Given the following, how would you make changes (if any).

I have a GeoMax2 with desuperheater.  The desuper is connected to a 40 gallon Bradford electric water heater, with an AirGenerate AirTap heat pump on the water heater.  My wife and I use minimal hot water, so 40 gallons is quite sufficient.

I would like to make sure that the faucet temperature is always at about 125 F.

It's currently about 140 F.  I do have the GeoMax on due to cool weather we're having.

Should I

install a mixing valve to mix whatever hot water output with cold water and set the valve to 125 F?

Set up a buffer tank for the geomax, and set the water heater/air tap to 125 F? (extra plumbing, causes for leaks)

Will the desuperheater, even in summer, ever heat the water past 125 F (I read somewhere that the desuper cannot go above 125 anyway, and/or shuts down at a higher temp...I'm not sure if that's true for all systems, or specific models)

Turn down the Bradford, turn up the AirTap, just play with the system until I get the right temp at the sink.  I'm guessing the Bradford should be set to about 120, but the AirTap to 125? 

If the desuper goes above 125, how can I control the water temp at the tap to always be about 125 without the whole 2nd tank buffer setup?

The Bradford has the upper and lower thermostats but they aren't labeled with temperatures...just "hot", "very hot", and "cool"

If it works for you, I'd turn off the water heater. That makes sure you don't use its inefficient elements.

A thermostatic mixing valve (or a faucet that does it for you) is a nice to have for keeping a steady temperature as the tank cools down with usage.

Oh, I have tried to ask my contractor, but he hasn't been able to help satisfactorily.

Warnings
When installing the AirTap™ on a gas or electric water heater, disconnect permanently the gas or power supply to the
water heater while the AirTap™ is being installed. The original gas/power supply to the gas/electric water heater shall
remain disconnected after installation.


I copied and pasted this from Page 2 of the AirTap installation manual.

Posted By heatoftheearth on 25 Mar 2010 11:13 AM

I copied and pasted this from Page 2 of the AirTap installation manual.

When all else fails, R.T.F.M.

Using a buffer tank -- shared by the DSH and AirTap -- makes sense to me.
No heat pump, regardless of source, is going to operate at peak efficiency
if it's trying to pump BTUs "uphill" against an electric resistance heater.

The AirTap and DSH might even turn out to be complementary. Air source
should be very efficient in summer -- when my geo system's DSH actually
produces very little DHW output.  In winter, it might be best to turn off the
AirTap and rely entirely on the DSH (followed by an electric finishing tank).

I am not having satisfactory resolution with my contractor, though. Should I call another guy to examine the situation? You're right...it clearly says that the electric can be permanently disconnected....except for me, it's permanently connected and I was told "the AirTap and electric work together" by the contractor.

Can be disconnected and shall remain disconnected are very different. The Air Tap sends several feet of copper refrigerant lines down into your water heater. These lines curl up along the sides and bottom of the tank. If one of these lines was to be in direct contact with the elements of a electric heater or the bottom of the tank in a gas powered heater,it would just be a matter of time before the air tap was destroyed.

Sorry, your contractor is mistaken. The electric coils are only fo backup.
The bottom element should turned off. The top needs to be below the temperature of the airtap.

I think it is clear that your contractor has solved a symptom and not the problem.
J

I'm not a big fan of AirTap owing to the lack of protection afforded by a double wall and vented heat exchanger.

That said, before getting too hung up on electric element thermostat setpoints I would try running with the Bradford White shut off completely - see if the DSH + AirTap carries the load. Since there are just two of you it might work.

I think the previous temperature shortcomings indicate when the heat pump isn't running delivered temps were less than desired.
J

I didn't infer that from the thread trail, but I likely missed something.

If in fact the AirTap falls short then resistance elements need to be invited to the party.

I think that for the AirTap to be efficient it needs to be given time to do its thing, so a bit more differential between the AirTap setpoint and the electric elements would be needed. Maybe set the upper at the minimum comfy showering temperature, say around 110, and the lower at 90-95.

Another option would be to increase the size of the finishing tank to 50 or 65 gallons, allowing the AirTap more recovery time. That would require the AirTap to be removed and reinstalled, which I can't guarantee would be successful given how it is installed - pulling all the high side refrigerant tubing back out of the tank might not work out.

I know from personal experimentation that reducing the btuh input to a water heater works if the water heater is sufficiently large: In an effort to cut demand I reduced the supply voltage to our 80 gal electric water heater. Its 5500 Watt elements then consume only 1375 Watts, and recovery time quadrupled. However the 80 gallons kept us from ever running out of hot water.

I've since retrofitted a Geyser Heat Pump Water Heater to the 80 gal finishing tank, and it works fine with the resistance elements powered down. However, my inlet water is generally 70+F and never below 65, a walk in the park compared with what northern water heaters have to deal with.

I feel like I'm cross-posting now, since I had another question about AirTaps on the forum as well.

Ended up I emailed the contractor about it (the installation/shutting off the electric permanently, etc.) and he hasn't replied.

I then cranked the AirTap to 135 (the max), shut down the Bradford at the breaker, and will see. As the weather warms, I won't be using the Geo much. Maybe the AirTap thermostat is just out of adjustment somehow. I'm getting 117 F water at the sink now, which is definitely decent. The pipes are all in an unheated basement, and I plan to wrap them tightly as much as possible.

