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Anyone look into heat-pump water heaters? My quest for oil-free house

Post in 'The Green Room' started by cbrodsky, Jun 30, 2008.

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  1. cbrodsky

    cbrodsky Member

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    I'm thinking more about how to go 100% oil free. Currently we are down to about 180-200 gallons/year after putting in solar HW to shut down our boiler about 8 months per year, and using wood heat as much as possible to keep it off in shoulder periods and minimize use in winter.

    The sad part of this, however, is that much of that 200 gallons is standby loss - I estimate at least 0.5 gallons/day, or about 60 gallons in that 4 month winter stretch when the boiler is on. This really aggrevates me in that a relatively low percentage of this expensive oil I buy ever makes it into my house as heat. But it also suggests that I could go with a low level of supplemental heating system even using electric sources and eliminate this altogether, maybe saving money in the process.

    I've been thinking about domestic water heat pumps as one possible solution, coupled to radiant tubing on our first floor (not installed, but easy to do and something I'm looking at anyway for better heating efficiency) and part of our second floor (tubing installed following opening ceiling for water leak). Two BRs would be kept at 50 on their own baseboard zone with relatively low temperature water.

    There is a nicely documented study from CT where these heat pump systems seem to achieve real-world COP of around 1.8:

    http://www.cee1.org/eval/db_pdf/277.pdf

    We have an unconditioned, large, and deep basement - only about 1 foot above grade. I suspect that even with one of these systems running, we'd be unlikley to change the basement temperature much beyond ground temperature, so we should have no trouble getting good performance - consistent ambient temperature.

    So if I work through this a bit, I could say that while I'm buying 200 gallons of oil a year, I'm really only getting about half that into my house as heat when accounting for combined standby and combustion efficiency losses. This would suggest that I need about (100 gallons) (138,000 BTU/gallon) divided by (4 months)(24 hours/day)(30 days) = 4800 BTU/hr on average for the 4 winter months that I run my boiler to supplement my wood heat. In reality, I know this will spike and drop over time, but I'm assuming that I would use a hydronic storage system to help balance this load, and would be looking at a heat source with a higher rating.

    Then the question is would any of this be cost-effective?

    My baseline case is just buy 200 gallons of oil. Currently, that oil would run $800/year if I'm lucky, plus I need to maintain my entire oil boiler system which adds a lot of cost.

    If I purchased equivalent of 100 gallons of oil with regular electric resistance heaters (i.e. a standard water heater) at $0.15/kwH, I'd spend about $606/year for the energy assuming 100% efficiency, and probably get about the same heat into my home as the 200 gallons of oil. This would have next to no maintenance or complexity. In theory it could even be done with cheap space heaters.

    If I was able to realize COP of 1.8 with a heat pump system, I think this means I could drop the price to $337/year. Probably more risk of unplanned maintenance/failure expenses, but probably still less than oil furnace. System cost is around $2000 for a 12,500 BTU system, so about 13-14% return on investment compared to a straight resistance heater.

    Anyone want to poke holes in this line of thinking or offer their thoughts?

    The other options I haven't sized include adding some high efficiency vacuum tube solar arrays and relying on existing electric resistance heater as backup when those don't do the job, and/or doing a water heat exchange loop behind my woodstove, integrating it into my solar control system as a second solar array that would take heat when stove is hot. The problem with the woodstove heating is that I'm not sure I can move that much more wood through it in the winter without burning faster/less efficiently, but I'd have to be burning at least 25% more wood to generate the required extra BTUs to supply water/radiant heating.

    -Colin

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  2. Red Sun

    Red Sun New Member

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    www.airgenerate.com.


    That's a link to a nicer looking unit than Nyle's.

    However, http://www.nyletherm.com/development.htm

    is developing a unit that might better help answer your questions based on their cost models or simply based upon what they have in development.


    I would compare the potential of the system under development with that of your consideration of eliminating the need for oil.

    I'm at 400 gallons of oil each year and can use small electrical panel heaters this year, to eliminate the load on the system to heat small upstairs bedrooms.

    http://www.eheat.us/

    Or, effectively eliminate perhaps 30% of the load the one upstairs zone requires now when heating with the antiquated baseboard.

    So my first question is to eliminate zones or supplement any zone in order to first eliminate the total heat required from the oil boiler.

    At this point I would rather consider a forced convection solar heating unit such as the http://www.cansolair.com/index.html or similarly the http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200339375_200339375 Solar Sheet systems and avoid buying any fuel.


    In eliminating zones from the oil system the best potential may well be to avoid buying any sort of fuels.

    I would like nothing more than to install a pellet boiler inline with the central heat system and use a second chimney to retrofit the boiler through. Truly a Maine sort of thing to do, because a pellet boiler if economical, could be installed where firewood was once stored in a room next to the kitchen. But, given the cost of pellet boilers and the inherent hassle of escalating pellet prices along with the need for bulk delivery, it is not yet worth the effort.

