How Green is My Recirc?

Mad Dog_2

Mad Dog

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Don in Albuquerque

water savings vs. energy waste in recirc

The local utility auditor told me that recirc systems are a huge energy hog, totally offsetting the water savings. All that pipe running hot water through the house radiates in the slab, walls, or attic, most of which is wasted in the winter, and fights home cooling in the summer. Then there's the electricity for the pump. A thermostat and timer only mitigate the waste. Is the ten gallons of water (and thirty seconds) wasted waiting for hot water worth the increased gas and electric usage? Has anyone done the math on this?

Gordy

recircs

My system is gravity induced. Not all houses lend themselves well to this. You don't need much flow to keep the pipes hot. I don't notice my water heater cycling between usess either. So in my case I don't believe it is an energy hog for the benifit I gain.


Then there is legionella lurking in there to.

Gordy

CC.Rob_3

no math

Haven't done the math, but I'm guessing recirc in a normal house is a waste.

Constantin

A matter of implementation?

Recircs do not have to run 100% of the time. Many recirc programs like the one on the Vitotronic can be triggered externally, and I will eventually upgrade my home system to pretty much only trigger whenever someone steps into a bathroom. That should cut down the amount of recirculation dramatically, reducing heat losses and energy consumption (by the time the clothes are off, the water is warm).

As for whether 10 gallons is a waste or not each time you run a shower, I guess it depends where you live, how many times a day that kind of water is wasted, etc. Plus, you could always use the shower/tub drain as a grey water supply for the garden (depends on local codes) and put all shower/tub water to double use.

Dave_4

DHW Recirc

Try looking at something like the Raychem HWAT heat tracing cables. Self regulating, low electrical draw, and you can wrap it around the DHW pipe to within a couple feet of your faucet for virtually instant hot water. Might be tough to retro-fit in an existing home, but it might be an effective application for new construction- save that copper recirc line (seen the price of copper lately?) and heat trace and insulate your DCW and DHW lines to minimize energy leakage and keep water waste low.

scott markle

recirc.

Good question, maby LEEDS publishes articles about this. It seems to me that as with other energy issues, regional considerations factor highly into appropriate building design. In the southwest water scarcity is a big concern, climate change could increase this problem. In these regions water may be more valuable than fuel. These areas also have huge potential for solar thermal, which could partly make the (heat loss from pipes) issue moot.

I have seen point of use motion sensors and activation buttons for recirc. systems. This sort of control would certainly minimize energy waste.

A house's size and layout ultimately decides if recirculation is a good idea.

1/2" PEX tubing = 0.0092 gallons / ft. From this I read that a 55' run holds about 1/2 a gallon. If smaller houses can accommodate this limitation, then recirculation is unnecessary. Smaller houses with smart compact mechanicals are greener because they use less resources In the first place. Designers need to think twice about about that detached bath room on the far side of the sprawling floor plan. Green is as much about common sense as number crunching .

jp_2

one step further

is to have a switch in the bathroom, before a shower or using hot water, flip the switch and the recir does its thing, a small indicator could tell you when hot water is there.

you stand and wait for hot water anyway, so nothing would really change.

only use when needed, then you save!

ALH_4

sensors

I was involved in a project that used motion sensors at the points of use with an adjustable timer that runs the dhw circ pimp for up to a minute. If I remember right, 15 seconds was enough. I have seen package systems for sale on the internet that do this. As I remember, they looked a little cheap.

Uni R_3

How much is your time worth?

Okay... for fun I'll try and do the math for my case. My layout is already pretty optimal. All hot water fixtures are already grouped in about a 30' wide area in the middle of teh house. The savings will only be a fraction of a long ranch or winged home.

Costs:

Added electrical costs:

Most recirc pumps are 25 or 33 watts. With a 25 watt pump running 25% of the time at $0.10/KWH that's $5.50 a year. With a 33 watt pump running 24 hours a day at $0.18/KWH that's $52. So I'll be using about $5/mo in extra electric.

Wasted DHW fuel costs for heating in the summer:

This one has the most variablity. If the house has a recirc loop of 80' and each foot emits an average (with less than 100% recirc in the loop) of 10 BTUs in the summer (180 days of unwanted added heat), then you'd need around 100 M3 of NG to heat that. At $0.50/M3 that's $50. That'll also need perhaps another $5 in electrical for boiler circ and blower requirements. In my case, there is no wasted heat for at least half the year because the hot water piping runs are all in the house.

