(If this is off-topic and can be bumped to right place, I'd appreciate it)

I have a GeoMax 2 GSHP with desuperheater, a 40 gallon Bradford water heater connected to the desuperheater, and an AirTap on the water heater, too.  AirTap is one of those mini-air-to-water heat pumps that sit on the water heater, pulls heat from basement air, and heats the water.  Supposedly between 150 and 250% more efficient than the resistive elements in the heater.

We've had plenty of great hot water all winter with the desuperheater.  Last weekend, I shut down the HP because the weather was just so nice out!  Later that day, we discovered, the hot water wasn't really hot anymore.  Temp at the sink was 105 F.  I checked the thermostat dial on the AirTap, it was set to 120 F.  The thermostats of the heating elements of the water heater aren't labeled with temperatures, just "low" "med" and "high".  They were set to the "low" side.

I called my contractor about no hot water, and he was very good and came out the next day.  He told me he made adjustments to the water heater and AirTap and now the sink water is 140 F, which it is.  However, I noticed the AirTap is still set for 120 F.   The heater thermostats are now set to "med".

So, if my water is 140, does this mean the AirTap isn't actually doing anything now and the resistive elements in the water tank are keeping the water at 140?  How would that save any money??

I had a conversation with a plumber about these water heaters yesterday. They are making hot water with heat energy in the home that you paid to create with your heat pump. Once you stop conditioning the space around your heat pump ("shut down the HP because the weather was just so nice out!") there may not be enough heat energy available (surrounding space is too cold) and electric elements may have to kick in. Perhaps some one can tell us what the cut out temp is (I know ASHP pool heaters don't cycle much below 50*).
Don't know that that is what happened here I encourage you to ask your contractor.

I've not weighed in on these water heaters yet but have mulled over them. My concern is a possible false economy as they necessarily add load to a heating system. How much load will depend on usage.
I can certainly picture applications for this technology, but I suspect a home in my area with three teenagers may not be one of them.
Picture a cool Michigan basement further cooled by ASHP water heater and long teenage showers causing additional deep winter auxilliary coil usage.......Can anyone tell me the approx. load per person it will add to my heating design?
Joe

I read somewhere that they are still 135% efficient at 40 F or something like that. The basement is unconditioned, so while it isn't stealing heat directly from the conditioned house, I supposed it is making the basement cooler than normal. The literature suggests that a certain number of sq feet of basement is sufficient, and some people duct the air output (cold) during the summer from the AirTap into their conditioned spaces.
I have more than enough unheated basement, and there is insulation between the basement and conditioned space.
Just seems that if the electric is set higher than the AirTap, then the electric will do all the work. I can't picture a scenario in which the AirTap would be on by itself at that setting.

I've been using an AirTap in my basement utility room for some time now in a water heater that has the elements disconnected, so that it runs 100% with the AirTap. I keep it set to about 130 and it works very well indeed. Yours should be keeping it much hotter than the 105F that you're seeing. It might be a problem with the thermostat.
You should hear the AirTap compressor for a couple hours a day. You could also buy a Kill-a-watt energy monitor to see how much it's running every day.
For those interested, I've been monitoring the usage on mine and it uses 1.5 to 2.5 kwh/day. I currently have it set dedicated to my wife's bathroom. The utility room gets chilly but not freezing - there's clearly enough heat getting sucked out of the basement living space, which is heated mainly by a super efficient Fujitsu RLS heat pump. My logic is that even if it does suck heat out of that space, it's vastly less costly than running an electric resistance water heater.

Depends what logic it uses. If you are really curious, measure the power draw to heat up a freshly filled tank.

Does that mean 40 is the lowest operating temp? Whay did unit stop heating? Was it too cold in basement or is something else wrong?
Your electric elements will have a differential don't know how large but, both settings may work together.
Again did you ask your contractor these things? We have so little info to help i.e. cut out temp, reason for shut down, thermostat differentials........
J

Sadly, I've asked my contractor and gotten very little info back. I've also contacted AirTap directly in hopes of more info. Their website says that even at 32 F, it's over 100% efficient.

My contractor told me that the AirTap is supposed to help the electric elements heat the water.

The AirTap is not supposed to be running at the same time as the electric elements.

Ah - the other thread.

Try measuring hot water temp with resistance elements off for awhile and the AirTap having had plenty of time to heat the tank. First thing when you wake up or upon returning home from work should do. If the temp is low that would suggest the AirTap thermostat is out of cal.

I would expect an AirTap left to heat 40 gallons with no hot water use for several hours to be able to bring that tank up to 125 or higher all by itself

I emailed my contractor about it, and he didn't reply.
I've since cranked the AirTap up to 135 (the max) and shut off the breaker for the water heater elements. Water at the tap is about 117 F which isn't bad, actually. Pipes are in unheated basement, so planning to insulate them as much as I can. May add a degree or two. I don't think the AirTap thermostat is quite correct, but I'm also planning to dump some water out of the heater drain vent and get a temperature reading there.

I've heard that you shouldn't set the water heater hotter than what the desuperheater will create. I have no idea what a desuperheater will heat up to, but I think I read about 125 to 130 F.

