Standby consumption on a plain electric tank heater would be about twice that (in $/mo) - so sounds right.

More interesting would be to to fill the tank with water of a known temperature and then measure how many kwh it takes to get it to 120F (keep it well mixed).

I'm skeptical that during a time of no DSH operation the AirTap would run every 4 hours for 1 hour. Rather, I'd expect it to run whenever there has been a substantial draw of hot water.

No way would I expect it to run from midnight until 1 AM and then again from 4AM to 5 AM if in the intervening 3 hours there has been no draw of hot water.

At a conservatively estimated COP of 2.0 it would deliver 4000 btuh to the tank in 1 hour's operation, and standby losses from that tank during the next 3 hours should amount to nowhere near that amount.

I know it's not the fridge...that blips on every 60 minutes for 10-15 minutes, drawing 110 watts during that time.

The AirTap was last on this morning at 3:40 AM for about an hour, and before that at 10:20 for about an hour, before that around 6 PM. That means its due on around 9 this morning. I can't think we're going to use much hot water this morning, so I'll unplug it and see if that truly is the spike I'm seeing.

I love a mystery!  Just read a review on Amazon about Airtap...Guy said his comes on once or twice a day for about an hour, depending on HW use.  He's in Texas in case that affects water temps/air temps/etc. 

*** UPDATE ***

It's definitely the AirTap.  We've used almost no hot water today, and the AirTap came on again (like clockwork) around 9, and has been running for the last 35 minutes.  (It last came on at 3:40 AM this morning.)  Turning the AirTap off stops the power drain, so it's definitely the AirTap.

Posted By stuart.wyss on 17 Sep 2010 01:32 PM
I've started graphing (Google PowerMeter) my electric use...

Which of the PowerMeter options you are using to record information?  Sounds neat. 

4000+ btu per 4 hours is about right for standby loss. This is why some people use tankless water heaters.

I have the Current Cost Envi hooked to Google Power Meter through a laptop.  Here's a screen shot!  Data is uploaded every 10 minutes.

I would have thought (from internet research) that btu loss is about 300/hour, or about 1 degree per hour.  Since the thermostat on the AirTap is set to 120 and the manual specifies a 10 F start/stop setpoint, that would mean my tank cools 10 degrees in 4 hours, or about 2 1/2 degrees/hour.  I'm losing about 1000 BTU per hour.  This *does* seem high.

JONR -

I drew upon all three of:

1) Department of Energy water heater test procedures and resulting Energy Factors
2) personally calculated heat loss rates from insulated tanks of hot water
3) personally measured energy consumption data for water heaters operating in standby mode

...in support of my remark that a conventional storage electric water heater dissipates nowhere near 4000 btuh every 4 hours to its surroundings. In fact all three of my supporting datasets/methods indicate that typical loss runs around 3000 Btu PER DAY

I don't know where you get off suggesting a 40 gallon storage electric water heater loses 4000 Btu every four hours. Would you care to share with us data supporting that claim? I'm quite certain you will not be able to field such data, so I can only conclude that you are once again shooting from the hip while passing yourself off as an expert in the field.

The First Amendment enshrines your right to write such posts. The trouble is that newcomers here might actually mistake your ill-informed opinions for actionable advice . I've avoided engaging in some of the prior controversies of this nature but this latest misinformation I simply cannot abide.

The water tank is located in an unconditioned basement, the basement is around 60 F at the moment.

I'm pretty sure someone else would do the plumbing of the water and DSH lines to the tank differently. For example, I have the cold supply from the main connected to the bottom drain. It's T-ed at that point also to connect to the DSH. The DSH hot return connects to the pipe labeled Cold at the top of the tank. The AirTap is connected with a T through the pipe labeled Hot and this T is also where the house hot water is pulled.

The DSH pex lines are 1/2" uninsulated across the ceiling, about 30 feet one-way. The AirTap cold air blows onto the DSH lines. I just checked the temperature of the pipes that stick out of the water heater...even though no water is being used, just to see what/where standby loss could be occurring. The Hot pipe is warm, but not very, about 80 F. The Cold Pipe which is the DSH red pex hot return is 115 F at 1 foot above the tank. All uninsulated. The red PEX pipe itself is still at 110 three feet away from the heater.

Pipe insulation will help your situation, but I'm not sure it is responsible for the apparent heat losses you are experiencing.

Is it possible something is up with the AirTap such that it isn't actually delivering the heat you think you are losing?

My Geyser, absent hot water draws, will run about every six hours for ~20 minutes. It isn't particularly efficient operating in this way since it takes 2-3 minutes to get up to speed and then loses heat from within its own cabinet after cycling off. I once sought to combat this behavior by moving the sensor from the boiler drain to beside the lower element. That had the desired effect of greatly reducing the cycle frequency, but the on / off temperature gap is so high with the Geyser (On at 96 off at 115) that the tank grew uncomfortably cool, so I had to return the sensor to the boiler drain. I'd like to be able to reduce the control gap to about 4 degrees and have the sensor near the lower element access.

