15% efficiency improvement on Weil-McLain WGO-5 boiler

jesmed1

Thanks again to the many heating pros here who have given me lots of good advice on our Weil McLain WGO-5 oil boilers. These are cold-start boilers without DHW, which are very oversized and as a result run only about a 30% duty cycle on a design day. Heat is delivered via a 100-year-old gravity conversion high-mass hot water system with original cast iron radiators.

In an effort to improve efficiency and reduce oil consumption, I made the following changes at the start of this heating season on one boiler:

1. Increased thermostat differential from 1.0 degrees to 1.5 degrees.
2. Programmed a 3-degree overnight setback (previously had no setback)
3. Added thermal post-purge to keep the circulator running until supply water temp reaches 90 degrees.

The objective of the first two thermostat changes was to lengthen the average boiler run time. Increasing the differential means the boiler has to run longer to make up a bigger temperature rise, and the setback eliminates some overnight boiler runs, causing a longer run in the morning. The result has been to increase average run time from around 30-40 minutes to around 50-60 minutes. In theory, this should increase efficiency. And the thermal post-purge keeps extracting residual heat from the boiler after shutdown.

Now that the heating season is halfway over, I've been able to compare run time data from our ecobee thermostat for a similar period last year, and divide the run times by the heating degree days for those periods to get an apples-to-apples comparison.

Last season's run time/HDD ratio was 6.72 minutes/HDD. With the above changes, this season's run time ratio is 5.72 minutes/HDD for the same period.

So the boiler is running only 85% as long per HDD this season vs last, which means a 15% decrease in oil consumption.

Since I made all three changes (increased differential, setback, and post-purge) at the same time, it's impossible to say how much difference each change made. But cumulatively the effect was considerable. I had been hoping for a 10% improvement, so 15% is a nice surprise.