    However, creating a heat loop for a radiant floor system or that one can boost the return water temperature to the boiler via a wood/pellet burner with a water jacket it would seem plausible to favor the development of smaller boiler free system.
  3. cbrodsky

    cbrodsky Member

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    If you aren't reliant on oil for domestic hot water, I think one of the biggest opportunities is find a way to strech the shoulder season where heat requirements are low - this can allow you to leave the boiler shut down altogether for more months, easily saving 15 gallons/month or more.

    For period when you're using your heat a lot, dollar for dollar, electric heat could cost just as much as oil. For example, if your oil heating system is overall 70% efficient, you need about 28 kWh to equal a gallon of oil. Depending on your local electricity rates, you could figure out at what point oil becomes more expensive for routine use such as shutting down zones. Or alternatively, with $4/gallon oil, electric makes sense only if you pay $0.14/kWh or less.

    But in a shoulder period, your overall percentage efficiency is much lower because your boiler spends a lot of time in standby hot and waiting, but not being called upon to deliver much heat to the house. So let's say on a given day, you use a gallon of oil, but half of that is just your standby loss to keep the boiler ready on call. In this case, you only put half a gallon of oil into heating the house, at 70% efficient. 138,000 BTUs*0.5 gallon*0.7 efficiency = 48300 BTUs = 14 kWh. So on a day like this with lighter use, you could spend 14 kWh to save a gallon of oil if you shut down your boiler - that will almost certainly pay off.

    -Colin
  4. Jerry_NJ

    Jerry_NJ Minister of Fire

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    RedSun, THANKS for the link to the airtap, looks great, one plus to add in is during the summer the unit will help dehumidify your basement, I run a dehumidifier anyway, and if I cut that off the hot water cost may just balance the saved power. I have a geothermal hp, and it has a hot water heat, but it has not worked well due to the constricted water flow required ... uses the drain tap on the WH to both take and return water to the tank..it becomes plugged in a short time. The airtap using a direct coil in the water tank will not have this problem.

    I am checking with my power and light to see if they offer any rebate, that would do the trick for me. Looks great on first inspection.
  5. Jerry_NJ

    Jerry_NJ Minister of Fire

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  6. Red Sun

    Red Sun New Member

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    The house is built turn of the century well before any sort of modern appliance let alone electricity... However, when they built the house (~1850-1890s) and Maine winters were real, they managed to stay warm. Probably and most likely better than I've been able to do when always relying on oil. (Although, a pellet boiler fits the scope of the house really well)

    The system has two domestic hot water systems. One off the boiler (on demand) and another using an electric hot water tank. In the shoulder season the electric is switched on the oil off.

    I'm with you on the cost of electric heat as a function of outdoor temperature or total consumption. However, when room size is small enough the cost per btu is significant, yet the formula regarding cost per btu does not consider the area to be heated or that the room is on the second floor... I ran two eco heaters last winter with one on all the time and the electric bill doubled ($105). The units were downstairs in order to keep a laundry room above freezing and a kitchen area to test the cost and efficiency of the heater to see if it performed as well as advertised. With the Lux plugin programmable thermostat ( http://www.luxproducts.com/default.htm ) I could manage the cost of running each unit. I'm sure I can reduce the load on the furnace in the upstairs area by supplementing the heating system with electric. Interestingly the converse of heating a space with oil versus electric is likely a function of the space (area) to be heated as it is the cost per btu.

    The real problem (me) is considering the use of sensors and a micro-processor of some kind that can learn the heating system in order to propagate the correct heat source to use or that it is optimized. When I discussed with a local heating professional the existing oil system, he went right for the configuration of its plumbing. Where I had measured the total return temperatures I failed to measure the temp for each of the zones. In short, the system has one circulating pump and two taco zone valves dedicated to an antiquated Crane series 70 -- 105,000 btu oil boiler. The advise given is to add in circulaing pumps.

    One leg of the downstairs zone runs only 1/2" pipe feeding two kickboard heaters in the kitchen. Perhaps a thirty five - forty foot loop. Since I cook with a microwave the kitchen is unusually cold. One question I have is how much would the entire system benefit if at all possible if I installed a pellet burner with a hot water jacket and fed that line through a heat exchanger into the return loop of the kickboard heater loop? At today’s pellet prices I’m told to expect pellet consumption to be as much as $200/mo.. I doubt it is at all feasible to install a small scale pellet system considering the added fuel cost.

    My remaining reasoning is left therefore to think green.
  7. Highbeam

    Highbeam Minister of Fire

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    Thank you all for the electric heat pump links. I hadn't seen these two versions and they are quite interesting. I am not too keen on dumping the cold air into the home as both models desire indoor installations. Interesting that they use the old water heater which would leave that tank's heating system intact. Dual fuel water heater!

