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Heat your water with a heat pump - 50% electric savings!

Post in 'The Green Room' started by mikeathens, Jan 22, 2009.

  1. DBoon

    DBoon Minister of Fire

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    Just keep in mind that the boiler AFUE efficiency rating isn't what you are really getting in the real world - it is essentially what the efficiency of the fuel conversion is before heat transfer and system losses. The heating of the hot water is probably not occurring at 86% efficiency.

    At 18 cents per kWh, it's hard to see how you could save money with a heat pump hot water heater, even with the savings on running the dehumidifier.

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  2. mikeathens

    mikeathens New Member

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    Keep in mind before you invest in one of these HPWHs:

    1. They do work, and they do save a lot of electricity.
    2. The heating elements can heat up 50 gallons of water to 120 F in about an hour. The HPWH will take two to three hours to do the same thing (depending on ambient air temp).
    3. The HPWH doens't keep your water at a constant setting like the electric elements. Instead, you have a 15 F drop before it kicks on.

    So, keeping in mind #3 above, consider this scenario (I went through this before in this thread, but I think it's important enough to reiterate).

    HPWH runs and shuts down. You now have 130 F water in the tank. Run a load of laundry. Run a load of dishes. Kids take a bath that evening.

    Now, you get up in the morning to take a shower. The water is running for about 30 seconds, and the HPWH kicks on (this is the first time since the bath, dishes, and laundry). After about 3 minutes in the shower, the water is still warm (comfortable, but not super hot). You have about 2 more minutes to finish up. After that, it will be somewhat cool.

    Now, you really don't have hot water for about another hour or two.

    Some may look at this as a big inconvenience, which I suppose it could be. My family just has to adjust schedules. Kids bath at night, wife showers at night. I take mine in the morning. We have a Bosch front load washer, GE dishwasher, both can heat it's own water if inlet temp is too low, so we run these at any time. I think it's a small price to pay for huge energy savings.

    EDIT: one last point about the cost. If you have to buy the HPWH AND a tank, both are components of the HPWH system, and both are eligible for the 30% tax credit. IF you pay $1200 for the HPWH and $700 for the new tank, you can claim a $570 tax credit on your '09 taxes. That's a good chunk of change.
  3. Wet1

    Wet1 Minister of Fire

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    Newstove, you should probably do just as you initially mentioned, add a 40 gal electric tank (with the Airtap A7 installed on it) inline before your indirect oil fired tank. I should also mention you can get used 40/50 gallon tanks on CL for next to nothing since most people don't want them. I picked up my 50 gal tank for $25 and it was only three years old... just something to consider.




    I've been mulling this over for several months now and I still haven't made any concrete decisions yet, but since humidity in our basement will be an issue soon, I need to start thinking harder about this. As I stated earlier, I have a NG fired indirect system and a separate electrical resistance DHW system. After spending thousands to convert from oil over to NG, I've not used the system at all as I've been burning wood/pellets and I don't see it being used in the foreseeable future. Since the NG system is not being used, it obviously makes it much less efficient to use NG to heat the DHW.

    My biggest fear with switching over to the HPWH is that it will not meet our normal demands, and based on Mike's gracious input, it sounds like this fear is justified. I know a 12k BTU unit is going to be released soon, but it will be more costly, and I'm still not sure this will cover our needs w/o any issues. Regarding the manufacturers, I suspect the NRT/Nyle unit is superior, but at twice the cost, I don't see how I can justify it.

    Since I want high capacity, low operating cost, and some of the dehumidification benefits of the HPWH, I'm strongly considering building a hybrid solar and HPWH system. What I'm thinking is making a solar DHW system similar to the one Gary built (http://www.builditsolar.com/Experimental/PEXColDHW/Overview.htm), but with slightly larger capacity and make it out of copper rather than PEX. Based on his numbers and my upgrades to his design, I suspect this system would cover at least 95% of our DHW needs. I would feed the large solar storage tank into my existing 50 gallon tank, which I would install the Airtap A7 on. The Airtap would hardly ever have much of a load on it, but it would do the job to maintain tank temp form tank losses, and work as a backup in the event the solar system wasn't keeping up with our demands (although this would probably be fairly unlikely). It looks like I could build the DIY solar system for around $1200 (with the more recent decrease in copper prices and adding in my upgrades), plus another $400 for the Airtap A7 ($600 shipped, minus the 30% tax credit). $1600 is a fairly large amount to spend on a DHW system, but assuming I spend $600 a year now to heat water and still have some issues with running out of hot water, this makes the payback period under 3 years... which isn't that bad IMO for a DHW system with nearly limitless capacity.