Conventional metal (but not Marathon) storage electric water heaters are not well insulated at the very bottom of the tank. This is OK in normal operation since element location and stratification combine to prevent significant heat loss from the bottom. Connecting an external heater via the boiler drain compromises this effect and likely increases standby losses and degrades EF, though by how much I don't know. I have my tanks sitting on discs of Blue Board to helps stop heat wicking down into the slab.

Posted By stuart.wyss on 19 Sep 2010 07:10 AM

I'm pretty sure someone else would do the plumbing of the water and DSH lines to the tank differently. For example, I have the cold supply from the main connected to the bottom drain. It's T-ed at that point also to connect to the DSH. The DSH hot return connects to the pipe labeled Cold at the top of the tank. The AirTap is connected with a T through the pipe labeled Hot and this T is also where the house hot water is pulled. 

Many ways to skin this cat.
Don't presume that since some of us do it differently, that it can't work in the way it's installed. If the connections you described were on a buffer tank, I would not be inclined to change them.
j

Joe- This is definitely not the buffer tank situation (not installed yet), just the regular tank. I've been reading around other boards and practically everyone recommends going against what's in the GeoMax installation manual (and many other manuals) and pulling the cold from the top of the tank and putting the hot into the bottom from the DSH (sediment, stratification, etc.)

I didn't know if the way mine was piped was causing the significant (1000 btu/hour) heat loss. The more I look at what my installer did, the more suspicious I am. There may be more than one way to skin a cat, but my installer wasn't even near the any member of the feline family when he installed my system.

I honestly don't know if the AirTap is producing the proper amount of heat. Is there any way to test now that it's installed. Would draining the water tank completely and starting it from a full tank of cold, then timing how long until shut-off of the AirTap be a good measure ?

Or turning off the Airtap, turning the breaker back on the resistive elements, then seeing what spikes they cause in power-usage?

That way I could tell if it's literally the tank cooling that quickly?

The AirTap (set at 130) is supposed to come on at 120 and off at 130 (10 F difference). It's clearly coming on, producing 10F of heat in an hour, then shutting off. It sounds like it's producing a decent amount of heat into the water. But the tank just loses 10 F every 4 hours....2 1/2 F per hour.

As we said before, bringing things to spec is a good way to start when you arent sure which wrong thing is causing poor service.
Circulate DSH from buffer cold in to drain Tee out. Not every one agrees on the sediment/stratification issue. But we are making too much of that.
Most agree on buffer (with R-410 systems). I think that has been pointed to for 4 pages now. Change that then worry about other stuff.
If you measure water in and out to airtap, you could calculate it's contribution.
j

Hopefully, this week will bring the replumbed system with buffer tank. I'll take new readings then and determine a course of action.

I'm not sure how I could measure the water in/out on the Airtap, though. It heats with a copper tube that's inserted into the water tank, filled with refrigerant. I know this tube gets really hot when the AirTap is running, and thus (we assume!) is delivering heat down into the water. There's been some concern (on these boards and others) that if the tube ever corroded, kinked, melted from the resistive elements, whatever, that the R-22 refrigerant would go immediately into the potable hot water supply and cause contamination.

I'm see that the so called engineer wants to engage in name calling instead of posting supporting figures - how immature. My gas water heater loses heat at about 2000 btu/hr while on vacation (easy to show from the bills). A low cost electric tank in cooler conditions would have less, but closer to 1000 btu/hr than the 125 btu/hr engineer claims. Others have reported 1 (as Stuart mentions) to 4 degrees per hour (~300-1200 btu hr) - again, way off from 125 btu/hr. I'd modify the AirTap efficiency expectation (COP=2 is overly optimistic in cool space) and factor in the quality of the heater and pipe insulation, ambient and tank temperature and thermo-siphoning. 125 btu/hr is "ill informed" for this situation - don't rely on it for thinking something is wrong with an AirTap. As a number that you could achieve in some different situations - sure.

I'm really late to the party here but as one of the few who actually has an AirTap and data loggers, I can give you some data. My unit was serving a household of 2 (my wife and I) with a 40 gallon tank with the electric coils disabled. So it was 100% AirTap heating it. I kept the temperature high 130-135F to give us more capacity, at the expense of knowing the standby losses would be higher. The unit would typically run only once per day, for 2-3 hours. I'll have to go back and look at the data files to see exact figures. The kwh consumption was typically about 3-4/day. At the high temperatures, the draw would definitely increase as the water heated, pulling up to 1200+ watts. The water heater it is installed in is a GE EnergySmart, which has a couple inches of PU insulation and a claimed 0.5 degrees/hr standby loss. The anti-thermosiphon apertures do their job pretty well, the cold water pipe remained luke-warm most of the time. Hot water pipes insulated with 1/2" rubatex. I can give more info, but I thought this would be a useful starting point. It definitely sounds like your unit has high standby losses if it's coming on ever few hours. Mine never does that unless we're using lots of hot water!

Tinoue,
Thanks! Mine's a Bradford White 40 gallon (new) so I'm guessing (!) that the insulation isn't bad. The tank feels basement temperature, so it must be pretty insulated.

Thanks for the data. That's helpful, and confirms that your tank loses about 10F to 15F per day, or about 1/2 degree F per hour - really good! Comparing your "once per day for 2-3 hours" to my "5 times a day for an hour" indicates again that I'm losing about 3x heat that most people do...

