but also provides a fair amount of dehumidification during the summer season, and harvests the boiler's jacket losses and distribution losses during the heating season, if installed in the same space as the boiler.

 

[Joe88]

Thank you once again for the detailed response, that was very helpful. The question I’m pondering is if it makes financial sense to replace the boiler with a properly sized one. I’ll have to learn more about heat loss calculations, but 175,000 BTU seems like way too much.

Regarding over sized boilers, what happens to the extra BTUs that the radiation can’t handle? My basement doesn’t feel particularly hot.

 

[Dana]

Thank you once again for the detailed response, that was very helpful. The question I’m pondering is if it makes financial sense to replace the boiler with a properly sized one. I’ll have to learn more about heat loss calculations, but 175,000 BTU seems like way too much.

Yeah, 175KBTU/hr is enough to keep my place warm (with lots of margin to spare!) at an outdoor temperature of -225F, a temperature not seen in southern New England even in the last few ice ages.

Regarding over sized boilers, what happens to the extra BTUs that the radiation can’t handle?

What happens to the "extra" heat is that the temperature of the heating system water hits the boiler high-temp shut off. When enough heat is emitted over several minutes to where the temperature is at the low-end of the differential setting the boiler re-fires, hit's the high limit and turns off, repeat until the thermostat is satisfied. If the burn times are shorter than 5 minutes, or there are more than 5 burns per hour it starts to take a real toll on system efficiency, and can wear out the ignition equipment on the boiler. This is commonly referred to as "short cycling".

My basement doesn’t feel particularly hot.

A boiler that idles at high average temp won't necessarily make a heat-lossy un-insulated basement feel hot, but even a relatively cool basement can have a high heat loss when the foundation walls aren't insulated. An 8" poured concrete wall moves about 0.75 BTU/hr per square foot per degree F of temperature difference. When it's 30F outside, 55F inside that's a 25F difference. If you have 2' of above-grade exposure on a perimeter of 125' that's 250 square feet, so just the above-grade losses would be 250' x 0.75 x 25F= ~4700 BTU/hr. That loss would represent a double-digit percentage of the fuel bill in most homes. The below-grade losses are real enough too, but the soil temps aren't quite that cold, warmer as you go down the wall.

Current IRC code-minimums for basement walls in CT would be R15 continuous insulation (not interrupted by thermally bridging studs.) Adding R15 to the wall would would reduce the above-grade losses to about 0.06 BTU/hr per square foot. With a boiler in the basement the basement would likely idle in the mid-60s unless actively heated, so call it a 35F delta-T. That would make the above grade losses of 250' x 0.06 x 35F= 525 BTU/hr, less than 2% of the fuel bill for most homes.

The real losses depend on your actual above grade exposures, deep subsoil temps, and weather averages, but don't let a cool or slightly warm basement lull you into thinking it's not a serious heat loss.

If you think you want to insulate the basement walls check back- there are ways to do it without creating a mold farm, and ways to do it that have a significant moisture risk, ways to do it right that cost a ton, and ways to do it effectively on the cheap. That topic gets covered on the remodel forum on this site fairly regularly.