'On-demand' DHW Recirculation?

corey

Does anyone have experience with or opinions on 'on-demand' DHW recirculation systems such as Metlund?
The idea here is that the DHW recirc is only activated when the user flips a manual switch located near each fixture.
The recirc pump is substantially more powerful (8-14GPM) than would be typical for an automatically controlled recirc system for really fast response.
Since there is no load during activation, they claim you can use the cold water supply line for the return, but I have some doubts about this aspect of the design.
One study I read found that even the 'best controlled' mechanical recirc systems (with both sleep timer and aquastat) still wasted 23% of total DHW energy consumption in additional standby losses.

Recirc Study:
http://hem.dis.anl.gov/eehem/99/991109.html

Metlund 'On-demand' system
http://www.metlund.com/

Mike T., Swampeast MO

System sounds workable

If a bit complex in operation.

The study you referenced was compared to very hot (180°) water through uninsulated pipes in large buildings--don't know if it really applies residentially.

Before I got my gravity loop running, my wait for hot water was hideous in some places (I have a 1" hot water main & large tank-type gas heater). After the gravity loop, wait is 17 seconds at greatest (2nd floor lav [1/2" branch] fed off main in basement with about 18' of added horizontal run). Most locations have nearly instant hot water. I insulated all the DHW pipes at the time I installed the gravity loop. Energy use did not seem to change--if anything it went DOWN a tad.

Unless everything is finished, a gravity loop can usually be installed easily in an existing home. As long as you insulate the loop carefully (no need to insulate branches unless accessible) I really think this is the best way to get hot water quickly without wasting energy. No moving parts--nothing to break--no pump, etc.

If a gravity loop is impossible, the system you mentioned seems a good idea (if it works--and stays working--like it says.) It would certainly be much more energy-conscious than a continuously powered, uninsulated loop.

EricR_2

Try this for recirc

Grundfos has a new recirc set up that works great and isn't costly or difficult to install,

EricR_2

Try this for domestic recirc

Grundfos has a pretty easy and inexpensive setup and they work! www.grundfos.com/Web/HomeUs.nsf/Webopslag/PAVA-56TMVA

kevin

how would you pipe this style of recirc. line.

and how would that work if you HW heater hasa thermal trap in it? ty kpc

ScottMP

Corey

Your concern over energy wasted should inlude all that cold water going down the drain while you and the misses what for it to get hot.

Scott

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Brian (Tankless) Wood

I've installed lots

of these and they work very, very well. I'm installing one this afternoon.

If you need one let me know. My supplier has very competetive prices.

Brian Wood. (337) 430 4649.

www.autocirc.com or www.lainginc.com

Mike T., Swampeast MO

Here's the link...

http://www.contractormag.com/articles/column.cfm?columnid=62

It's a 2000 article by Dave Yates.

corey

3 Recirc Options

Thanks for all of the responses.
Many good points and links.

I see 3 recirc options:

1) Gravity loop
2) Mechanical 'standby' loop
3) Mechanical 'on demand' loop

Gravity Loop:
Costs: Tubing/fittings/valves + piping labor
Benefits: Simplity, No moving parts. No electricity consumed. Easiest install/lowest install cost. Will work in my home (appropriate rise to induce circulation).
Downside: Always 'on'. No nightime shutoff or aquastat control. Probably highest standby losses.

Mechanical 'standby':
Costs: $250-300 (Grundfos) + Tubing/fittings/valves + piping labor + circ wiring labor
Benefits: Automatic. Very controllable- nightime shutoff and aquastat to minimize standby losses.
Downside: Moving parts. Consumes some electricity (not much). More expensive than gravity. Some extra labor over gravity (mostly wiring).

Mechanical 'on-demand':
Costs: $345 (Metlund) + Tubing/fittings/valves + piping labor + circ wiring labor + switch wiring labor
Benefits: Lowest standby losses. Somewhat less electricity consumed than mechanical 'standby'.
Downside: Extra manual step to activate system and 5-10 second wait before draw. Most expensive. Moving parts. Consumes some electricity (not much). Extra labor of installing several manual switches. (Their wireless remote control option does not appeal to me).

Note: I have decided against using the existing cold supply for the recirc return, which could give both mechanical systems a piping labor advantage over gravity. Sometimes you really want cold water. Trade waiting for hot water for waiting for cold water? Does not seem sensible in most cases, unless access for installing a dedicated loop is really bad.

Decisions, decisions...