    Then again, with a solar system covering 95% of our hot water needs, I wonder if it would even be worth adding the HPWH at all and just keep the existing (and reliable) electric DHW tank as it sits. I guess the biggest reason I'm really leaning toward adding the HPWH is because I don't know that I'll have time to build the solar system this summer and I'll need the dehumidification soon... plus I don't think we'll have much of a demand for DHW until fall. Decisions, decisions...
  4. newstove

    newstove Member

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    Yeah, that's what I'm leaning towards. Of course, my wife thinks I'm nuts. ;-)

    I just feel better conserving energy if I can. Kinda like the free wood argument - yes, it's work, but otherwise most of it will rot, so why not take it, stack it and burn it. Gives me free exercise...

    Robert at North Road Technology (http://www.northrdt.com/index.html) has been great - has answered every question and even sent me the installation manual so that I can review how the install would go.

    As far as I can tell from specs and such, the Geyser heat pump will consume about the same amount of power as my current dehumidifier, and should have a better efficiency. So, I think that might wind up being a wash during the seasons when I run the dehumidifier. Now I just have to wrap my mind around the cost of the unit!

    Thanks for all the feedback.
  5. DBoon

    DBoon Minister of Fire

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    It's not always about economics - do what feels right to you if you have the spare cash and can afford it. There is a lot to be said for doing what you can to reduce oil usage.

    Let us know how it turns out.
  6. newstove

    newstove Member

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    Exactly. The "spare cash and can afford it" part is where I have to wrap my head around the cost of the unit. I actually love the idea, even if it were economically neutral if it saved energy. I just have to convince myself I can afford it right now.

    At the same time, in the next year or so, there may be vast improvements in the technology, so who knows what will happen. I'm not in a huge rush, now that I have a plan. ;-)
  7. OpenSky

    OpenSky New Member

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    I just installed a new AirTap A7 on a 62 gallon electric WH last weekend and re-engineered the thermostat setup by bypassing the thermostat in the A7 and using the lower heating element themostat instead. First morning after the A7 install I woke up to 98 degree water and decided that wasn't going to cut it. So far, I have 130 degree water at the closest tap about 10 ft. from the tank. And now it makes no difference if the tank has been setting all day or all night with no usage, I still have hot water. I think by using the thermostat that is lower on tank it can heat more consistantly since the cold will stay in the bottom of the tank and the thermostat on the tank is faster reacting. I am new to the forum but I have enjoyed reading all of the input...Thanks
  8. Wet1

    Wet1 Minister of Fire

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    Welcome to the forum OpenSky... and great idea! I'm sure several people would appreciate and benefit from more details on how this is actually done, any more info or pictures would be great.
  9. mikeathens

    mikeathens New Member

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    I thought about messing with mine, too. I had the same thought about the supplied airtap thermostat bulb being toward the top of the tank...

    I'm curious...does your unit kick on pretty much each time you draw a few gallons of water using the lower t-stat? It seems that the lower tstat won't give you that 15 degree drop before it kicks on...and in fact, will only allow a couple degrees before kicking the unit on. And I wonder if you use some of your efficiency/energy savings? Just thoughts, and by no means do I necessarily know what I'm talking about.

    Hey, if you could fill me in on how often the thing kicks on, I would appreciate it. As mine stands, we get one shower, one load of dishes, and maybe a load of laundry before it turns on...and then someone might be "left in the cold", so to speak. I was just thinking this morning how it would be nice to have a t-stat override button, so I could manually tun it on to reheat the whole tank while I'm gone at work...
  10. Wet1

    Wet1 Minister of Fire

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    I'm sure the unit kicking on and off would cause a little drop in efficiency, but I doubt it would be enough to lose an sleep over. These things are really nothing more than a basic window AC unit, and those turn on and off all the time. Also, hot water demand/consumption is typically batched (in my household at least) so I doubt the unit would be kicking on and off all that often. But you're correct, the more the unit kicks on and off, the greater the hit in efficiency.
  11. mikeathens

    mikeathens New Member

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    I actually called airgenerate about this today...they emailed a tech sheet on the procedure for connecting to an external thermostat. Not rocket science, but it is something they're aware of and have looked into. So, according to them, I guess it's not a problem. I think I'm going to do this, too (PITA...I just wrapped the thing in r-13 fiberglass and a lyer of foil bubble wrap). This will probably change the on/off temp differential to 5 or 10 degrees, too.
  12. OpenSky

