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water heater recir pump, is it worth it?

Posted by stash-hdy (My Page) on
Fri, Jun 29, 07 at 19:47

I need to replace my water heater, I am interested in information on attaching a circulating pump on it so that I get hot water faster. Do they work? cost? difficulty in installing. I am not interested in a tankless, it's not cost effective for the two of us.


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RE: water heater recir pump, is it worth it?

Worth it? In terms of pure economics, no. The heat-loss though the standing pipes (unless specifically and incredibly insulated) is tremendous. Big money-loser.

They do, indeed, work for getting "instant" hot water at your taps. They're typically not too hard or expensive to install. Many people have them on timers so at least there's less waste while you're sleeping. Depends on how important that convenience is to you. You'll see the difference in your energy bill at once -- and you won't like it. I had one of these in a house I bought. It worked just fine but the expense was nuts. I took it out. By comparison, water-waste was MUCH cheaper.

Tankless heaters are not the same thing at all. You still have to wait for the hot water to hit.


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RE: water heater recir pump, is it worth it?

Recirculator Economics

Many people have argued that installing a hot water recirculation pump will increase energy cost for hot water. Let us examine the true facts.

In current state of the art construction high-end custom-built single-family residential structures are averaging between 1800 and 2400sq.ft. For the sake of argument let us consider the worst-case scenario and base our calculations on a 2400sq.ft home.

Using the square root formula we could then say that if the house is square it would have a 49 x 49 base, whereas if it is a standard 1/3 x 2/3 oblong shape it would be approximately 30 x 80.

Let us once again consider a worst-case scenario. The water heater is in one corner and we have a bathroom in the opposite corner.

We then have a Hot water main line that runs parallel to the end wall until it reaches the center of the structure and runs the full length of the structure down the center. On the opposite end we have a 15 branch line from the main to the bathroom. We then have a 105 main line. Now for illustration let us further assume that the hot water main line is a " pipe.

A " pipe has a volume of 5.32cubic inches per linear foot therefore the total volume of the line from the water heater to the bathroom is 5.32cu.in/ft x 105 = 558.6cu.in. One gallon of water occupies 231cu.in therefore the total volume of the line 2.41gallons. If we then consider that the code standard flow rate for a bathroom lavatory faucet is .5gpm it further stands that if the water in the line we at room temperature we would have a delay of nearly 5 minutes while we wait for a fresh supply of hot water to exit the water heater and arrive at the lavatory.

Now let us consider the economics a moment. Not only are we pouring 2.4 gallons of water down the drain, water which we no doubt bought from a municipal supplier, we are also paying sewer tax on that water, not to mention that we previously also paid to heat that water.

Let us now consider what it cost to heat that water. Assuming the cold water is entering the structure from a buried water line then the average temperature of the cold water supply will be 55deg.F. Typically the water heater is set to the code-mandated 125degF so the differential temperature is 125deg.F. 50deg.F. = 75deg.F.
One BTU (British Thermal Unit) will raise one pound of water one degree of Fahrenheit.
Water weighs 8.34lbs/gal. Therefore 2.4gals will weigh 20lbs and we have a 75degF differential so it would require 20lbs x 75degrees = 1500BTUs to reheat the water in the water heater that replaces what we just ran down the drain.

Typically a 50gallon natural gas fired water heater has a 36,000BTU/hr burner which would then be 36,000BTU/hr / 60min/hr = 600BTU/min. thus it would require 1500BTU loss /600BTU/min.= 2.5minutes burn time just to make up the water that we poured down the drain.

Now let us consider this fact. An un-insulated hot water line can cool to room temperature in as little as 15 minutes and if these lines run through a cold basement or crawl space that time would be considerably less.

Typically a 3BR residential structure will have a family of four and statistically each resident will go to the bathroom at least 4 times a day. This means that the loss could occur 16 or more times per day, thus our net loss will be a minimum of 38.4gal of water and 40 minutes of gas burn time per day. If we then consider that we statistically consume an average of 100gal of water per person per day a family of 4 would consume 400gal a day, yet we are wasting nearly 40gal, which accounts for 10% of our daily water consumption.

