Trying to determine best secondary-side pump speed to get coolest return water temp

28W

Just a curious homeowner here, so please excuse any dumb questions.

My system has Grundfos 15-58 3-speed pumps, and a single heating zone. I'm trying to determine whether running the secondary circuit pump (i.e., out to the radiators) on speed "2" vs speed "3" makes any difference in terms of the return water temp going back into the boiler. I have a strap-on pipe thermometer on the primary-side return right where it goes back into the boiler (i.e., after exiting the LLH).

It doesn't seem to make any difference whether I run the secondary pump at "2" or "3". For any given boiler output temp, the return temp back to boiler does not change when the secondary pump speed changes.

Paul48

Finned-tube baseboard?

28W

mixture of that and convectors, which are also fin-tube

Paul48

If you look at the rated output for baseboard, there isn't a lot of difference 1gpm or 4 gpm . The key is to remove the same gallons per minute on the secondary side, that you are moving on the primary side. Then get your reset curve right, so you are producing the right temp, and sending it all to the emitters. IMHO

28W

That's what I figured, but I don't know how many feet of head I have on the secondary side. Based on the Viessmann manual, and previous discussions on this forum, I think I'm moving 4.7 gpm on the primary side with the pump set on "2"

28W

Based on the pump curves for the 15-58, there is a pretty big gpm difference between "2" and "3". I have a small, 4-bedroom house, so i'm guessing (just guessing) that the head height can't be all that high.

Paul48

I'll toss this out there..... Why not use a Caleffi balancing valve with flow meter, and dial it in? Maybe hot rod will jump in? SWEI can help you dial in your reset curve.

icesailor

How important is it?

For a third world method, just take two Multi-testers that have temperature probes on them. Tape one to the supply side and the other to the return. Insulate the sensors. Read the differential on High. Read the differential on #2. If the return is now lower than it was on high, heat is being given off. If you turn it to #1 and it drops even lower, it's working. I've seldom seen a 20 degree differential unless someone made the length of the loop way too big.

If it is really important, install two cheap Pasco Tridicator gauges on the supply and return. Take the pressure differential between the supply and the return. Look on the pump curve for the pump and see how many GPM's the pump is delivering by the differential "head". Turn the speed switch up and down. Does the differential pressure change? Yup? The pump works.

Is the room warm? Yes? That's all anyone cares about. At least the ones that sign the checks.

28W

Thanks for the suggestions. I'm interested in figuring it out in order to maximize boiler efficiency without running the pump faster than needed (and hence using more electricity).

Empire_2

Just a side note, if your base are all in series that 20* design temp will be the difference between heat or no heat at the end of your run. Can you give a description as to how your convectors are laid out? Is there a supply pipe going to each convector and the other side going into the return? or are they all one after another?

hot_rod

Generally increasing the flow thru a heat emitter like fin tube, radiant loops and air coils increases the heat output. Asset increases the average temperature from beginning to end of the loop or circuit.

The limitation if the velocity, or speed of the fluid. Excessive flow rate shown as feet per second fps, will cause noise, velocity noise, and excessive wear in the piping components.

Up to 2" pipe size the generally accepted flow velocity hydronic for quiet trouble-free operations 4 fps.

Knowing what you are flowing thru will determine the flow velocity.

Here is a table for commonly used hydronic piping.

Ideally flow rate should be just enough to meet the load on the system, never dropping below 2 fps to assure air removal and good heat transfer from the fluid to the tubing wall.

This is why there is so much interest in variable speed circulation. Match the flow rate to the ever-changing load, adjust it as conditions change. Saves energy, eliminates on/ off cycles and provided best comfort.

Empire_2

Hot Rod, you may be interested in "Circuit syzer by B&G. It is a nice plastic slide rule type to find gpm fps, velocity, accepted pipe sizes etc... I like it because it's sturdy and accurate. I think you can go to B&G.com and inquire about it. You will love it. Beats a piece of paper or book IMO.

Mike T.

hot_rod

Got one, they're great. Mine has a date of 1967 on the jacket!

Ever see an old timer B&G rep wheel one of those things They do calc quicker with the wheel than I can with the computer or phone ap

Empire_2

I have the old style, but all the lettering and numbers have either faded off or rubbed off. I left it on the dash for some time and the sun destroyed it.lol

Mike T.

SWEI

System Syzer is available as an app for both Android and IOS.

Empire_2

Thanks SWEI, I'll have to check that out. On the job is usually where we need it.....

Mike T.