Pump Speed for Indirect

Chester

I have a 3-speed Grundfos pump feeding a Smart-30 indirect and am wondering what the optimum pump speed setting should be. Is there a rule of thumb I can use or do I really need to figure out how to measure the Delta T?

Thanks in advance!

Chester

Thanks!

SWEI

Assuming you have a condensing boiler, running the pump at a slower speed will decrease boiler return temps and increase efficiency. Lowering supply temps will also help. It can take a fair bit of tinkering to get the performance you need at the lowest temps, especially if the demand is variable.

On larger commercial systems, we incorporate demand signals to dynamically manage this. I's still a work in progress, but the results so far are quite promising.

Chester

Yep, it's a Greenstar 57 that's heating the house just fine with supply temps below 105F, even when outside temp has been in the mid-20's this winter. (I have lots of cast iron radiators.) The thing is condensing like crazy during heat calls and we don't use that much hot water, so I'm not really worried about squeezing every last % of efficiency out of the DHW.

Is it normal for these pumps to be a little noisy, especially at the highest speed? It sounds kind of like a rattle. The Alpha that feeds the heat zones is dead quiet.

Gordy

I've often wondered if fast production of dhw may be better than a slower production recovery for efficiency. If tank temp is raised to extend usage, and supply not exhausted then slow low temp recovery may be it.

hot_rod

Don't most of the mod cons fire up to 180 or 190F for DHW priority?

I would run the hottest possible SWT and the highest speed on the pump, notice how the BTUs jump off a fast moving train .

I've tried low temperature DHW recovery and it takes hours and hours. Use the large delta T, highest SWT and fastest speed, get 'er done quickly, then move back to heating loads with low SWts.

SWEI

It really depends on the indirect sizing and the system dynamics. We have a SMART 30 in a church that typically recovers in 6-8 minutes using a 174°F SWT. The thermal mass of the building is such that during cold weather it takes ~8 hours to recover an 8°F heating setback. Once the building gets up to temp, interrupting the heating for a few minutes is unnoticeable.

SWEI

If you set your SWT at or below your DHW target temp, what do you expect?

On the TT boilers, you don't directly set a DHW supply temp anyway -- you set using an adder to the DHW target temp. If someone adjusts the DHW setpoint, the supply temp tracks it.

hot_rod

SWEI said:

It really depends on the indirect sizing and the system dynamics. We have a SMART 30 in a church that typically recovers in 6-8 minutes using a 174°F SWT. The thermal mass of the building is such that during cold weather it takes ~8 hours to recover an 8°F heating setback. Once the building gets up to temp, interrupting the heating for a few minutes is unnoticeable.

Recovers to what setpoint temperature? 120F?
If so 174- 120 is a good delta T. What kind of flow rate?

Chester

My Smart 30 takes a good 10-15 minutes or more to recover to 140F after a nice hot bath. But that's with "only" 57,000 Btu pushing it. When there's a DHW call the boiler goes to high fire. It takes a few minutes but I end up with 185F supply temp out of the boiler. The boiler seems to have figured out almost exactly when to shut the burner off in order to let system momentum finish the job. It usually cuts off when the tank gets to around 130F and then keeps the circulator on until the tank tops out.

You can tell I've been spending way too much time hanging out in the boiler room.

SWEI

hot rod said:

SWEI said:

It really depends on the indirect sizing and the system dynamics. We have a SMART 30 in a church that typically recovers in 6-8 minutes using a 174°F SWT.

Recovers to what setpoint temperature? 120F?
If so 174- 120 is a good delta T. What kind of flow rate?

I'd have to go back and look at this point, but IIRC the DHW setpoint might be 126°F. Small draws from hand washing most of the week when the preschool (which that boiler heats) is in use. Thursday AM the men come in at 0530 to cook breakfast and they use a fair bit of hot water for that, but the SMART 30 replaced two 50 gallon 50k tankers that had rotted out. They didn't believe it would work when we first proposed it, but we have had zero complaints.

Flow rate on the DHW is mostly hand sinks during the week, plus one mop sink (early AM cleaning crew.) The kitchen has a big triple sink that draws about 3.5 GPM at full open.

Flow to the boiler is via an HEC-2 set for 30°F ΔT. Short runs of 1" black iron and F1960 PEX. Haven't measured anything there because it just works.

NYplumber

Experiment a bit if you care to. This tank is a tank in tank style not a coil in tank. The coil in tank design thrives off high velocity to scrub btus out of the water and through the coil. The Smart 30 is a tank in tank and has a bit of thermal mass as well as large passageways. While you can run it in speed 3, try other speeds as well as other supply water temperatures. Dont forget to keep a case of beer and a bucket to sit on handy for watching the boiler recover.