Now let us examine the economics of a re-circulation system.

Some argue that we must now pay to operate a pump, but let us see what that actually cost of operating that pump is?
Typically the pumps used for re-circ systems are a 1/40hp pump. Allowing that 746watts of electrical energy equals one horse power we can then say that a 1/40hp motor will draw 746watts/HP / 40 = 18.65watts.

For maximum efficiency the pump should be controlled by differential temperature on the return line by example, If we have the water heater set for 125degF water we would then set the pump control to start the pump when the water in the line cools to 115degF.

The plumbing code requires that when we install a re-circulation pump all hot water lines, both the distribution lines and the return lines MUST BE insulated. With proper insulation the pump will run an average of 3 times per hour. The pump only needs to run long enough to move a fresh supply of hot water from the water heater to the point where the control sensor is placed. In the above example if that sensor was placed in the bathroom the total volume of water that must be moved would be the 2.4gal of water which stands in the line however to make it easier to connect the control system we typically place the sensor right next to the pump on the return line at the water heater. This means we would have 2.4gal of water in the " line. If we then have a " line as a return line from the bathroom to the water heater we have approximately one more gallon of water in the return line (1/2" pipe has a volume of 98 per gallon), for a total of 3.4gal/cycle.
A 1/40hp pump is rated for 4gal/min so the actual operating time would then be slightly less than one minute per cycle and allowing an average of 3 cycles per hour we get a run time of 3 minutes per hour.

The pump consumes 18.65watts x 0.05hrs or 0.9325watt/hours per hour of operation.
This would then be 0.9325watt/hrs x 24hours= 22.38watt/hrs per day. This is then 0.2Kw/hrs per day and allowing 30days per month it would then be 6kw/hrs per month. In my community we pay $.10 per KW/hr for electric so the actual operating cost of the pump is 60 cents per month or $7.20 a year.

Let us now consider the other cost factors. Without the re-circulation system we are typically pouring 40gallon or more of water down the drain daily or 1200gallons a month. Keep in mind that we are not only paying for that water, we are also being charged sewer tax on it as well. With the pump the water is being returned to the water heater so the water loss is zero. In my community the cost of water is $5 per 1000gal thus the net savings on water alone is $6/month + 66% for sewer tax which would be another $3.96 for a total net savings of $9.96 a month. As we can see the first months water bill savings alone will more than adequately pay the annual operating cost of the pump.

Previously we concluded that if we do not have a re-circulation system, but rather pour the water down the drain, we then had to input an additional 15000BTU to make up the heat loss.
With the pump our differential temperature is now 125degF 115degF = 10degF per cycle.

We are now moving 3.4gal/cycle x 8.304lbs/gal = 28.23lbs/water and one BTU will raise one pound of water one degree of Fahrenheit so our net energy loss is now 28.23lbs x 10degF = 282.3BTU per cycle for a net energy savings of 15,000btu 282.3BTU = 14,717BTU/cycle. The energy savings is then 99.99% per cycle.

Oh yes. And did we mention that that upside is instant on hot water at the remote location?

When all things are considered I personally dont see how we could pass up a re-circulation pump.


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RE: water heater recir pump, is it worth it?

Very interesting and due respect. However, your "true facts" never paid my bills.

The savings I realized from removing the recirculation pump were sizeable and instant.

If it works for you, bravo. I shall wait with interest for the plethora (doubtless!) of respondents telling us how much they saved by installing such systems at their own locations.

Good day.


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RE: water heater recir pump, is it worth it?

My home is a single story with the WH in the garage at one end and the kitchen at the other end. Used to take almost a minute to get hot water to the kitchen sink.

I installed a retrofit Laing Autocirc pump under the kitchen sink about 4 years ago. Plotting my utilities since the install I save about $100 a year on my water bill and never saw an appreciable increase in my electric bill.

Hot water at the kitchen sink in less than 10 seconds worse case and usually instantly. All the fixtures back from the kitchen to the WH have hot water immediately.

Only criticism is that there can be warm water for a couple seconds at the cold side of a fixture because the pump uses the cold side to recirc. This is easy to live with to gain instant hot water and save $100 a year on the water bill.

