Having Trouble with VDT Circ.

Rich Davis_2

I have a Taco VDT circ on a system that is 1 zone and 4 loops using a TT PS 60 boiler. For the last couple years that I've had the system everything was fine. Up until tis cold spells zero to -5 I started noticing that the 20 degree diff was widening I thought it was just because of the cold. This morning I checked and with a 15 degrees out side the diff is nearly 40 degrees. I haven't done anything to change anything. So the circa is slowing down, it's been running nearly constantly to keep the house to set temp. I've checked the positioning of the sensors and they seem ok,So is it time to look for another circ or should I check something else. Thanks

Gordy

Interesting

Maybe goes back to life expectancy of a vdt circ.. Has to be sensor one or both, or circuit board. Can't believe the circ is bad maybe not getting correct inputs. Have you tried to change the delta t lowering it more to say 5 to see if there is any compensation?

Rich_49

VDT

Lotsa heat being extracted would be my thought .  Is the % light solid or flashing ? If it is solid the pump is more likely than not pumping at full speed and the widening Delta is a byproduct of the pump just trying to keep up .  I suspect another pump with a similar performance curve would perform similarly at conditions like this .  Try setting the dial back to the 2 setting ( 10*) in the board and observe what happens , then set it at the 6 (30*) and again observe . If the fluid stays at a 40* Delta there is nothing wrong with the pump and the Delta T is just because of the heat loss being greater than the conditions which this pump was chosen for . If the test shows that it is just operating at full speed maybe a 0012 or 0013 VDT would be the solution . But then again , maybe it's broke . 

  Another question is , what is the design heat loss for this house and what temp was it designed for ? I have seen many systems designed for 14* in my area that are just not capable of heating the structure at lower temps . Maybe this is your issue .

Joe Mattiello

VDT circulator

totally agree with Rich's comments regarding the VDT pump struggling to keep up with demand when outside ambient temperature drops.  The VDT pump model was not mentioned, so it's difficult to suggest a pump with more capacity.  I would prefer to do the math, and select the most appropriate pump so you do not have another issue.

If the pump has been running fine for the last couple of years, another consideration is a faulty sensor.  You can use the ohms/temp matrix found in the VDT pump instruction sheet to validate the sensors.  It's entirely possible to have a bad sensor. 

If you need help with sizing a pump you can call Taco Tech support for assistance. for your reference I attached a tutorial titled selecting circulators document TD10 found in the radiant made easy application guide at www.taco-hvac.com