dom ht water loop pump stat

MrHVAC Mechanic

When installing a pump to circulate the domestic hot water around a building, where is the proper location for the aqua stat??
To clarify, the pump would keep hot water at all fixtures so you don't have to wait for it to get ther at the fixture.
What also would be the proper temp setting?

Doug_7

DHW Recirculation Suggestions

I always figured the best location for the aquastat was on the recirculation pump suction. If it hot at the pump suction, it is hot at all fixtures.

Proper temp would be about 20* F less than the supply temp.

While you are at it, suggest you install a throttling valve on the recirculation pump discharge to set the proper flow to get a 10* F temperature drop from supply to return. I have a 3/4" globe valve that is 1/2 to 3/4 of one turn open.

This is not a case where more flow is better - it is a case where less flow is better. Too much flow will cause velocity-assisted corrosion and pin-hole leaks. You want pipe velocity of below 2 to 3 ft per second to avoid velocity-assisted corrosion and pin-hole leaks.

You always need to control and minimize the flow in DHW recirculation systems.

An AZELTEC DIGIT-STAT DS-60 DIGITAL TEMPERATURE GAUGE installed on DHW supply and recirculation return would be perfect for watching these temperatures and setting the flow to minimum. Alternately just strap the outdoor sensor of a Radio Shack digital indoor/outdoor temp sensor to the return piping.

Depending on the building, you can also install a timer to shut-off the pump at night or other times it is not needed. This saves energy two ways - pump energy and energy from constantly radiating heat from the DHW pipes. This may not be practical in multi-family buildings, because someone always gets up at 4 AM and wants instant hot water.

I always thought the best idea was a motion sensor in the bathrooms and kitchen to turn ON the pump whenever someone was there. I have never done this - just wish I had.

You can buy DHW recirculation pumps with the aquastat and timer built in.

Some food for thought.

Doug

MrHVAC Mechanic

Doug,
Thanks for your input.
I understand your point at the inlet of the pump.If the stast is satisfied at this location, obviously the pump will stop. Could this cause the lines at the last fixture to cool before the pump starts again?? Will it start before this happens?

Doug_7

Aquastat Experience

I did all kinds of experiments using an aquastat on the DHW recirculation system in a 21 suite condo. I first tried a Honeywell strap on the pipe type - but that didn't give very good temperature control.

Then I installed a tekmar 150 to get one degree control of the setpoint and differential. Works better but still not great.

The problem is - when the aquastat reaches setpoint it shuts off the flow and you are dealing with the rate of cooling of a stagnent DHW line. Will the stagnent line at the last shower cool down faster than the stagnent line at the aquastat ? Cooling rate depends on line diameter, insulation or not, ghost flows.

Also about the setpoint - If I set the aquastat 20* lower than supply it doesn't take that long to cool 20* so it doesn't stay OFF very long.

In our multi-family building, there is over 700 feet of piping involved, some of it large (2") diameter, creating a very long time lag between turning the pump ON and getting hot water to the last fixture. Finally I just put the pump back to running 24 hours a day - really because I couldn't get reliable enough temperature control in a multi-family building to avoid complaints.

In a single family building it would be a lot simpler. But rather than use an aquastat at all, I would wire up the pump to run based on a motion sensor in the bathroom - or even simpler - to run only when the bathroom light is ON.

What is the time lag between turning the pump ON and getting hot water to the last fixture ? If it is only one minute - no problem. I can't get undressed and jump in the shower that quick.

The aquastat will try keep the piping hot 24/7. A timer can shut the pump OFF at night.

But if actuated by a bathroom motion sensor or when the bathroom light is ON - then the pump is OFF at all times except when actually needed. You can go away for the weekend and save money and save wear and tear on the pump and piping system - because the DHW recirc system is OFF.

Think about it. See if the time lag between turning the pump ON and getting hot water to the last fixture works in your faavor. Depends on your house.

Doug

Doug_7

Question for MrHVAC Mechanic

MrHVAC Mechanic - Are you doing this in a large multi-family building or in a single family house ?

What kind of building ?

MrHVAC Mechanic

Job is existing 7 floor building. One apt per floor with their own htw storage tank and pump and boiler. Perimeter baseboard off same boiler.

You last post was very infomative. Thanks Doug.