Smokin’ In The Boys Room

Technically this isn’t a 1974 tune, but these guys did rock the house on The Midnight Special that spring…

Brownsville Station with their one and only hit.

And what’s with the Harry Potter glasses?

Residential hydronics never fails to fascinate.  Despite our best efforts at diligent design (accurate heat loss calculations, proper boiler selection, appropriate pipe sizing and circulator selection), what we actually install is pretty much always a tad (or a tad-plus) more than what we calculate.

And if we take into account the inherent “fudge factor” (or what engineers call “load safety factor”) built in to any heat loss calculation, that “tad” becomes even greater.

To repeat a phrase used in last week’s blog (click here), we measure with a micrometer, mark with a piece of chalk and cut with an axe.

Had a question at a webinar this week that sent me into a deep thought sequence, and since that happens only once a decade it requires a blog post.  The question was simple, blunt and outstanding:

“If mod-con boilers have turn down ratios, is an extensive heat loss calculation necessary to size one?  As long as it’s big enough, won’t it just pretty much find its “happy place” and work from there?”

Like I said, darn good question!

The “let’s-get-this-job-done-quick-and-get-the-heck-out-of-here” part of me agrees wholeheartedly.  That is, after all, how most residential boilers in Europe are sized (and as we all know, the Europeans know everything there is to know about hydronics).  But the “anal-retentive-numbers-cruncher” in me can’t let go.

Let’s say you’re looking to replace an old boiler with a new mod-con.  The old boiler has a net IBR rating of 120,000 BTUH.  Using this spec chart from NTI Boilers, which would  you choose?

Seems like the 150 is the right beast, doesn’t it? And get this, even if it’s oversized, with that 5 to 1 turn-down ratio it can modulate down to an input of 30,000 BTUH. Simple, fast, no math and you know it’s going to be big enough.

What’s not to like?

What if you actually run a heat loss calculation and the numbers say the load’s only 58,421 BTUH?  Would that change your mind?

Well, the “modulating down” theory says it’ll be fine, since the turn-down ratio will make sure the boiler output matches the load as close as it can. And it’ll do a fairly decent job of it.

But there’s always an “However” lurking.

As mentioned last time (click here), there is an awful lot of “fudge factor” built in to actual heat loss calculation formulas, not to mention whatever other fudge we add, just to be safe.  A conservative estimate may be around 10 to 20%.  Also understand that nearly 50% of the heating season is spent at roughly 1/3 of the total heating load or less.  So for instance…

If the “real” load of the building is closer t0 50,000 BTUH, then 1/3 of that load is right around 16,500.  The Ti150 modulates down to a low-fire input of only 30,000 BTUH.  What do you get? A boiler that, even though it can modulate, is still at least twice as big as it needs to be at least 50% of the time.

Choosing the Ts80, with a low-fire input of 16,000 BTUH, can modulate along with the heat loss much more closely. It won’t short-cycle nearly as much as the 150, so the moving parts will last longer and the overall system will operate more economically.

It’s also going to be a few hundred dollars less expensive and 30 pounds lighter. So you got that going for you.

Which is nice.

The only reason you might still go with the Ti150 is if the boiler was also making domestic hot water (it’s equipped with a flat plate heat exchanger for DHW).

If not, then follow Hydronics Efficiency Tip #42: When in doubt, size the boiler to match the heating load as closely as possible.

And a boiler that can modulate down to 16 (thousand) is beautiful…

Ringo hit #1 in January of ’74 with this classic remake.

Well played Mr. Starkey. Well played…

Leave a Reply

You must be logged in to post a comment.