Bypass and Delta T pump?

152NP

I am building my own system that will include a boiler loop, a Tekmar 361 controlling a variable speed  injection pump, and a heating loop. The heating side will have a home run manifold setup, and each cast iron radiator will be controlled by a TRV. Pumps will always be running during cold weather. I understand that I will need a differential bypass on the heating loop to reduce flow to the radiation circuits as TRVs close (and I am sure there will be periods when they are all closed), but I got to wondering how a Taco delta T pump might work in the heating loop so the pump isn't running full out all the time.  Then I got to wondering how the delta T pump and a differential bypass might interact with each other. Would these two components fight each other some how? 

CMadatMe

You wouldn't

Need the differential by-pass with the VDT pump.

152NP

Dead head?

I am concerned about dead heading the pump.  I don't know if the delta T pump will slow down enough under a no temperature change situation (when all TRVs are closed) that it could sustain a prolonged dead head period.  Maybe an always open 3/8 bypass loop would be just enough to protect the pump?

lchmb

vdt



The 00-VDT Variable Speed Circulator adjusts its speed to maintain the differential temperature (based on the RANGE dial setting) between the

supply sensor and the return sensor whenever a heat demand is present. This will increase overall comfort and sharply reduce boiler short-cycling.

The 00-VDT will also control velocity noise issues in the system, eliminating the need for a pressure differential bypass valve.



That is from the manual from the unit..I have never used one so really cant say much about this unit...

Mark Eatherton

Greatful Dead Heads....

Trucking, got my chips cashed in... (Do Dahh Man).



Unless it is an extremely large pump (read full horse power pumps, not fractional) Dead Heading is an over blown myth when it comes to small, fractional h.p. pumps. Too much impeller bypass to dissipate the mechanical energy and heat.



I had a Taco 007 that was dead headed for 3 (THREE) years. And it survived. When I opened the manifold isolation valves, WHOOSH, it took off like a brand new pump. No explosions. No burnt up motors.



Using a P.A.B. with any V.S. circulator will cause the circulator to think that there is a large hole to be filled with water, and will cause it to run continuously in an effort to fill the "hole" that it can see. Simply stated, you should NOT use a PAB with a VSP. You can, but the net effect is the pump will not cycle back like it is supposed to. PAB's are restricted to use with fixed speed pumps.



My vote is to use a VSP, and skip the bypass.



Our "systems" are made for "design" conditions that occur for about 2 % of the time, meaning it is grossly oversized for 98% of the time. Would it make sense to keep your car idling at 5,000 RPM and using the clutch to modulate the speed based on the need to accelerate 2% of the time? Does it make sense to keep the accelerator pedal glued and screwed to the floor, and use the ignition switch to modulate the speed of the vehicle?



I didn't think so.



EVERYTHING in MODULATION (boiler, pumps, flow control valves, TRV's etc...)



Our loads are not a constant. Why should our heat production systems be based on a constant?



Happy Labor Day!



ME

152NP

Dead Heading a Delta T Pump

I think I disagree with a part of your argument.  The delta T VSP would not be trying to "fill a hole it can see". We are talking delta"T", not delta "P".  This pump would only be looking at temperature drop,not pressure. If a bypass was open, it would only "see" a lower temperature drop because hot water would cycle right back to the pump. This would cause the pump to slow down, not speed up.



I am still not sure I would want to dead head the pump.  You may be totally right, but I just don't like the possibility that the thing could explode when my little boy is "helping daddy" in the basement.



I am still thinking maybe a 3/8 always open bypass loop would get the job done.

lchmb

ask

Why not call Taco and ask? They make the product, they know the best way to set it up/pipe it... And they are more than happy to help with any design's involving their products...

Mark Eatherton

You are correct...

It is different with a DP pump versus DT. With DT and your bypass situation, the pump will ramp down, thereby shorting high pressure drop loads flow. No wasted energy, just uncomfortable occupants....



If you put a 3/8" bypass in, you should valve it off so it can be eliminated so you can evaluate the good, bad and ugly of it in that application.



ME