Help me understand

More details will help you get a better answer: boiler type and BTU rating, size of house (sq ft), age of house, location, total linear feet of baseboard. All relevant factors in assessing (1) whether your boiler is oversized (probably), (2) whether you have enough radiation (maybe not if the house is always cold at design temperature), and (3) how to optimize or fix what you have.

You mentioned the boiler is about 10 years old and was originally oil-fired. You bought the home in 2019 and converted the boiler to natural gas in 2023.

If I understand correctly, you have a Lennox COWB3 series boiler with a gas-fired power burner that replaced the original Beckett oil burner. At the same time the gas burner was installed, a buffer tank was also added. Correct me If I am not understanding your system.

Does the Lennox boiler provide domestic hot water at all? In other words, what supplies your hot water for showers and faucets?

To confirm that the tank is truly a buffer tank and that it’s installed correctly, could you provide the brand name and model number of the tank? Also, please share a few photos taken from far enough back to clearly show the boiler, the tank, and the piping from floor to ceiling. That would be very helpful.

Based on how you described the system operating, it would waste a lot of fuel if it were actually set up that way. I don’t believe someone would intentionally design a control system like that. A proper setup would have the boiler operate as a cold-start boiler using an L8148A control, which only fires the burner when there is a call for heat.

Because your system has two small (micro) zones, a buffer tank can make sense. Without one, if only a single bedroom zone calls for heat, the boiler water temperature will rise very quickly, hit the high limit, and shut the burner off. The zone then sits idle while the water cools, and the burner restarts for only a few minutes at a time. That short cycling never allows the burner to reach efficient operation.

When properly installed, the buffer tank adds water volume to the system. This increases burner run time and lengthens off time, which improves efficiency and saves fuel. That’s the main reason a buffer tank would be used in your situation.

The normal sequence of operation should be this:
A thermostat calls for heat → the zone valve opens → the zone valve end switch activates the boiler control → the circulator pump and burner turn on. Cooler water circulates through the boiler, buffer tank, and radiators while the burner gradually raises the water temperature. When the thermostat is satisfied, the system shuts down, possibly after a short thermal purge to use any remaining heat.

If there is no thermal purge, the system shuts off immediately, and any leftover heat in the boiler or buffer tank is wasted unless another zone calls for heat soon afterward.

So a house of that vintage with decent replacement windows, 2600 sq ft total, in Philadelphia, your design day heat loss is probably under 20 BTU/hr/sq ft, so 50,000 BTU/hr or less total.

Since you did a gas conversion, your BTU input rate isn't necessarily what the boiler rating plate says, but what the gas conversion burner input rate is. If the input rate is, say, 100,000 BTU/hr, with your non-condensing boiler running at, say, 80% dry gas efficiency, then you lose an additional 15% in the latent heat of the water vapor, so you're down to 65% overall combustion efficiency, or 65,000 BTU/hr output. Then you have additional thermal losses when the boiler shuts off and residual heat goes up the chimney. Then you're at 60,000 BTU/hr or less of useable heat.

So (for example) a 100,000 BTU/hr input rate might not be unreasonable for your house with that boiler. But If your input rate is higher, it's probably more than you need, and that contributes to the short-cycling problem. The input rate of conversion burners can be changed, but your bigger problem is the incorrect setup of the buffer tank system that @EdTheHeaterMan and other pros can help you correct.

Trivia of the day… while the normal efficency for natural gas in a non-condensing application, is 80% (LP is slightly lower), the normal efficiency for fuel oil in the same application is 83%.

I looked at your post @jesmed1 about Dry gas efficiency and latent heat loss in the flue gas. If you read the information throroughly you will see and understand that a combustion analyzer that the industry uses for testing combustion efficiency of a cast iron boiler will give you the combined dry gas + latent loss for a total combustion efficiency. This illustration clearly demonstrates that a 80% efficient combustion includes the 13% Dry Gas Loss and the 7% Latent heat loss for a total of 20% stack loss

In your statement, it looks like the 80% combustion efficiency was combined with an additional 15% latent loss to arrive at a 65% effective efficiency. That can be a bit misleading, since the 80% combustion efficiency already accounts for latent heat losses based on the standard test methods and equipment used to establish that rating. Adding the latent loss again ends up double-counting it. Your earlier post bears that out with a 13% Dry Gas and 7% latent loss for a total combustion efficiency of 80% (See illustration from your post above)

The specialized laboratory equipment needed to separately measure dry gas and latent losses is very costly and not something that’s practical or necessary to use in the field on a regular basis. For most real-world evaluations, the published combustion efficiency numbers are sufficient.

There are, of course, other losses that combustion efficiency doesn’t capture. These include standby losses when the boiler is connected to a chimney, where draft can continue to pull heat from the cast-iron sections after shutdown. There are also jacket losses to the boiler room, along with potential losses from leaks, makeup water, and piping or pickup losses in unconditioned spaces. All of these can affect overall system performance and are worth considering.

My main point is simply that efficiency losses can be easy to overestimate if we’re not careful about how the numbers are applied. Sticking with established testing methods and documented performance helps keep the discussion clear and accurate.

I hope this better explains what I meant regarding your numbers.

My atmospheric gas boiler begs to differ 😅

@EdTheHeaterMan is correct.

I thank @EdTheHeaterMan for correcting some misunderstandings on my part about how combustion analyzers work, and thus actual vs. reported combustion efficiency. I've deleted one post and edited another one to prevent promulgation of incorrect information.

I am here to learn, and thankfully Ed and others here are good teachers!

your emitters can only output about half the output of that boiler as @hot_rod calculated.

one more question, are these indeed just fin tube baseboard or are these convector cabinets? fin tube on monoflo is a bit unusual although not unheard of.

If he spent all his time in the basement and didn't measure each room upstairs, he is doing you no good.

Look. You can measure , you can calculate. You can do what you want.

84 feet of baseboard will only do 47,000 btus. You can put a 500,000 btu boiler in and you won't get any more heat out of the baseboard.

If the rooms are cold its one of two things you have a water flow issue or your short on baseboard. I was only pointing out that you boiler is double the capacity of your baseboard.

If you want to find out if you have enough baseboard or not there is only one way to do that and that is with a heat loss calculation.