January 18, 2009

How much exactly would an Autocirc1 save in energy costs?

Previously I discussed a hot water recirculator pump called the Autocirc1 in the article Autocirc pump to save water, money and get hot water faster.

The Autocirc1 is made by Laing. The Autocirc1 pump is designed to keep hot water in your pipes at all time. The pump saves money by not wasting hot water so the manufacturer claims that it will save you on your energy costs. However the pump keeps the water in your pipes relatively hot most of the time and this will cost you money.

A commenter Tony said:

"I am not convinced that this device will really save money for many people. By keeping hot water in the pipes, constantly drawing it from the tank when the pipes get cool, you're forcing the heater to heat more water than it would otherwise do. That will reduce or even outweigh the savings from the water consumption."
Thats a good point and I wanted to get a better idea for myself about how the Autocirc1 would or wouldn't save money.

So the question today is: How much money will an Autocirc1 actually save and how do you figure the savings?

Before I begin, let me state specifically that these are very rough numbers and I'm making estimations here. There are some variables I'm ignoring on purpose to simplify things. I'm doing a fairly crude estimate on purpose here because I can't realistically do a perfect model or it gets way too complex.

This pump keeps hot water in your pipes all of the time. This means you get hot water at your faucets instantly and you don't have to run the water for a minute or so to get hot water. The maker of the Autocirc1 claims it saves homes $100 to $300 annually. But unfortunately they do not go into specifics on their website about HOW you save energy.

First of all you have to look at how it saves money. The key concept is that it takes more energy to heat water up to the 125F that your hot water heater keeps it at than it takes to keep water at a 85-95F level.

Normally water comes into your home at 40F and then your hot water heater heats it up to 125F. When you use some hot water in your home and then turn off the faucet all the water in your pipes is still hot and just sits there. That water cools of pretty quickly and is effectively wasted. The longer your pipes are the more water will be wasted. In addition to not having to heat the water you also don't have to pay the water utility for the water itself.

On the other hand, with the Autocirc1 it works to keep the water in your pipes at 85-95F all of the time. That means it only has to heat up the same amount of water by 10F degrees periodically when it cools down. Its a limited amount of water that it is heating as well.

Lets look at a couple families with different homes and different situations. These two examples are meant to represent two extremes and I think most homes will fall somewhere in between.

I start with some assumptions. First of all I assume that it costs 2¢ to heat a gallon of water by 80F (as discussed previously). I also then assume that it costs about 1/4¢ to heat a gallon of water by 10F.
I assume that municipal water bills cost 0.2¢ per gallon of water on average. I assume that the pump recirculates the water 3 times an hour average.

The Smith House:
Their hot water pipe is 3/4" and their hot water heater is about 90' from the bathrooms on the other end of the home. They have an electric hot water heater and pay 11¢ a kWh. Their hot water pipes hold about 2 gallons of water and it costs them 2¢ to heat a gallon of water 80F.

With normal pipes:
Normally you run water 20 times a day spread out about evenly through the day. That means that any time you use hot water you will pull 2 gallons of hot water out of the tank which then fill the pipes. You then use up some water and leave those 2 gallons of hot water sitting in your pipes. That hot water then cools down over time. This is effectively 2 gallons of hot water you're wasting every time you use hot water. A gallon of hot water takes the Smiths about 2 cents to heat from 40F to 125F using their electric water heater. So you're wasting around 80 cents a day. Thats $292 a year. You're also wasting 40 gallons of cold water that you run down the drain waiting to get hot water at the tap. You have to pay your water bill and that water costs money. At 0.2 cents per gallon that amount of water would run you about $29.20 a year in extra water bills. So total cost is $321.20

With the Autocirc1:
Now lets say you have the circulator setup. It keeps the water in your pipes constantly at 85-95F. The circulator pump has to kick on every 20 minutes to circulate that water. So in effect you're heating up 2 gallons of water 10F degrees every 20 minutes all day. You have the timer setup so it runs 16 hours a day and shuts off when you're asleep. That means you're heating it about 48 times a day. That would cost about 24 cents a day or about $87.60 to heat the water.

The Autocirc1 would save the Smiths $233.60 per year.

