Small Delta-T hydronic system

bauer

Morning!

I’m having an issue with very small heat transfer on a hydronic unit. The unit is a weil-mclain gas boiler, two zone valves and a Taco 007e pump. Confused as I am getting only a 3-4 degree difference in temperatures between the supply & return piping and I can’t figure out why. It is also my understanding that if I were to try and throttle down a zone via a manual valve, that the 007e would account for this and defeat the purpose?

Needless to say, the baseboard radiators are warm but the house is very slow to heat up.

Any thoughts appreciated!

hot_rod

The delta T on a system is a moving number. We choose a ∆ for a design but a system may never operate at the exact number. The delta varies based on the load of the space. In a cold room expect to see a wide delta on start up.

As the room warms the ∆ will tighten up.

The determine the actual operating delta the system needs to get to thermal equilibrium. This is the point where the supply and return temperature have stabilized. 10- 25° is a typical fin tube design ∆.

Fin tube spec sheets usually show BTU output at two different flow rates. Slantfin shows a 1 and 4 gpm.

Here are some graphic of how various ∆s effect the heat output of a strip of fin tube. As flow increased the ∆ closes up. That equates to a higher AWT average water temperature across the heat emitter, higher BTU output. For 3/4" fin tube, 4 gpm is about the max, without getting velocity noise.

What is the temperature at the first fin tube on the loop? Two ways to increase heat output are to raise the supply temperature, or increase the flow rate.

bauer

Thanks for data, love that stuff. The first emitter was around 150 degrees. I could increase supply temp for sure… My main concern was that there was a high demand — the house was at about 56 degrees at the time of startup. The circulator would run for a long period without the burner on because the water was still coming back so warm… it was taking an extraordinary amount of time to increase the temp of the space a few degrees.

hot_rod

Various brands of residential fin tube will have similar out put. 150 SWT isn't giving you much BTU output. and that temperature drops along the series. Most fin tube designs are, were, based on 180 SWT

You are sure the pump is actually circulating when you take measurements? The tight number you observed could bve the pump has just turned off, no water is moving?

I don't believe that 007E is a ∆T function? Maybe a ∆P function for zone valves?

Bottom line you need temperature differential and flow to transfer heat hydronically.

bauer

It sounds like water is moving and I can see small variations in the pressure gauge when it is on vs. off. So yes, the circulator was running when I recorded temps. (Note, this system is not/could not be set up pumping away as it has an indirect DHW coil and install instructions limited the location of the circulator to the return side).

It seems the 007E pump makes adjustments based on inferred head pressure (inferences made via electrical draw on how difficult it is for the impeller to move water) so no, it does not directly adjust baed on delta T.

I will crank up the SWT to 180. I experimented with restricting some flow on the zones and was able to get up to about a 10 degree differential, despite other info suggesting that the 007E pump would account for this…

hot_rod

The ∆P circulators that adjust flow do so based on zone valves opening and closing, so a multi-zoned system, is that what you have?

It will not make any flow adjustments on a single loop circuit.

Here is an example of how delta P circs looks/ works

bauer

yes, two zone system controlled by Taco zone valves

hot_rod

If the pump is programmed properly you should see, hear it modulate as the valves open and close. Some pumps have a display that shows you the change in W or gpm.

plumbworker

Also Check the orientation of the finned elements within the baseboard enclosure. I recently ran into a job where we did a boiler replacement and the initial installer had positioned the fins incorrectly and there was poor convection across all the baseboards with very low delta T.

DCContrarian

https://forum.heatinghelp.com/discussion/202927/small-delta-t-hydronic-system

The question I would start with is, has it ever worked satisfactorily? If the answer is yes, let's focus on what has changed since it last worked. If no, that's a little trickier, we're looking at some sort of design issue.

The heat output of a radiator is given by the flow times the temperature drop. If the heat output is less than what is wanted, either the flow or the temperature drop has to increase. Generally, to increase the temperature drop you have to increase the water temperature.

If it worked at one time, my first suspicion would be flow has become diverted or obstructed. I would make sure the system was free of air before checking anything else.

hot_rod

Plugged strainers is another common flow blockage that sneaks up on you slowly. Any Y strainers in the piping?