    OpenSky New Member

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    By using the lower thermostat, the A7 does kick in rather quickly since the cold inlet water is streaming into the bottom of the tank. So the A7 runs until it can get the temperature back. I ordered a Kill A Watt today to track the electricity. The electric meter is spinning very slow with this thing running. It's just the wife and me at our household but, for two people we sure wash allot of clothes. We are using the Whirlpool Duet front load. As far as lifestyle, I can't tell any difference between the heat pump and the elements....
  13. mikeathens

    mikeathens New Member

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    This past saturday (April 11), I rewired the airtap to run on my water tank lower thermostat. Now, after drawing a little water (let's call it 5 gallons) from the 40-gallon tank, the HPWH will kick on. It runs more frequently now, but for less time each run. Hearing the HPWH kick on doesn't mean we're about out of hot water now!

    The hugest pain in the ass doing this was routing the wires out of the cabinet and to the thermostat.

    Anyhow, it works!! I think this is a much more effective way to run this thing.
  14. OpenSky

    OpenSky New Member

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    I installed the Kill A Watt inline with the AirTap on April 10th. My current electric contract I am paying 10.8 center per kwh. It is showing my daily cost to run the AirTap is .62 cents. So far so good! This thing is working...
  15. mikeathens

    mikeathens New Member

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    Uuuuuggghhh...I'm such a procrastinator. I still have to get one of those kill-a-watt things.

    So you're using about 5.75 kWh/day to heat water? By my estimate, based on the energyguide included with my tank, I would be using aobut 14 kWh/day if I used the in-tank elements. If mine also shows about 6 kWh, I'll be pretty happy (two kids + me and my wife using hot water). 6 kWh will be about 25% of my solar array output on a good, sunny spring day.
  16. diyer

    diyer Member

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    Make sure you run through the numbers before you buy.

    Assuming a 50 gallon tank, 50F to 100F rise. 50F * 8 lbs/gallon = 400BTU's.

    Most AC/dehumidifiers are probably 5000 to 12000 BTU's. Also as you have a closed system, the temp delta between the water and cellar will increase, causing loss of efficiency.

    You may make it if you use 500-1000 gallons of hot water / day.

    My guess is you would be best trickling the hot water all the time, but where is the savings?

    Carl
  17. DBoon

    DBoon Minister of Fire

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    1 BTU = the amount of energy to raise 1 lb. of water 1 degree Fahrenheit.
    So a 50 gallon tank has 400 lbs. of water (50 gal * 8 lbs./gal) and to raise that 50 degrees F would take 20,000 BTUs (50 degrees F * 400 lbs.) What you indicated would be true if the temperature was only raised 1 degree F.

    There are 3.414 kW/BTU, so this would mean about 6 kW of electricity.
  18. diyer

    diyer Member

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    Oops.

    Well at least the numbers have been gone through. Sorry for the dropped delta T.

    Carl
  19. DBoon

    DBoon Minister of Fire

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    No problem. There are two kinds of posters - those who have already made a mistake, or those who will.
  20. diyer

    diyer Member

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    Can I hijack the thread a little?

    I am looking at storing cold in my thermal store and using it for air conditioning. I am looking for recommendations.

    I have a Tarm pressurized system in an outside storage unit. This runs a heat exchanger loop into an unpressurized 1500 gallon insulated milk tank. There is another loop for heat(forced air) and DWH.

    I currently use 2 window airconditioners during the summer. They run continuously during the day. I'm guessing that they are about 20K BTU's total (about 1.5 tons). If I find a water-to-water heat pump, I can run the hot side through my DWH superstore, continuing outside to the boiler loop(boiler off). This would heat my water and dump excess heat outside. The cold side would be put through the heat exchangers in the 1500 gallon tank. I will be running Propylene Glycol in the system and would like to get down to 25F or so on the cold side.

    The cooler temperatures in the evening with some radiational cooling will allow me to harvest the cold "off peak". During the daytime, I would pump the cool out through a heat exchanger into the forced air system. It would be sweet to add off peak electric rates for this if I can get them.

    I would also like to swap the hot/ cold loop and heat the tank with the daytime temps during spring and fall for use during the colder evenings.

    Does anyone know of a water to water heat pump in the 1 - 1 1/2 ton(12K - 18K BTU's) range?

    Also, I was wondering what the best coil arrangement would be for this water cooled "chiller". I currently have a hot water coil in the up flow furnace plenum for heating. I may want to find a chiller to put in above the current coil. This would have the cold added at the top, flowing downwards, possibly plumbing the hot water coil in also.

