Rock The Boat

Very first 45 I ever bought…

Some 1974 funk from The Hues Corporation. Try getting that out of your noggin today.

Rock on witcha bad self!

Here’s an interesting question that’s come up at least a half-dozen times in the past few months:

What does a Delta-T variable speed circulator do in a panel radiator job when a thermostatic radiator valve (TRV) closes?

That’s a great question.  And the answer is…

It depends.

On what the entire system is doing. Let’s examine…

First off, what causes a TRV on a panel radiator to close?  Obviously, the room is approaching whatever temperature the TRV has been set for.  The room doesn’t need the radiator to deliver as much heat, so the TRV is going to start closing (it’s important to note – TRV’s are rarely, if ever, open or closed.  They modulate, so they’re either opening or closing).

When a TRV is closing, it’s reducing flow through the radiator.  In reality, at that moment in time, the Delta-T across that single radiator will get a little wider.

And what does a Delta-T circulator do when it senses a widening Delta-T?

It speeds up.

Umm….that’s bad, right?

Well, if there’s only one radiator and one TRV in the entire system, and we were talking about only that one moment in time, then yeah, that would be bad.

But the thing is, there’s never just one radiator and one TRV in a system.  There are lots of radiators and lots of TRV’s in a system, and they’re all doing different things at different times.  And we don’t just deliver heat at one single moment in time, we deliver heat over an entire heating season.

Saying a Delta-T pump doesn’t work in a TRV application because of what happens in that one moment in time is like saying LeBron is a crappy basketball player because he missed a jump shot in the 2nd quarter in a game against Philly last December.

We need to look at what happens in the system as a whole.

A Delta-T pump doesn’t worry about the Delta-T across just one radiator.  Its sensors are on the boiler supply piping and the system return piping.  As all of the TRV’s in the system do their modulating thing – opening and closing throughout the day – the water temperature coming back from the entire system is going to be lower.  A Delta-T pump varies its speed to maintain that designed-for system-wide temperature difference, and lets the TRV’s handle flow through each radiator.

And while the Delta-T circulator is maintaining the system-wide Delta-T, you’ll experience all of the system-wide economy of operation benefits that go along with it, including making the boiler operate as economically as possible.

And the chances of all of the TRV’s doing exactly the same thing at exactly the same time for even a minute? Roughly the same as a 53-year-old hydronics trainer (and high-school JV career benchwarmer) schooling LeBron in a high stakes game of one-on-one.

In other words, not good.

How does a Delta-P circulator handle this same situation? Differently, and similarly.  We’ll explore next time.

1974 was a pretty interesting year for music.  Van Halen and The Ramones both played their first gigs.  The Captain & Tennille got married and Sonny & Cher filed for divorce.

And, shockingly,  this tune was the song of the spring…

“Don’t look Ethel…”





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