So $60/year in extra resource costs.

Fixed costs and total costs:

With a $400 investment for a circ, flowcheck, insulation and return piping, I'll factor in $40 a year to cover the fixed costs.

Variable and fixed costs total around $100/year in this case.

Savings:

Water consumption savings:

The water here works out to just under $5/1000 gallons, very inexpensive. If 0.6 gallons are wasted 4 times a day all year it is 900 gallons or $5 after taxes in saved water. This could easily be double or more but it is still a small number. regardless, it is hard to save when it is already a bargain.

Fuel savings for warm water not going down the drain:

There is heat in that water that's running down the drain. If we can assume that the water that is wasted is on average half-heated then there is 900 gallons to half heat, and we're also leaving 900 gallons of fully heated water in the piping each time. This works out to around 20 M3 of gas being wasted or $10. I think this may be understated.

So for possibly up to $100 per year in additional costs to have recirc, there may only be a savings of $15.

Time savings:

But then again, this net of $85 or less will save 2 minutes a day or more and that means about 12 hours a year not spent waiting for the tap to get warm. That works out to around $7/hr for freeing up time spent waiting for taps. Still not bad at all.

I actually think recirc still could actuatlly be far more neutral than this example, possibly even green positive.

It's a cheap luxury, and if it isn't green it's not exactly wreaking havoc with the planet - especially if you consider the true value of water. It's also safer for kids young and old if the shower temps or bath temps don't keep ramping up if you have simple taps.

*********************

Okay, so what insulation gives the best bang for the buck on this, and how many BTU/ft should you factor with 120° water inside the pipes and a 75° indoor summer temperature?

hr

look at this recirc

Plumbers in my area don't believe in insulation, timers or t-stat. This plumber tightly fastened the pex hot and recirc, both 3/4 for some reason, against the concrete basement walls. About 180 feet worth. Basicaly a radiant wall heating system!

Luckily the mod con boiler has a "hours in DHW" mode so I can show the homeowner the cost of running that system before he even moves into the home.

I asked the plumber WHY. It's the way the boss told us to do it!

hot rod

Bob Gagnon plumbing and heating

Brush your teeth

with the hot water before it gets hot. No wasted water, no wasted heat. I have lost more recirc jobs by telling people that. Bob Gagnon

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Dave Yates (GrandPAH)_1

some info

Don,

Here's a link to an article; http://www.contractormag.com/articles/column.cfm?columnid=584

And, a link to an on-line discussion that deals with the numbers too: http://forums.taunton.com/tp-breaktime/messages?msg=80109.1&redirCnt=1

Mad Dog_2

That's how we set them up.......Regular switch as you enter bath

The other crucial factor is insulation of all hot water lines and especially the recirc - which isn't always practicable due to inaccessible areas of the building. On a new installation, its the way to go, As Constantine said, you will have to weigh whether water or the energy used for you particular area of the country and the rates of each. It would be nice to see a cost analysis, but who has time for that???? Mad Dog

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Scott Lind_5

Additional Reading Material on Hot Water Distribution

The following links are to a series of 3 excellent articles on the topic. For my own home I chose to use two pole switches in all the bathrooms and I use the second set of contacts to control my hot water recirc pump and my heat recovery ventilator via a timeclock.

It's certainly not the cheapest alternative but gave me the ultimate in flexibility. The recirc pump is presently set up to run for 15 seconds after the switch is turned on and then it shuts off. The ventilator is not installed yet but it will run much longer - probably something like 30 minutes.

http://www.psdmagazine.com/pdf/Mar_Apr_04/36_39.pdf
http://www.psdmagazine.com/pdf/May_Jun_04/16_20.pdf
http://www.psdmagazine.com/pdf/Sep_Oct_04/14_19.pdf

jackchips_2

Nice

suggestion, Bob.

I fill the humidifier three times a day with the hot tap until running warm and I do it at peak usage times. My heaters are 30 feet from both my kitchen and my wife's bathroom.

It's not as much a problem in the summer with the incoming water and the room temperatures warmer.

One additional cost for wasted water around here (MA) is we are billed almost an equal amount for sewage usage which doubles most formulas based on water usage.

Jack