Split The Difference

Still thinking about 1970 and what an eventful year it was.  For instance, this song, inexplicably, was a hit…

Gimme Dat Ding,” as performed by one-hit-wonder The Pipkins, reached #9 on the US charts in June of 1970.  The lead singer was Tony Burrows – who also sang lead for Edison Lighthouse on last week’s 1970 blog hit.  One of the co-writers of this memorable classic was Albert Hammond, a one-hit wonder in his own right.

After 41 years and extensive therapy, I can admit it.  I owned this on 45.

And I know the words.

A few blog posts ago we discussed the benefit of adding outdoor reset to a standard cast iron boiler.  A study by the Center for Energy and Environment showed resetting a cast iron boiler could reduce fuel consumption up to 15%.

That ain’t hay, my friend.

This very topic came up for discussion at a training class recently.  A student asked – very perceptively – why the boiler wouldn’t short cycle during milder conditions.

After all, a standard boiler doesn’t have a modulating burner –  it’s always firing at the same rate. If it’s making 1500 water instead of 1800 water, it’s reasonable to presume the boiler would short cycle, killing any efficiency gained by lowering the water temperature.

A very perceptive question!

The answer lies in splitting the difference.

A high limit aquastat/relay allows the user to manually adjust the boiler’s setpoint temperature (typically 1200 to 2400).

There’s also something called a “differential.”  The Honeywell L8148A, for example, has a fixed differential of 80.  The differential is designed to minimize short cycling.  If the high limit is set at 1800, an 80 differential means on a call for heat the boiler won’t fire until the water temperature drops below 1760. It’ll keep firing until the temperature reaches 1840.  That’s 40 below and 40 above the setpoint, for a total of 80.  Hi-limit aquasat/relays usually have a fixed differential.  Triple-acting aquastat/relays – on boilers that maintain a minimum temperature – usually have adjustable differentials.

A small differential is good when it’s really cold out and you’re at 90-100% of the heating load (and the boiler’s not ridiculously oversized, but that’s another blog entry). You’re taking a ton of BTU’s out of the boiler, and you want to replace them quickly to keep the house warm.

The problem is that most of the heating season is spent between 35-85% of the heating load.  Under those conditions a small differential actually promotes short cycling.  The boiler’s creating BTU’s much faster than you’re using them, so the water will leap from 1760 to 1840 in a heartbeat.

If you throw reset into the mix, short cycling would get worse.  If it’s 200 outside and the reset control says you only need 1600 water, a boiler with that 80 fixed differential will bop back and forth between 1560 and 1640 like a ping pong ball.

Smart boiler reset controls (such as the Taco PC-700) have something called an automatically adjusting differential.   The brains programmed into the control’s microchip tighten up the differential as it gets colder out (so there’s always enough heat) and widens the differential as it gets warmer out (so the boiler won’t short cycle).

So when it’s 200 outside and the control says the boiler only needs to make 1600 water, the differential automatically adjusts wider, to maybe 160.  The boiler won’t kick on until the internal temperature hits 1520, and will run until it hits 1680.

The benefit grows as outdoor temperature increases.  At 450 outside the control determines the system only needs 1350 water. We can’t let a cast-iron boiler do that because the flue gasses will condense, so the PC-700 is factory programmed to a minimum boiler temperature of 1500 - more than enough to satisfy the heat demand.

In this example, say the differential automatically adjusts to 220.  On a call for heat, the boiler won’t fire until the water temperature hits 1390, and will keeping firing until the water temperature reaches 1610.

More often than not you’ll satisfy the heat demand using residual heat still in the boiler, without ever starting the burner.

And the most efficient boiler is the one that satisfies the heat demand when it’s off.  It doesn’t use any fuel then.

There’s another element to this equation that could help fuel economy even further.  We’ll discuss that next time.

And until next time, here’s another classic from 1970…

Ride Captain Ride” by Blues Image was a million seller, hitting #9 that summer.

Had that one, too…


One Response to “Split The Difference”

  1. Next time!!! Why you gotta leave me in suspense?? lol…

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