Recirculating HW energy loss

ChrisJ

I'm curious,

Is there any info out there regarding the cost of recirculating domestic hot water in a single family residential building vs water lost down the drain getting hot water to fixtures?

JUGHNE

I share your curiosity, I have pondered this for my non-pumped gravity recir system.
In the wintertime it is just more heat added to the envelope.
Other wise in the summer perhaps lack of SS pump/aquastat investments and power usage offset the water down the drain.
But the water is hot within 2 seconds at the shower and lav.

ChrisJ

JUGHNE said:

I share your curiosity, I have pondered this for my non-pumped gravity recir system.
In the wintertime it is just more heat added to the envelope.
Other wise in the summer perhaps lack of SS pump/aquastat investments and power usage offset the water down the drain.
But the water is hot within 2 seconds at the shower and lav.

I calculated around 1000 btu/h @ 130F loss in the setup I'm doing worse case, in the winter when the basement is chilly. Like you said, this will just add to the house.

In the summer it'll be warmer down there, so the loss should drop slightly but then of course I'm paying to have the air conditioner remove the heat I paid to supply. So it gets difficult to figure out.

I came up with a rough estimate of $3 per month averaged over 12 months.

hot_rod

here is the math to calculate recirc energy loss.

I'm not sure you could easily calculate water down the drain and cost, every faucet would need to be calculated, and at what temperature in the pipe and in the space.

I think the point in CA is water waste more so than heating cost?

https://www.caleffi.com/sites/default/files/file/idronics_11_na-r2.pdf

JUGHNE

Also, another factor is the groundwater supply temp into the house. It would take a lot of micro measuring to calculate all of this.
But a simple red neck method would be to determine the slowest drip out of your farthest hot faucet that would keep the supply pipe hot. That would give the GPH flow needed to have hot water at the ready.
With the gravity recir there is a nearly closed ball valve at the base of the WH tank then a 1/2" check valve flapper that it must crack/lift in order to maintain flow.

Larry Weingarten

Hello, This doesn't answer your question directly, but might get things closer. I've a hot water nerd-friend, Gary Klein, who tells me that running a 24 hour recirc line roughly triples your water heating energy bill. The most efficient system he's found is demand control. Here are some articles: https://www.garykleinassociates.com/writings.html

In the Monterey CA area, we have the most expensive water in the US, so efficient water use matters. An interesting tidbit is that you need to flush out about twice the volume in the lines before getting usable hot water, so keeping pipe volume low is important. If codes would let us use 1/4" tubing (assuming decent pressure and short runs) we could save a lot of water and energy for nearly everything but filling the tub!

Yours, Larry

ChrisJ

> @Larry Weingarten said:
> Hello, This doesn't answer your question directly, but might get things closer. I've a hot water nerd-friend, Gary Klein, who tells me that running a 24 hour recirc line roughly triples your water heating energy bill. The most efficient system he's found is demand control. Here are some articles: https://www.garykleinassociates.com/writings.html
>
> In the Monterey CA area, we have the most expensive water in the US, so efficient water use matters. An interesting tidbit is that you need to flush out about twice the volume in the lines before getting usable hot water, so keeping pipe volume low is important. If codes would let us use 1/4" tubing (assuming decent pressure and short runs) we could save a lot of water and energy for nearly everything but filling the tub!
>
> Yours, Larry

Hi Larry,

Triples? How can he make that statement? Wouldn't the loss be completely system specific?

When I added a Taco 006 and Aquastat to my parents Bradford White 50 gal power vent heater in 2016 they said they didn't even notice a change in lpg usage.

mikeg2015

To complicate the calculations, you have to subtract the wasted water and the time pipes are hot from NOT having a recirculation system.

Larry Weingarten

Hi @ChrisJ, Gary has been involved with and consulted on thousands of systems, so likely has an average in mind. His work is science based and data driven, so I’m confident it didn’t get pulled from a hat.

You’re, Larry

hot_rod

A DHW recirculation loop is essentially a hydronic ic heating loop.

The heat loss and energy consumption would be dependent on flow rate, temperature difference, and insulation value.

Loss to ambient could be about 1/3 more with uninsulated copper compared to 1/2 wall insulation.

There may come time when some buildings or states require 24/7 recirc at 140F for legionella prevention. Or at least a 1 hour blast daily at 140F.

Uninsulated tube would be a real energy hog, and additional load in AC season.

Jamie Hall

Be just a little cautious about energy loss. If the pipe in question is in an unheated space, then yes -- it is lost. But if it is in the heated areas of the building, it isn't. It's just another source of heat. Now depending on how the heat is generated, it may cost a small fortune... but it isn't lost heat.

Jean-David Beyer

I have recently installed a Taco HotLinkPlus-e recirculation system. It uses the cold water supply line as the return, with the hot to the cold connected by a clever valve that closes once the hot water reaches 108F. I have a problem with this because part of the supply circuit runs through the concrete slab at grade that is an enormous heat sink, and it takes quite a few minutes to get to the sink where that valve is. By the time the water gets there to close the valve, so much hot water has been pumped into the cold water supply that I must run the cold water for a minute or more to get it cold enough to drink. Even though I put 125F water out of the mixer valve at the indirect water tank output. So I may waste a little less hot water, but I waste a lot of cold water now.

The circulator plugs into a SmartPlug that monitors hot water use for a week, then replicates pumping for those times the next week. That is the theory, but it does not work that way. It does nothing the second week, and then in the third week it starts the circulator from time-to-time. I have been keeping a chart, and I cannot see a correlation in the water use in the past and the times the Smart Plug chooses to run the circulator.

And it does seem to run the boiler more often than before to keep the indirect hot, but I have not decided to sit in my garage for a week to watch the thing run.