Oversizing

John L

What are the long term effects of oversized equipment from the perspective of energy consumption, equipment malfunction or other side effects.

Brad White_9

If non-modulating (on-off) fire

The first short-term casualty is efficiency.

Each time your boiler fires it takes about two minutes to reach peak efficiency and to get to steady-state combustion stability. (CO, CO2, etc. in balance). Within that is the fact that you are warming up a block of iron before it imparts peak heat to your medium (water). You are "firing without effect" for at least the first minute of each cycle.

Then- all of a sudden- the setpoint is reached because the firing rate and radiator output and/or heat loss are so mis-matched. If the space radiation is equal to the boiler, that is better, your space reaches its setpoint faster. If the radiation is appropriate to the heat loss but the boiler is say, double that output, it reaches high limit and shuts off, starting another cycle. It will ride the limit until the space is satisfied. Like using an 18-wheeler to make local deliveries.

Keep in mind too that this might happen during the coldest day of the year, your design day which is usually less than 0.4 percent of the year. As it gets warmer, cycling gets worse.

Personally, I can tell you this is costly in gas usage. Oil too of course.

The second effect, longer term, is the wear and tear on ignition components, vent dampers everything that goes to work during a cycle. This may not show up for several years depending on degree of over-sizing.

Modulating fire is God's Gift next to condensing. Mod-Con rules.

Long Beach Ed

Steam

If it's a steam system, an oversized boiler will cause all sorts of problems including short cycling, not heating all the radiators, water hammer and general poor performance.

It will all add up to increased fuel costs. We see them all the time. We've halved fuel bills by removing oversized steam boilers and the places heated much better.

Long Beach Ed