The pump paid for itself in two years and from then on it's found money.

If I was building new or could retrofit a proper recirc I would do as lazypup laid out but not wanting to rip out walls and with the house on a slab the Laing Autocirc does a fine job.


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RE: water heater recir pump, is it worth it?

Well, fair enough. Perhaps I've got an education coming. Shall hope for others.


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RE: water heater recir pump, is it worth it?

Rechecking my records, the $100 savings per year I realized from installing the Laing Autocirc were mostly lower water bills but also included using less KCl (@ $7.50 per bag) for my water softener.

I did watch my electric bill carefully for about a year and really never saw an increase. The pump is low wattage and only runs about twice an hour for a minute or two. The pump has an appliance like timer built in so it can be set as inoperative for "x" hours but I never bothered.

The Laing Autocirc is a non-intrusive install so anyone can try one without any cutting or soldering.

I think they sell them at Lowe's and Home Depot sells the Watts version of the same product. Both have liberal return policies.


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RE: water heater recir pump, is it worth it?

Location I referenced is older block construction (huge single-floor rambler) with uninsulated copper tubing throughout. Older re-circ. design without timer. Suspect that configuation would result in greatest heat loss and other inefficiencies...perhaps the reasons for my dismal experience.

Maybe time for re-think for this old boy. Thanks for post.

Yo lazypup...thanks to you, too. I'm over it.


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RE: water heater recir pump, is it worth it?

thanks, I did a quich search on the Laing Autocirc and found 2 models, will need to research which one is best for my application. The other question I had was also answered, where to place the pump. I had assumed that it should be attached to the water heater but it should go at the longest run in the supply line. Thanks again


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RE: water heater recir pump, is it worth it?

I should have read the 2 Laing Autocirc model write-ups. Any thoughts on which is a better application. Attached at the water heater or at the furthest point from the water heater.


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RE: water heater recir pump, is it worth it?

Appears to me the two offerings may be intended to simply cover whatever the easiest install-site may be due to outlet availability or room. Notice the wattage consumption of the tank-install model is less but not by enough to get excited about. Both appear to operate on the same principal.


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RE: water heater recir pump, is it worth it?

The basic question is do you have electricity under the sink which is the farthest from the WH. If yes, then the model 1 that is pump and valve works nicely and is the easiest install. If no, then the model 2 is what you need.

I've had the opportunity to use both at my home and in my house they both work the same.

But, as I posted, my house is perfect for this type of recirc retro-fit being long with the WH at one end and the kitchen sink at the other.


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RE: water heater recir pump, is it worth it?

Lazypup,
I can't help but disagree with you based on the very fundamental fact that the heat loss is in the piping and is proportional to the difference in temp between the ambient temp outside the pipe and the pipe temp.
Now for your instant hotwater, the average temp in the hot and cold water piping to the usage point is, say 110 deg and if the ambient is say 65 deg, the delta Temp is 45 deg. On the other hand, with the normal system that delta Temp on average would be close to zero, save for the occasional heating due to usage. If you assume 10 deg of delta T on average, the loss ratio of the instant over the standard is about 4. This doesn't even account for the doubling of the pipe due to heating the "cold" water line in the loop.
Now you can try to make the case that you lose replacement heat when you open the hot water tap, but that is the result of the cooling down mechanism described, nothing more. Your cooling down is more insidious and drastic due to the higher delta T.
I will grant you that the loss of water is high in the normal case but from an economic standpoint water cost is insignificant.
I'm with Asolo on this one.


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RE: water heater recir pump, is it worth it?

What is recommended for a home system with a softener(Autocirc or Autocirc2)? Where in the system do you install the pump (Autocirc2) on a system with a softener?

-TM


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RE: water heater recir pump, is it worth it?

As I posted earlier in this thread... "the basic question is do you have electricity under the sink which is the farthest from the WH. If yes, then the model 1 that is pump and valve works nicely and is the easiest install. If no, then the model 2 is what you need.

I've had the opportunity to use both at my home and in my house they both work the same".

I have a water softener and it has nothing to do with these retrofit recirc pumps. If your water softener is connected directly to the cold water inlet of the WH then you still have hard water and your softener is a waste of money but that's another subject altogether.