Pipes in unconditioned space will definately affect tap temperature.
The more flexible you can be about tap temp the less you'll spend making hot water.
Taking heat out of the air surrounding your plumbing does sound a tad counterproductive.
J

I spoke to AirGenerate, and they confirmed that the electric elements can be left connected, but I should turn down the bottom element to 90 F or better, turn it off, and leave the top set to just below the temperature of the AirTap. AirGenerate said there might be a way to reset/recalibrate the thermostat, too, so that if it's set to 125, the water is actually at about 125.

Hve you ever measured water temp at the tank? ...or just the tap?
How do we know you are not acheiving set point?
J

I agree with the counterproductivity of sucking heat out of space near HPWH and thus having to pump in or otherwise add back replacement heat. Air source HPWHs are probably counterproductive much north of I-40.

Posted By engineer on 01 Apr 2010 12:48 AM
I agree with the counterproductivity of sucking heat out of space near HPWH and thus having to pump in or otherwise add back replacement heat. Air source HPWHs are probably counterproductive much north of I-40.

I respectfully disagree.

Much of the year requires cooling and/or dehumidification, in many climates considerably north of I-40. Even if one runs straight electric for half the year and activates the HPWH the other half, the net gains can be substantial.

During operation, you're getting a COP of 2.5-3 from the HPWH, excluding the benefit from cooling and dehumidification. If you throw that into the mix, then you're well over a net COP of 3. So worst case, if you use the system just 6 months a year, you're getting an average COP of 1.5 for the year which is a substantial win over straight electric resistance.

I happen to use mine year round. Why? The rest of the house is efficiently heated (COP >3.5 geo), so even if I'm drawing heat from the house, that heat is much more efficiently generated than the electric resistance coils in the water heater. Even if we factor in large efficiency losses, the net efficiency of the system is in excess of 2. That means that my year-round COP is more on the order of 2.5+ which is a major win compared to straight resistance water heater.

Yes, it would be even more optimal to have the water directly heated by the GSHP and have a desuperheater, but that wasn't an option at the time.

The point is, as techies, we musn't throw out good solutions in pursuit of the perfect.

A HPWH operates "free" during the cooling season, about 3x better than electricity during the open window season and somewhere in between the cost of interior heat and electricity (closer to the former) during the heating season. I agree, a major win over an electric water heater, specially with tax credits.

Posted By tinoue on 01 Apr 2010 05:06 AM

Posted By engineer on 01 Apr 2010 12:48 AM
I agree with the counterproductivity of sucking heat out of space near HPWH and thus having to pump in or otherwise add back replacement heat. Air source HPWHs are probably counterproductive much north of I-40.

I respectfully disagree.

Much of the year requires cooling and/or dehumidification, in many climates considerably north of I-40. Even if one runs straight electric for half the year and activates the HPWH the other half, the net gains can be substantial.

During operation, you're getting a COP of 2.5-3 from the HPWH, excluding the benefit from cooling and dehumidification. If you throw that into the mix, then you're well over a net COP of 3. So worst case, if you use the system just 6 months a year, you're getting an average COP of 1.5 for the year which is a substantial win over straight electric resistance.

I happen to use mine year round. Why? The rest of the house is efficiently heated (COP >3.5 geo), so even if I'm drawing heat from the house, that heat is much more efficiently generated than the electric resistance coils in the water heater. Even if we factor in large efficiency losses, the net efficiency of the system is in excess of 2. That means that my year-round COP is more on the order of 2.5+ which is a major win compared to straight resistance water heater.

Yes, it would be even more optimal to have the water directly heated by the GSHP and have a desuperheater, but that wasn't an option at the time.

The point is, as techies, we musn't throw out good solutions in pursuit of the perfect.

I don't think the idea is automatically discarded when we say it is not great for universal application, but I think the claims of efficiency are greatly exaggerated and neglect to mention other consequences.

During heating system when you cool heated air, you add load to the house that must be accounted for with either larger heating equipment or longer run times. That doesn't change the efficiency of your heating system, just your water heater. IOWs if I spend extra heating my home this winter, that is part of the water heater operating cost.

The water heater may help dehumidify some, but it could also impede dehumidification by cooling basement. IOWs extra dehumidification cost could be part of the water heater operating cost.

The suggestion that electric elements are sometimes required for back-up greatly reduces the average cop as well, the numbers touted thus far are peak cops.

Remember these are not touted as great for geo owners only, so the make up heating cost will vary significantly.
Therefore if I have someone with an old 68% efficient furnace, lots of dehumidification requirements and no insulation between basement and ground floor is this water heater still a great idea?

Lest we forget, extra operating hours also mean more equipment maintenance. This is part of your water heater operating cost.

Throw in the cost of installing this water heater vs. conventional electric resistance water heaters and you add another terrific burden to ROI.

Meanwhile, I see some very obvious beneficial applications for these units. One that comes to mind is a manufacturing facility, where there is a significant PHW demand and the equipment is installed in an assembly area that has lots of heat generation from machinary.

I won't join you in pulling numbers out of a hat, I'm not trying to support any claims. I've no horse in the race other than wanting to give my customers the best possible advice. My concerns are legitimate and should be addressed by the makers of this equipment.

Joe