"Faster, slower…"

Welcome back, gang!  This week’s song is all about feeling good about yourself…it’s an anthem that says that no matter what you may think, I’m not that bad a guy.  It’s from a folk-rock/country-rock/storytelling specialist named Todd Snider (biography here), from his 1994 debut album “Songs for the Daily Planet.” The song is “Alright Guy.” (Lyrics here)  Enjoy….

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

Someone once said that when the only tool you have is a hammer, you tend to treat every problem like a nail. And when it comes to circulator selection for residential hydronic heating systems, there is that tendency to hammer away with our favorites.

Had a discussion last week with one of my favorite wholesalers who, nearly 3 years after its release, still looks at a Taco 00R 3-speed as a 007 at speed 1, 008 at speed 2 and 0010 at speed 3.  Take a look at the following pump curve charts and tell me what you see…

Taco "00" 3-Speed Pump Curves

Taco "00" Fixed Speed Pump Curves

First, consider the 00R 3-speed (tech info here).  At low speed, the 00R pump curve very closely matches the fixed-speed 008 – it’s a lower flow, higher head, steep curve circulator.  At different speeds, the pump curve doesn’t change its “steepness.”  It’s still the same curve – just as steep – it’s just a little “stronger”  (more flow, more head pressure). Changing the speed of a 3-speed circulator simply bumps the pump curve up or down. It doesn’t change the shape of the curve or the characteristic of the pump.

Compare all 3 of the 00R curves to the Taco 007 and the Taco 0010 curves.  Both are high flow, low head, flat curve circulators.  Their curves don’t in anyway resemble the 00R.  As we said in last week’s blog entry (click here to review), flat curve pumps work best as zone pumps or with zone valves.  Steep curve circulators – high head, low flow – work best with radiant heating.

But what about a radiant heating job that uses zone valves or manifold actuators?  The head pressure produced by a steep curve may be required, but when only one or two zones are calling we run the risk of velocity noise, banging valves and other, less noticeable but more insidious problems.

This is where a variable speed circulator can be of value (there’s also value on non-radiant zone-valve type applications.  More on that later).

Variable speed circulators vary their speed (meaning the impeller spins faster or slower) based on some sort of outside information.  Some vary their speed based on something called “Delta P” (difference in pressure), while Taco’s varies its speed based on “Delta T” (difference in temperature).

Delta T is an interesting concept in that the temperature drop in a piping circuit accurately reflects how many BTU’s are being taken out of that circuit at a given point in time.  Remember the Universal Hydronics Formula?

GPM = BTUH ÷ (ΔT × 500)

Remember the BTUH heating load will change based on how cold it is outside at any given point in time.  A fixed speed circulator will always operate on its performance curve (energy in = energy out – it’s the law!), so in reality, the GPM is going to be what the circulator dictates.  If the BTUH changes with the weather, and the 500 is always 500 (why wouldn’t it be?), then the only variable left is the Delta T.  It has to vary.  With a fixed speed circulator, it’s the only thing that can.

So in the real world, it would not be unusual to see a baseboard or radiator zone that was designed to a Delta T of 20 to have, at best, a Delta T of 15 or 16 degrees (meaning the water leaves the boiler at 1800 – or 1400, if you prefer – and returns to the boiler at 1650 – or 1250 – instead of 1600 or 1200).  That’s a 25% difference in return water temperature, which can impact overall boiler efficiency.  And that’s the BEST you could hope for.  As the heating load drops (it gets warmer out, or zones satisfy, or both!), the actual Delta T in the system gets smaller and smaller, further affecting boiler efficiency.

With a Delta T variable speed circulator, the goal is for the Delta T to stay fixed, which accurately reflects the BTU’s taken out of the system at a given point in time.  As more BTU’s are being taken out  (it gets colder, zones open, or both), the circulator will speed up to provide the right amount of flow.  As fewer BTU’s are needed (it gets warmer, zones close, or both), the circulator slows down to deliver the right amount of flow.

Now, we saw last time what happened with a Taco 00R 3-speed (and the Grundfos 15-58 3-speed) in Ed Taylor’s zone valve job.  As zones closed, the pressure differential in the system built up so when a zone valve closed, it did so with a bang!  Both 3-speed circulator are steep curve circulators, so as zone valves closed and less flow was required, the new system curves would ride up the pump curve, creating greater pressure differentials.  Installing a flat curve 007 did the trick! (Review the last post here).

If a variable speed Delta T circulator (find Taco 00-VDT info here) had been used, the results would have been similar.  As zone valves closed, the circulators sensors on the supply and return would have noticed a narrowing of the 20 degrees Delta T, since the system was using up fewer BTU’s.  The circulator would then slow down until the sensors again read a Delta T of 20.  Fewer zones calling equals fewer BTU’s required to heat the house – fewer BTU’s required equals a lower flow rate required – lower flow rate required equals a slower spinning impeller.

Taco 00-VDT

We’ll let this sink in for now.  The next couple of entries will focus on the pump affinity laws, the effect of system Delta T on boiler efficiency, and electrical consumption of 1/25th HP circulators, variable speed or not.

Let me leave you with some questions, and thanks in advance for your feedback.  It’s very helpful and helps me with ideas for future blogs.  Some please, keep it coming!

  1. Have you used variable speed circulators (ours or others, don’t be afraid to say which ones)
  2. What was your reason for using a variable speed circulator?  What were you hoping to accomplish?
  3. Did you find that whatever it was you were hoping for was worth the added price tag?
  4. Does Todd Snider rock, or what????!!!???!?


2 Responses to “"Faster, slower…"”

  1. We are currently looking at using a 0010 vdt for an eight zone baseboard system w/possible addition of one zone of radiant. Question is, does the vdt start out at full speed and then slow down or the other way around? If it does start out at full speed, would a pressure differential vavle be required?

  2. Hi James…

    The VDT starts at full speed for a period of about 30 seconds and then adjusts to find its proper speed. I don’t think that would require a differential bypass valve.
    The VDT is available in a 008, 0012 and 0013. If you’d like to use a 0010 — if that’s the right size for your application, you can order a 0010 VS circulator and set it up in the Delta T mode, with DIP switches 1 and 3 on, and the rest off. As always, make sure you do the proper load calculations and head loss calculations before selecting any circulator.

    Thanks for reading and posting!


Leave a Reply

You must be logged in to post a comment.