With the Autocirc(1) you install the pump/valve combo at the farthest sink if you have electricity there.

With the Autocirc (2) you install the valve at the farthest sink and the pump at the WH, but you need electricity there.

You can find the detailed install docs here... http://autocirc.com/

Try it, you'll like it.


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RE: water heater recir pump, is it worth it?

Thank you very much!

-TM


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RE: water heater recir pump, is it worth it?

I trust lazypup to know code, chapter and verse. On the other hand, I have seen him seriously misuse "facts" to make a case against tankless water heaters. I think there is a great bit of stretching in the model that he has created above.

The truth is that the cost of running a recerc system on your hot water is going to vary, considerably, based on the total cost of water (water and sewer), the cost of energy to heat that water, the design of your plumbing and water usage patterns.

To assume that hot water lines are not insulated in a situation without a reticulating pump and are fully insulated in a situation with a pump isnt reasonable particularly in retrofit applications. I understand that its code, but many of these people are DIYs that are installing a pump BECAUSE there is a demonstrated problem such as uninsulated pipes and excessively long runs. In these situations, they are often using the cold water line as the return to the heater (with no insulation).

There was a person who posted about not having hot water in the shower in the master bath. The water would eventually get hot if he ran the tub faucet on the same fixture, but not the shower. It turned out that his hot water line was in the slab, was a long run from the heater to the master bath and the pipe wasnt insulated. In other words, at 2 gpm, he was losing most of the heat in the line to the slab (this was the winter also). Now, you put a pump on that and I assure you that you will see a really hefty bill for heating water.

Before going with a recalculating pump, insulate your hot water pipes. This will greatly reduce the amount of water that is wasted waiting for the water to get hot because it wont cool off as quickly. If you arent willing to insulate the pipes, dont put in the pump.

Also consider a timer, or better yet a motion detector for activating the pump. One person just wanted the pump on his master bath that was just used in the morning and evening. A motion detector worked fine for heating up the water just when he needed it without the standby loss otherwise.
If it's impossible to insulate the pipes, go with the motion detector to activate the pump.

Justalurker claims to have saved a $100 a year in water savings. Thats a lot of water. He doesnt say how his water is heated, but its clear that the cost of running the pump isnt the issueits the cost of the heat loss in the pipes.

The right answer is much better designed plumbing in the first place, including adequate pipe insulation and more centrally located water heaters.


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RE: water heater recir pump, is it worth it?

Jake,

As I stated, IF I were building a house from scratch or had access to my plumbing I would opt for a conventional recirc system BUT my house is on a slab and I don't want to tear out a bunch of walls.

My water is heated with an LP water heater. There are two of us in the home. The home was built in the mid 80's and is at 7000 feet above sea level so it is better insulated than some homes of that vintage.

Our water meter clicks over every 1000 gallons. My average water usage was 4000 gallons per month and every 5th month or so it would drop to 3000 gallons on one months bill then back up to 4000 gallons.

With the Laing Autocirc the monthly bills dropped to 3000 gallons for four months and then one month at 4000 gallons. That savings coupled with the savings in KCl for my softener (less water used, less softenring needed, less KCl used) @ $7.50 a bag and having hot water at the kitchen sink in seconds was a win-win all the way around.

LP usage for 4 years with the Autocirc was within $25+- of five previous years so I didn't look at it in detail. Remember, the Autocirc only runs for a minute or two a couple times every hour so you're not necessarily firing the water heater more often.

The OP seemd to be asking about a retrofit recirc pump and that's what I replied to. Obviously plumbing a conventional recirc at time of build is the right way to go but for those who can't easily retrofit and would rather do something the Laing is a complete success with no downside I can see after 4 years.

While I respect the math, and for sure forethought and premeditation are wise, I am living this experiment and I am writing the checks. I use less water which costs me less money and is more enviromentally sound. I use so close to the same electricity that I can't calculate the difference. I have hot water at the kitchen sink in seconds rather than waiting a minute plus.

With respect, my four year, real world, everyday experience trumps supposition and conjecture.