Mine's never pulled 1200+ watts, though ever. Most ever was about 650 watts.

Name calling? The only name I called out was JONR...

There is no doubt that a conventional center-flue gas fired storage water heater WILL in fact lose up to 10x more heat in standby mode than a completely enclosed electric storage water heater. Whenever the unit isn't firing the center flue continually draws cold air along several square feet of metal surface designed to transfer heat. It is a fundamental limitation of that heater design and accounts for their abysmal EF ratings around 0.6. They are economically justifiable only when gas is much cheaper than electricity.

Supporting figures: If an electric storage tank water heater with an EF of 0.95 is compared with a gas-fired center flue storage tank water heater with an EF of 0.60, then the gas unit has 8x greater standby loss. ((1.0-0.6)/(1.0-0.95)) = 8.0.

Applying standby loss data from a center flue gas water heater to a system with an electric storage water heater is ill-advised and mis-informed.

How did I get into this battle of wits with an unarmed opponent?

stuart, I don't know if this will help you any but here's my experience and setup using an Air Tap. I have it mounted on a 60 gal WH with no power to the resistive elements. The first thing I discovered is the temperature probe drops down into the tank only about 18" to 24". The Air Tap really only keeps the top 2' of water at the set temp. Air Tap has instructions available for bypassing the built-in thermostat and using one of the WH thermostats. I wired mine to the bottom thermo so now the entire WH comes up to set temp. I think the built-in thermo had an on/off gap close to 15°. With a fresh fill of cold water the AT will run for several hours getting the water heated. Keep in mind the heating capacity of the AT is only about 1/2 that of the resistive elements. I think right now it only runs about a couple of hours a day. I have to change the supply wiring to use a Kill-A-Watt monitor on it.

I run the water from the AT heated tank into a second, 50 gal tank that has the resistive elements running. The second tank keeps the water at temp, makes sure it's at the temp I want, and provides surge capacity if hot water draw is ever so great the Air Tap can't keep up with it. How much the second tank elements operate I have no idea yet, but I'm sure not much since the AT heated water comes out at 120 to 130°F. I also needed the second tank to meet the plumbing code requirement for first hour recovery rate which the Air Tap can't meet. That requirement is enforced for new construction.

Heat source for the Air Tap in the winter is a valid issue. That's why you don't want it inside the heated envelope of the house unless the cost of space heat is less than the cost of electricity. Mine is in the garage, which is constructed of ICF. I think it's primary heat source, when I'm not actively heating it, is ground heat coming through the floor slab, and daytime warming of the walls from the sun.

I really think if you want absolute control over what heat source has priority in heating your DHW you need three tanks, the first for DSH heated water, then the Air Tap heater to make up what the DSH doesn't supply, and then the resistive heated tank for finishing. Or use a 60 to 80 gal for the first tank and use the Air Tap to heat the top half of the tank and circulate the DSH heated water in the lower half. You might have to do some fancy custom piping to accomplish that, but I'm sure it can be done.

Re: the refrigerant tube of the AT. According to info I received from Air Generate it is a double wall tube to comply with code requirements for protecting potable water from contamination. Now what happens or how the system responds if either the inner or outer tube develops a leak, I don't know. I was only interested in getting what I had to from them to satisfy the state plumbing inspector the system was safe for potable water use. If I hadn't gotten that info from them the inspector would have made me remove the AT.

dmaceld,
The idea of using a different thermostat is intriguing. Is it on the AirTap website where anyone could find it? (I looked and couldn't, but it was 6 AM in the morning when I was looking). It's definitely something to consider. Interesting about the single/double wall. There's been a lot of debate on whether it's truly single/double and how it is banned in some states. I like your set up with just the AT and then the finishing electric. Probably the electric never comes on and when it does, it's a very occasional "maintenance" use. And you could drain the entire tank and have it replaced with almost at hot water from the AT tank! Nice!

At this point, a 3rd tank (!) would not be for me, but I see where you're going with this. Some people leave just the bottom resistive heater on their 2nd finishing tank with AT. Not as ideal, I guess, in terms of electric use, but better in keeping the water hot throughout.

It will be interesting over the next week to study the electric use and see how often the AT is going on now, and for how long. The new installer completely changed the arrangement, and I'm sure that the DSH is actually working now with the buffer tank, and that the AT is coming on fewer times. See my thread about my desuperheater IN is hotter than my OUT. It's the opposite, now :-)

Posted By stuart.wyss on 27 Sep 2010 06:37 AM
dmaceld,
The idea of using a different thermostat is intriguing. Is it on the AirTap website where anyone could find it? (I looked and couldn't, but it was 6 AM in the morning when I was looking).

Quite possibly not. Send me a PM with your email address and I'll send you a PDF of it. It's too big to post here. Use the PM link at the top of the page.

As for double wall all I have is an email from them with this statement, "Here is a statement from AirGenerate in response to the inspector's concern on the Uniform Plumbing Code in Idaho: "The AirTap uses a double walled copper tubing which meets International Plumbing Code.""