boiler efficiency

susan_3

We have an oil-fiired steam heat system (approx 50 years old)) in a house recently purchased. The boiler
efficiency was 74% 2 years ago; last week it was calculated as 68%. Since we understand the lower the efficiency rate the higher the oil useage, are there any prudent steps that con be taken to =increase= the current efficiency rating so as to make the boiler more economical to use?

* We use a "full-service" heating company which performs its own annnual boiler cleaning & tune-up in addition to supplying us with oil - as such, we are concerned there may be a "conflict of interest" here, since a poorly tuned boiler would therefore use more oil, which would result in increased oil sales by the company concerned.

Any advice or general comments would be appreciated. - thanks.

Occasional Interloper

Increasing Old Boiler Efficiency

Without knowing the details on this old boiler, here are some general improvements you could try (I'm assuming it's a high mass cast iron boiler):

1- Soot is a barrier that reduces the ability for the water in the heat exchanger to absorb the heat from the flue gasses. Make sure the service person has opened all of the cleanout doors to access all areas of the boiler and cleans it thoroughly each year.

2- If the burner itself is 50 years old, it should be replaced. Any retention head type of burner will improve the percentage of oil that you actually burn, at a richer mixture than the old burner, and this would be a drastic improvement.

3- If the burner is a newer type, or you replace the old burner, you can probably reduce the firing rate of the nozzle from the specified gpm on the rating plate. Most boilers are oversized for the home's heating load, and you can always go back to the larger nozzle, if the heat is inadequate. Keep in mind that the air adjustment must be properly set for the nozzle size, to have clean and reliable ignition.

4- If there is a combustion chamber, usually made of fire brick or a refractory material, it may need to be repaired or replaced. This helps establish a better flame, and if it is collapsed or weak the flame could be affected, or damage to the boiler could occur from the flame.

5- The flue passages of older boilers are typically very wide and unrestrictive. Some designs encorporated baffles that disintegrate over time. If there are remnants of baffles found in the boiler passages, you could try getting replacements of the same type and installing them. There are baffles available that can be retrofitted into older boilers that did not originally use them, but you will need a qualified person to decide what is appropriate. A heating supply house may be able to offer the right baffles, based on the boiler make and model. Two tricks that my father used on old boilers with large swinging cleanout doors in the front were a hanging baffle (supported with a chain over the flame) and bricks slid into the upper flue passages (if large enough). Any thing (besides soot) that restricts the flue passages and forces the hot gasses up against the heat exchanger walls will reduce the heat lost up the chimney. There has to be some room left for passage of the gasses of course, so the service man should check the pressure in the combustion chamber and readjust the burner settings, if necessary.

Hope this helps, I have seen poor efficiencies raised to over 80 % with these practices performed.

Jim Davis

All excellent ideas except downsizing the nozzle. To heat the large mass of a boiler the gases produced must fill the whole area to produce the proper pressure drop and slow velocity for maximum heat transfer.

Occasional Interloper

Reducing Nozzle Size

When I made the nozzle size reduction suggestion, I was wondering to myself if this was appropriate on a steam boiler, due the amount of energy needed to not only get the water heated, but to change it's state (from a liquid to a vapor). My experience is almost solely with hot water boilers - and have never had a complaint of insufficient heat after downsizing the nozzle. I especially believe in this practice when the old burner that was running a dirty flame at less than 10 % CO2 is replaced by a new retention head type, running with a clean smoke reading and a higher CO2 - effectively you get more BTUs of burnt fuel energy for the cleaner heat exchanger surface to absorb. The only other issue is the chimney, where the cooler flue gasses might now be condensing, which could cause damage. But probably the reduced flue temperature of this older boiler would not be as low as any newer high efficiency boiler, anyway (once at steady-state).
Does anyone use any particular CO2 vs. Net Flue Temp points for oil, as a basis for deciding what might be prone to causing condensation in the chimney?

Leo

Properly tuned boiler

Unless your oil company isn't honest they will be tuning the burner to do it's best. If a burner isn't tuned properly it runs dirty and the next guy has a mess to clean. If it is tuned to run too clean it can blow the flame out when it starts causing a no heat call. Having worked for two different oil companies the old statement of putting in a larger nozzle to burn more oil is a wives tale. Now your unit is early 1950's technology, highly efficient not really. Maybe the oil company is trying to sell you a new unit in which case you will more than likely use less oil, have new technology in your basement, and have parts under warranty for a few years. Do you still drive a 50 yr old car, have 50 yr old kitchen appliances etc? Factor out the cost of a new heating unit for 20, 30, 40 plus years and it is really a deal.

If you have an efficiency tag left by the oil company the co2 reading is probably under 10% usually around 6-8 for a lot of older units. This is caused by air infiltration at all the old joints. It just tends to go down with time.
Good luck whatever you decide to do.
Leo