    I don't know how to add the condensate drain pan under the coils in the up flow. It seems like the pan would block the air.

    Another concept would be to buy a downflow or side flow heat exchanger and place it before the furnace. This could have the drain pan on the side of the furnace. I could plumb the hot water coil, but it would be last in the cooled water loop to avoid condensation on it.

    I would really like to have the first solution (all coils in the furnace plenum). This would be the same plumbing for heating and cooling. Also with the extra cold water coil, I could get more heat out of it during the winter.

    All of my coils/plumbing is 1" PEX. My Tarm boiler is in a storage container. I am considering adding a solar exhanger to the side of the container. This would add to my daytime heat gain for hot storage, and the radiational cooling during the cold storage season.

    This is really just a geothermal heatpump, but only using the diurnal ranges for short term storage.

    Thanks
    Carl
  21. Wet1

    Wet1 Minister of Fire

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    diyer, start a new thread for your questions. You're way off topic and you'll get little or no response within this thread.
  22. jh6u

    jh6u New Member

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    Back to the HPHW topic:
    I've been comparing the Geyser to the Airtap:
    A neighbor has the Airtap; it works well, except that he has had icing problems. He's fixed it by running a 100w lamp in parallel with the unit to heat the condensor, a kludge at best. The better solution is that the newer units apparently include an anti-icing sensor. I have two concerns with this unit: 1) It apparently doesn't have UL certification: I haven't been able to find out whether this would be an issue with homeowner's insurance if there were to be a problem/fire. 2) It seems that it's a tight fit to get the heating tube into the tank. My neighbor commented that it would be difficult to get this out once it got crudded up. I worry that the lifetime of the unit would be limited by the time to tank failure.

    The Geyser: more expensive, but I've been more impressed with the engineering, at least on paper. It includes an anti-icing cycle. It works by replacing the lower electric element- so the upper electric element is still there as backup in case of high demand. I suspect this is why they can claim such a high first hour rating. It also looks to be much easier to take out of an old tank. So I'm leaning towards the Geyser.

    We have a great setup for this- a 1000 sq ft unfinished basement, so lots of area to transfer heat from soil. By my calculations, this should only take ~200 sq ft.

    I'd appreciate feedback, especially on the insurance/UL issue.
    Thanks all for an interesting forum thread.
  23. mikeathens

    mikeathens New Member

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    Well, I'm not sure about the UL rating issue. I would suggest calling Airgenerate. I'm sure they could answer any question you might have. Their customer service has (so far) been excellent to me. If I remember, I'll have a look at the unit to see if I can find any UL label.

    I had no issue getting the heat exchanger/tube in the tank, and I can't forsee any issue removing it in the future. I would expect the unit to fail before my tank does. The one issue I did have with it was the tank be ing nearly out of hot water before the unit kicked on. I solved this by wiring it to the lower thermostat and disabling the supplied bulb.

    Icing has not been an issue whatsoever for me. The low ambient operating temperature has been in the low 60's. I would assume that your neighbor has been operating his in a cooler basement, maybe in the low 50's range? If so, I would imagine this will be an issue with any of these types of units. Running in a colder room will cut way back on efficiency. Deicing will cut back even more. Runnning a 1500-3000 Watt in tandem with one of these will really put the hurt on your savings (like using the upper element).

    Since I've rewired to the lower t-stat, my airtap has supplied all of the hot water we've needed for my family of four. So far, no complaints. Now that the temps are climbing into the 80's outside, my house is warming up, too. That air from the airtap that made the bathroom a little chilly a couple months ago sure feels good now!!
  24. jh6u

    jh6u New Member

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    Here are direct quotes I've gotten from each company: I'd appreciate some help with interpretating these & how these certifications relate to National building codes & insurance coverage:

    AirGenerate:
    "The AirTap has multiple industry certifications: NTS (National Technical
    Systems), GAMA (Gas Appliance Manufacturers Association, which recently
    merged with AHRI, the Air-Conditioning, Heating and Refrigeration
    Institute), and CSA, and we are in the process of getting Intertek (ETL)
    certification, too."

    North Road Technologies:
    "QPS has performed our UL and CSA Certification compliance in accordance with QPS/SGS Listed (UL-1995 / CSA C22.2 No. 236). This is “guaranteed UL compliant”."
  25. mikeathens

    mikeathens New Member

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    Airgenerate emailed their label to me. It does mention UL standard. Unfortunately, I don't know what it all means.

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