I concur with Tinoue's findings. I've plugged our AirTap into the Kill-a-Watt for months on end and it uses 44kW in summer and 58kW in winter for a yearly average of just under 50kW/month. The breakers for the hot water elements are switched off so the AirTap has provided all of our (2 adults in Central FL) hot water for 18 months at a cost of $6.50/month. It uses 630W during operation and runs about 3.5hrs/day. It will condense nearly a gallon of water on a humid summer day. During the coldest week in January, the AirTap used 20kW (double it's summer use) Since our water tank and AirTap are in the garage I experimented with using the cars as heat sinks, and switching the AirTap on after pulling a hot car into the garage. Dark car in the sun or hot engine-both worked well and by the end of the month we'd used 58kW. Same for February. If you find the AirTap doesn't satisfy your hot water demands, I did read about one owner's solution to bypass the AirTap's thermostat and use the lower thermostat for his hot water tank instead. That info was posted on the AirTap forum at one time so the technique is known to the folks at AirGenerate. The AirTap thermocouple bulb dangles down into the tank on the end of a copper tube. Maybe the bulb is 3' down or maybe it got hung up during installation and is only 6" down so the unit only heats a few gallons before tripping off the thermostat. By using a thermostat that's lower in the tank, he forced the unit to heat a larger quantity of water within the tank. In short, I've been satisfied with the AirTap and impressed by it's efficiency. Unit longevity is still an unkown.

At this risk of resurrecting this thread!

A different contractor came by my house today to evaluate my system and problems (see multiple other threads, sigh) and his strong opinion about AirTaps was...
Great in the summer - cools & dehumidifies the basement, and thus indirectly the house (unconditioned basement, but no insulation in basement ceiling)
Very Good in the fall, spring when you don't really have heat on - basement is cooled, minimal dehumidification
Poor in winter unless desuperheater is doing most of the work - any heat you pull out of the basement into the water heater will be replaced by heat from upstairs through the floor. Better to use elements in winter in conjunction with desuper. In winter, too, the time required to raise the water after a long bath would be incredible with just an AirTap.

He loves the concept of the AirTap, just not in a heating dominated climate like PA.

Meanwhile, we ran out of hot water today...AirTap set at 120, 40 gallon tank, no geo (it's been so nice it's off). After my wife and I showered, I caught the tail end of no heat.

Cranked the AirTap up to 135, but at that temp, even when everything gets fixed, the desuper would never come on (125 cut-off).

Now I'm thinking buffer tanks, too.

40-50 gallons doesn't sound like much capacity, especially considering that maybe only 80% of the tank is actually used. Either a buffer tank or a larger water heater would increase capacity. A buffer tank makes more sense here. :-)

Yeah. I'm thinking of the following now:

40-gallon buffer tank (unpowered) hooked to desuperheater.

40 gallon AirTap (existing) becoming the 'finishing tank'. AirTap set to 135 F (max). Possibly connect the BOTTOM electric element of the finishing tank to a timer so it's available during peak times *as needed* to help the AirTap....maybe 6 AM to 7:30 AM, and 8 PM to 10 PM, also set to 135. I don't want the AirTap and/or Electric to 'compete'.

Maybe not ideal, but certainly better than what I've got.

I wonder if it's better to leave your current setup as is (AirTap and DSH) to the existing tank, and make that your buffer tank, and use the new water heater for the finishing tank always set to the finishing temperature (no timer needed.) The only time the finishing tank elements will run is to either raise the temperature of the water (from the buffer tank) if it is not hot enough, or to maintain temperature due to standby losses. Engineer?

Geome,
Intriguing! So, the finishing tank would be resistive heating which would be the least efficient, but would only be maintaining the temperature or possibly heating it a few more degrees. The AirTap and DSH would make sure that no matter the season or whether the Geo is on or off in nice weather, the buffer tank would also have hot water. I can see that in Spring/Fall when the Geo isn't on, that would still give me 80 gallons total of hot water.

That's how I see it, but since I'm not a geo installer it would be great to get their opinions considering your AirTap. This is a standard buffer tank arrangement that I mentioned, except that you have the AirTap as well.

I assume a more efficient (lower heat loss) finishing tank would result in less resistance heating being required, but higher efficiency (like a Marathon) can cost more. I am not aware of anyone here that has done an analysis on the payback (since there are lots of variables.)  Of course, a Marathon has a lifetime tank warranty to the original purchaser (and perhaps limited to the original installation location too.)

I assume the electricity needed to maintain temperature in an electric water heater is low, but I have no data. 

Aren't typical standby losses 7% or so (with average usage - 100% if you are on vacation)? Other than this, one tank per heating method is ideal - ordered from most efficient to least efficient. Some people use a timer to give one heating method a shot at doing the work before a second method kicks in - not perfect but it does save a tank.

Just curious. I've started graphing (Google PowerMeter) my electric use and I can confidently say that the AirTap comes on once every 4 hours for 1 hour at a time to maintain the 40 gallons of water in the water tank. When it runs, it uses 600 watts. The desuperheater/geo is currently off because the weather is so nice here in SE PA, and the AirTap is pulling unconditioned basement air for the heating cycle.

I know straight electric resistive would pull more watts, but also operate for a shorter period of time, or perhaps more/fewer cycles? It's a Bradford 40 gallon tank.

Anyone have any regular hot water numbers to compare, and/or can say if my situation sounds pretty good?

Electricity is 15 cents per KWH which includes generation and transmission/taxes/etc.
I think mines works out to about 54 cents per day, or $15/month for hot water. Obviously, it runs a little more when we're showering/dishes/etc. But for "standby", is $15/mo pretty good with no desuper currently in operation?