Funny No More

-16 wind chill in Minnesota this morning,  and Nick Lowe captures my mood perfectly…

First day of Spring my you-know-what.  “I don’t think it’s funny no more…”

Okay, now that the venting is over, let’s get back to business! Last time we discussed how variable flow rates and water temperatures affect fin-tube baseboard output.

The verdict?  Some, but not enough to matter, for one simple reason.

While the per-foot output may change, the amount of feet installed does not!  We’re gonna have enough output.

Okay, Captain FloPro, but what about panel radiators? Surely your Delta-T circulator will send one of those systems into litigation land!

Hold your horses, Perry Mason.  Read on.

For reference, let’s use the Runtal VLX 56 Panel Radiator.  This radiator has a rated output of 1,840 BTUH/FT at a 180* AWT  (Average Water Temperature – 190* Supply, 170* Return).  However, in our setup we’re using a Supply Water Temperature (SWT) of 180*.  At a 20* Delta-T, the return water temp would be 160*, for an AWT of 170*.

That changes the output of the radiator slightly.  Runtal supplies the following factors to figure the output at lower AWT’s:

180* = 1
170* = .89
160* = .78
150* = .67
140* = .57
130* = .48
120* = .38

Using the .89 multiplier for 170* AWT, we find the VLX 56 will have an actual output of 1,638 BTUH/FT.  To offset the overall heat load, we’d need to install 25 feet of VLX 56, for a total potential output of 40,950 BTUH.

So when it’s 0* out with 170* AWT (average water temp), we’re good to go as long as we deliver 4 GPM at a 20* Delta-T, as designed.

When it’s 35* outside, the system only needs 20,000 BTUH and only 2 GPM.

A variable speed Delta-T pump would slow down to deliver just that.


But what in the name of LA Law would happen with outdoor reset?

Same thing only different, friend.

We know from the factors above that the lower the AWT, the lower the output of the radiator.  Will we be able to keep up?

Let’s take a gander at an outdoor reset curve, courtesy of Heat-Timer, and see what’s what:

The SWT at Design Conditions (0* outside) is 180*.  At a 20* Delta-T, that’s an AWT of 170*.  Using this particular reset curve (shown in red), when it’s 35* outside the SWT would be 140*, the AWT 130*.

And at 130* the potential VLX 56 radiator output would be 22,080 BTUH.

At 35*, we’re at exactly half load, so we only need 20,000 BTUH.

I’d say we’re good.

Some simple fundamentals to remember: with outdoor reset, water temperature goes down as outdoor temperature goes up.

But when outdoor temperature goes up, the BTUH heating load goes down.

When the BTUH heating load goes down, the required GPM flow rate to satisfy that heating load also goes down, regardless of the supply water temperature.

With ANY circulator (fixed speed or variable speed) the water temperature has to be high enough for the radiation to deliver the required BTU’s.

But in every case – the amount of radiation installed REMAINS THE SAME. Can’t get around that one.

Nowhere does it say that the higher the water temperature, the lower the required flow rate. Nor does it say anywhere that the lower the water temperature, the higher the required flow rate.

Lots more to kick around on this topic, which we’ll do next time!

Until then, dream of the beach!

If you can’t get up and dance to that one, then you’re definitely frozen!


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