Reelin’ In The Years…

stowin’ away the time

Steely Dan from 1973.  What a great band! And none other than Jimmy Page says Elliot Randall’s guitar solo is his favorite of all time.

And how cool is the Cos on the intro???

Let’s continue our discussion (click here to review) of using a Taco BumbleBee in a panel radiator system. The question on the floor today is what would happen when it’s warm out and you don’t need much heat at the radiators?

Well, if you have outdoor reset on the boiler, everything chugs along as it would if it were colder out – with one exception. The BumbleBee runs at even lower speeds when it’s milder out – like in late November – than when it’s colder out, like in mid-January.

Why?

Because GPM = BTUH ÷ (Delta-T × 500).

Always has, always will.

Sure, the water temperature will change.  So will the potential output of the radiator, along with the BTUH load of the room (more on this is the next blog entry).

If there’s no outdoor reset, the TRV’s will still regulate flow through the radiators, they just won’t open as much. Since there aren’t a lot of BTU’s being taken out of the fluid, the Bumblebee will react accordingly. Will the pump dead-head if all the TRV’s are closed? Yep – so will a Delta-P pump. Don’t forget presumption from last time was continuous circulation. The impeller is always turning – even when there’s no flow required.

What will the Delta-T function do if all the TRV’s are closed? That’s a pretty rare occurrence, because TRV’s aren’t made to be “off.”  They’re made to modulate – so the likelihood may be completely off the radar, but just suppose…

Let’s say it’s mild out and reset says the supply water temperature needs to be 120 degrees, and you’ve set the system Delta-T to 30 degrees. That would mean the pump would run at a certain speed in order to bring the return water back at 90 degrees. As TRV’s close – as in completely closed – the BumbleBee will slow down accordingly.

When the last TRV closes completely, the pump will be running fairly slowly. Now there’s no flow. The pump will run at that same speed until the it senses that return water temperature dropping. The water’s not moving, after all, so its temperature will probably drop some…eventually. The pump will try to get that return temp back up by ramping up — slowly.

Once one of the TRV’s opens up again – the sensors will get a reading of return water temp changes, and the pump will do what it needs to do.

Again, this is one of those occurrences that is mostly theoretical – and if it does happen, the time period is, as they say back home, wicked short.

How would a Delta-P circulator handle this same situation? Look at a Delta-P pump curve chart. Delta-P pumps work on a fixed pump curve — they’re funny looking pump curves, but they’re fixed.  If all of the TRV’s are closed, the Delta-P pump will work at the spot where the low end of the pump curve intersects the 0 GPM line – it’s a fair bet the BumbleBee would be running at a lower speed because the pump adjusts to what the system needs, with a constantly-adjusting pump curve. Check the Delta-P curves to see where that intersection point is.

The other thing to remember, a Delta-P pump runs the same speed in January as it does in November.

Lots more to talk about in this fascinating topic.  As the song says, let’s “Do It Again…”

“…in the land of milk and honey, you must put them on the table.” 

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