DHW recirc loop?

bruce_21

Are you sure its separated tub vs sink and not different sides of the building? I've done them with two returns when everything was really spread out. It might need a couple valves installed to balance the flows.

Timco

Large building with DHW recirc loop. Leaves boiler room as 1.5" and comes back as 1/2" from 2 directions and into 3/4 through an old 15-42. Barely warm at pump. Odd thing is sinks in building are fine, tubs are all luke-warm water. Has anyone heard of 2 loops one for the tubs and one for sinks???

Tim

Timco

Only sure of 2 returns, both lukewarm. DHW goes out at 140* and comes back about 90*, and ties into the supply of the Laars mighty therm making the HW. Cleaned the FC, but I think the small 15-42 may be going out. Not bronze so it should be changed anyway...

Tim

Larry Weingarten

If you can...

... valve it so you can purge the two returns individually using line pressure... If you then get hot water while purging, it's a flow issue, suggesting a new pump. Purging might clean out the lines a bit also. Once that's done, temperature balancing the returns will want to be done. Any chance of access to check whether or not there is insulation on the lines?

Yours, Larry

Unknown

I'm with Larry

I'm with Larry on this one. Did a 3 stories home with 3 returns and NO pump. Yet all return have bal valve.

Doug_7

Recommend Insulation, larger lines and smaller pump

I has a very similar situation in a large building with a 15-42 DHW recirculation pump and 3/4" DHW return lines.

Building had seven 3/4" return lines coming back from seven suites per floor in a three story building. System involves about 600 feet of 2" and 1 1/4" DHW supply piping and about 600 feet of 3/4" DHW return piping.

Original DHW recirculation flow with the 15-42 pump was 9 GPM and line velocity was 6 feet per second in the 3/4" common DHW return piping. The DHW temperatures were all fine because the 3/4" return lines allowed enough flow.

The problem that showed up after 4 or 5 years was multiple pinhole leaks in the 3/4 common DHW return lines. Too high a flow velocity caused velocity assisted corrosion.

The solution was:

1) Insulate all the DHW supply and return piping which reduced the piping heat losses by over 60% saving money.

2) Replace the 15-42 pump with a SMALLER UP 15-10 B5 pump which provides 4 GPM and line velocity under 3 feet per second in the common DHW 3/4" piping.

3) Install throttling valve on DHW pump discharge and balancing valves on the shortest recirculation loops. Balance flows to get about 10º F drop on each loop.

Comments - If you attempt to solve this problem with a larger pump you will end up down the road with multiple pin-hole leaks caused by velocity assisted corrosion in the 1/2" piping.

!/2" piping is not recommended for DHW recirculation flow in larger buildings due to the velocity-corrosion problem. Look at replacing 1/2" piping with 3/4" minimum.

Insulating the DHW piping may seem expensive - but the payout is about two years on reduced DHW heating energy costs alone.

Insulation also solves the cold DHW supply problems and greatly helps avoid the velocity assisted corrosion problem.

Velocity assisted corrosion of DHW piping is a nasty and expensive problem. Building water damage.

You can get into DHW recirculation time/temperature flow controls but that is secondary - the basic design has to be correct to start with.

I have a lot of backup documentation on the before and after design of this DHW recirculation system.

I agree with flushing the 1/2" lines as a first step. After that there are no simple solutions or shortcuts.

How big and how old is the building ?? Any DHW leaks yet ?

Doug

Doug_7

Short Version

The 15-42 pump will be fine if you insulate all the DHW piping, replace the 1/2" return piping with 3/4", install balancing valves and balance the return flows to get a Delta T of 10º F.

Customer saves money by reducing DHW heating costs and avoiding expensive pinhole leaks down the road.

Doug

Doug_7

The trouble with 1/2\" DHW recirculation lines

I wanted to say a bit more about the use of 1/2' copper for DHW recirculation systems.

1/2" copper is OK for domestic water supply where you have a 60 psi driving force through a short length of 1/2" to a sink or tub. Lots of flow - 10 GPM no problem.

But 1/2" copper is not OK for DHW recirculation systems in larger buildings, because a pump such as a 15-42 only puts out 5 psi of differential pressure maximum. You can have some fairly long runs of DHW recirculation piping and the pressure loss per foot of 1/2" copper is fairly high.

If you had a 100 foot run of 1/2" copper - the most you could push through it with a 15-42 pump is about 2.5 GPM.

With a 50 foot run of 1/2" copper - the most you could push through it with a 15-42 pump is about 3.5 GPM.

To offset the piping heat losses you probably need about 9 GPM with uninsulated piping or 4 GPM with insulated piping.

If you try to use a bigger pump to push more flow through 1/2" copper DHW recirculation lines, the flow velocity gets too high and velocity-corrosion will get you.

So the best solution is to have minimum 3/4" lines, with all piping insulated and flows balanced with balancing valves.


What is the length of the longest continuous run of 1/2" recirculation piping ??

Is this a new building, or an old building with a new problem ?

Doug

JJ_4

Cross connection...

A little background: In my 3 story bldg there are existing re-circ circuits(3/4" that feeds back into the cold water supply for the water heater) on the 1st floor. When we installed low flow aerators the 2nd and 3rd floors took forever to get hot water (never on the 3rd floor). There are two lines (1" ea.) running up at each end of the bldg, so we installed a small recirc pump (Laing) on the 3rd floor in each restroom. Everything was going OK...until we got cold calls at one end, 2nd and 3rd floors. The plumber asked if there were any cross connections...turns out there is a mixing station for cleaning products on the 2nd floor (thus my OT post). When we turned off the faucets (separate hot and cold valves) the problem stopped...hot water again. Maybe in your situation a thermosatic mixing valve at the tubs is the cross connection? Just a guess.

Timco

Wow! Thanks for the replies. This is a 3 story, 40 unit building. One large rectangle. The two 1/2" returns disappear above the middle hall and a row of apartments, no way of determining how far they go. They both are about 90* as they enter the hall & laundry room prior to hitting the BR. I could disconnect above the WH's and back flush the 1/2" lines from above the FC. I thought of a bal valve but both returns are cool. The 15-42 turns fine when removed, but might it be dieing and not able to move the water? No noticable sound if I throw a iso valve. Hint...there ws a series 100 B&G on there at some time, not sure when the grund was installed, but looks within 10 years. Complaint is that it has been going on for some time, not all of a sudden, but there. I was just asked about it...just the tubs are cool. Sinks are fine.

Tim

Mark Eatherton

Sounds like...

possibly the anti scald adjustment needs set on the tub mixers, or the iron body pump is so full of crap that it is barely moving ANY water.

Try the swoosh test and see what happens.

ME