This has been covered in other threads on this forum and while some have looked upon the retrofit recirc negatively I can't recall a single poster who had a retrofit recirc with anything negative to say about it.


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RE: water heater recir pump, is it worth it?

I love my recirculating hot water system. The kitchen sink is 40' away from the 50 gal electric hot water heater, and there is a rocker switch at the sink to activate the pump. Very simple. It was installed when the solar hot water system was added in 1980.

We live in the country, have a well and septic sytem, and are all electric (but soon to switch to LP bec I want a gas range). The electric useage is nil, still the orignal Gunfros pump, and I'm not wasting precious water or overloading my leach field.

Now I need to research whether to replace the 50-gal electric with a 50-gal LP or a tankless water heater ...


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RE: water heater recir pump, is it worth it?

There was a person who posted about not having hot water in the shower in the master bath.

that person was me. i thought about putting in a recirc pump, but have since abandoned that idea unless i totally replumb the house. even now, with outdoor temps in teh upper 90's, you cannot get HOT water at the 2 lavatories in that bathroom unless BOTH are running. the water gets luke warm, but not hot. i have not measured with a thermometer, but i would say that they are a good 10 degrees cooler than the tub spout hot water temps. using LP's figures, that means a minimum of 1 gpm flow now, and during winter at least 2.5 gpm since the old shower head flow was just a hair too low to get hot.

with the price of copper right now, i will live with it. plus we rarely use hot water at the sinks anyway. i shave while my wife is in teh shower, so the water is hot enough for me then. if i need to shave at a different time, i simply turn on both sinks. no sewer fees for me, and my water is never over 29.00 a month, though i am sure my septic would appreciate using less water!


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RE: water heater recir pump, is it worth it?

Lurker,

The bulk of my criticism was directed at the model that lazypup laid out. It looked good, but the most glaring issue, as I pointed out, was the assumption about insulated pipes that wasn't at all reasonable.

The other criticism was a general one for the trend of bad original design / poor workmanship -- particularly putting a water heater at one end of the house and a faucet 40-50 feet away. Add to that uninsulated pipes, particularly in a slab, and it's just ridiculous both in terms of wasted energy and water. We shouldn't be designing residential plumbing systems that way.

I am glad that you found a solution to your problem. It sounds like you had a situation where you were dumping a lot of water down the drain and/or water was very expensive. One thousand gals a month seems like a lot and may be an extreme case. My point was that it won't be the same for everyone and will vary by usage pattern.

Most of the people who are looking for a solution to their hot water delay are focused on the delay and the water usage. If they solve those problems, they may be very happy.

I am trying to point out that the tradeoff is in standby heat loss -- and that can be mitigated in some cases with a motion detector switch, a manual switch -- as napagirl has-- or other means. Certainly insulating the pipes when they are accessible is the first place to start.


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RE: water heater recir pump, is it worth it?

Here is a data point for you all.

I installed a Laing recirc pump (uses cold water for return) and a solar water heater here in Southern California this summer. I liked getting the hot/warm water sooner. I didn't like my energy bill doubling from about $130 to $260/month (and that was with the pump ws only operating during daylight). The heat loss from recirculation was more than the sun could handle, forcing the backup heater to work pretty hard.

I have a single level 1700 sq foot home built in 1970.

The pump is now turned off. Oh well.


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RE: water heater recir pump, is it worth it?

"...the assumption about insulated pipes that wasn't at all reasonable."

Since you are required to insulate lines for a recirc setup. it is completely reasonable.

All insulation will do for a regular system is stretch out the cooling off period of the water, it cannot stop it from cooling off.

If folks are installing recirculation systems and NOT going back to insulate the lines as required it is there problem, not an issue with an analysis that takes into account code required issues.


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RE: water heater recir pump, is it worth it?

I wanted to bump this message because I have just installed a Watts and do not see any feedback here. I will keep track of my electric bill and inform you next month.


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RE: water heater recir pump, is it worth it?

Disregard my comment on the Laing autocirc. The problem is not the pump, but my water heater.

Now that I have my heating problem fixed, I'm using the pump and enjoying the extra convenience it provides. I'll come back and provide some new real world data in a couple of months.


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