The Jones House:
This house is the same design as the Smith home. But the Jones family is a couple with no kids who uses their hot water faucets just 6 times daily.

With normal pipes: Normally they run the water 6 times a day at 2 gallons each. It costs them 2¢ to heat the water and 0.2¢ per gallon to buy it from the water utility. They waste 12 gallons of hot water a day or about 4,380 gallons a year. This costs them a total of $96.36 a year.

With the Autocirc1:
Their costs with the Autocirc1 would be the same as the Smith family which were $87.60 a year.

Total difference for Jones family would be $8.76 savings per year with the Autocirc1.

Keep in mind that these are very rough calculations and the figures will vary a lot based on the exact variables. If you vary the size of the pipe, water costs, electricity or gas costs, and average ground temperature of the water than that will impact the figures.

How did Laing get their numbers?

On their website Laing says that the Autocirc1 will save people energy costs in the $100-$300 range per year. They have a study they did to come up with these numbers. Their website however doesn't give the specifics on how they figured it. About their study they say: "Above summary based on study and analysis prepared by Edward Saltzberg and Associates, consulting mechanical engineers. Copies of this detailed analysis are available on request from Laing Thermotech, Inc." So I went ahead and sent them an email to ask them for a copy of that study and they quickly sent me it. I looked through their study to see how they figured the savings. They used the same basic ideas that I did. The Autocirc1 would save you money by not having to heat the water and based on water bills but it would cost you money based on keeping the water in the pipes relatively hot.

I'm not going to go through their entire study line by line or examine everything they did. If you want to look at their study then I'd recommend you just email them and ask for a copy. I will say though that it seems their conclusions are fairly realistic and I don't see any major faults in it, but it is based on certain set of assumptions.

They figured that a family would waste about 34 gallons of hot water daily. They are looking at a family of four and assuming they'd use the shower 4 times daily and sink uses of about 11 times a day. Thats a fairly heavy water use so if you have a small family and/or use water infrequently then your experience will differ.

They assume a water cost of $2.02 / 100 cu ft and $1.35 / 100 cu ft. Theres 748 gallons in 100 cu ft so this equates to about 0.2¢ for water and 0.1¢ for sewer. Those assumptions are in line with average water costs, however water utility cost varies a lot from city to city so again your circumstances may differ.

They state that heat loss from a 3/4" copper water pipe is 25 BTU per hour per linear foot for uninsulated pipe and 10 BTU /hr/ft for insulated pipe. This is a key piece of information for the calculations. With this number you can easily figure the cost of keeping the hot water warm in the pipes. With this you can figure the costs pretty easy to operate the Autocirc1.

Lets say your water pipes are uninsulated and 60' long and you keep the Autocirc1 on 16 hours a day.

= 60 ft x 25 BTU /hr /ft x 16 hr/day x 365 day/yr = 8,760,000 BTU /yr

There are 3413 BTU per kWH. So that means you'd be using 2566 kwH a year or about $256 a year with electricity of at 10¢ /kwh. There are 100,000 BTU in a therm so it would take 87.6 therms and if therms are $1 each then this would cost about $87.

For insulated pipes the costs would be 40%. So that would be $102 for electric and $34 for gas.

From Laing's study the cost of keeping the water in the pipes hot would be $34 to $256 for 60' of pipe.

On the other hand they are figuring that your water savings are about 12,000 of hot water per year. With hot water costing 2¢ to heat and 0.3¢ for utility costs thats about a $276 cost for the wasted water.

My methodology and that used in the Laing study differ a bit. But it is clear that the savings from an Autocirc1 will vary a lot depending on the usage level of home. The more water you use and more often you use your plumbing then the more the Autocirc1 would save. However if you do not use your plumbing very frequently then the Autocirc1 may not really save you much if anything. And any application of an Autocirc1 is assuming that the home has fairly long pipes running between the hot water heater and the sinks.



Some references:
Online discussion on Autocirc savings
BTU conversion figures

1 comment:

  1. Jim,
    Thanks, this was a very helpful analysis. Our main concern is not wasting water (especially if we have to ration some year) and convenience. I was concerned I might be wasting energy with the reheating of water in pipes but am now convinced that I may actually save a little energy.
    Kevin

    ReplyDelete

Blog Widget by LinkWithin