    The heat pumps claim 50 dB noise level. Is that about like a refrigerator? They heat to 130.

    A guy could put one of these on his hot tub instead of the 8000 watt element.
  8. cbrodsky

    cbrodsky Member

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    I'm thinking that if you have a below-grade basement that is un-insulated, you'll stay at 50-55 which seems to be OK. But without that, I think the applications are limited to warmer climates as you note.
  9. Jerry_NJ

    Jerry_NJ Minister of Fire

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    Sorry about my double post, the second just a quot of the first. Maybe the moderator can remove the second post if he/she see this note.

    I hadn't given much thought to the noise level, but in my basement there is a dehumidifier running much of the time in the summer, and I can live with its noise level, I have a desk and work shop in the basement and spend a lot of time there.
  10. Redox

    Redox Minister of Fire

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    Interesting question, Colin! I suspect that you might get more answers if you repost this question in the Boiler room. Lotsa hot water there!

    I think you are on the right track in wanting to get rid of your oil burner, or at least shut it down and keep it as a backup. It sounds like you have made some serious investment in alternative energy and are ready to take the next step. The more load you take off your oil boiler, the less efficient it becomes and using it for DHW with a tankless coil is probably the most expensive way to generate hot water right now. There are a lot of people who have installed a conventional electric water heater and are saving money over a tankless oil setup. The numbers are probably real, IMHO.

    HPWHs are still in their infancy right now and this is the first I've seen of the Airtap unit. It looks like a good idea and is significantly less expensive than the Nyle unit. It is also the first unit I've seen that puts the heat exchanger in the tank with the hot water. Time will tell if this is a good idea as a failure of either the heat exchanger or the water tank will mean having to replace the entire unit. I believe codes will eventually require a double wall heat exchanger as mixing refrigerant (and oil) with water isn't a good thing. The only other unit I've seen available is the E-tech from AES and it is an add on unit similar to the Nyle. These are the only three units I have ever found for residential use, and I have been looking for years. Now that oil is $4 a gallon, we might be seeing more.

    Being so new to the market, they don't have much of a track record yet, but I think they will. They will definitely see more use in the southern parts of the country than up north. Since they are not really designed for outdoor use, you will have to pull heat out of your home to put into your water heater. I suspect that putting one in the basement is going to pull the air temperature well below 55 degrees in the winter as most seem to have about a 6000 BTU compressor in them. That amount of cooling can drop a good size room down 20 degrees in the summer and I suspect that your basement might end up dropping into the 30-40 degree range unless you duct the cool air somewhere else. Maybe it could be ducted into the room with the wood stove? Just a thought.

    I really wouldn't count on it to act as much of a dehumidifier as they aren't designed for this. Most of the specs I've seen show only a few pints of capacity and your need for hot water probably won't coincide with the need for dehumidification, but anything it does is just a benefit in the summer. In the winter, you don't really want dehumidification anyway. I don't think it is going to have the capacity for space heating as the specs top out at 8000 BTUs or so and they are really going struggle to put out more than 120 degree water. Stealing heat out of the basement to heat another part of the house is going to be a lost cause in the long run.

    I'm not sure what the future holds for these units. Just like a regular heat pump, their usefulness drops the further north you go and I believe a full solar system is going to be more attractive in the South. Since you already have the solar part, I would suggest an electric water heater right now in your situation. Install it after the solar heater for maximum efficiency and wrap it up with extra insulation and it will be very cost effective. The extra storage is a benefit with solar anyway!

    Jerry, you oughtta fix that desuperheater setup. You are missing out on the "free" hot water. Just put an unpowered water heater in front of your existing heater and pipe the heat pump into this new tank. Just my .02.

    Chris
  11. Redox

    Redox Minister of Fire

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    50 dB is a LOT of noise if it is in your basement and running for hours on end. 130 F is probably the top end for these units and 100-110 is going to be more realistic for efficiency's sake. I would use a separate heater as a preheat for your existing water heater if it's electric. Gas would have a lot more standby loss.

    8KW = 27000 BTU and these units seem to top out at 8000 BTU or so. It will work, but you will need a heat exchanger to eliminate corrosion issues. I'm sticking with the electric heat on the hot tub for now. They don't really use that much heat as long as you keep the top on. Disregard if you are having all night tub parties...

    Chris
  12. Jerry_NJ

    Jerry_NJ Minister of Fire

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    I hadn't thought about the risk of putting the copper tubing directly in the hot water tank, but that simple approach is what makes the AirTrap so economical. EDIT: I did watch the video for installing the AirTrap, and as the copper tubing is "T'ed" into the hot water exit pipe it can be removed and reinstalled in a new tank if the tank needs to be replaced.

    As for getting my goe-hp working again, I hadn't given any though on an aux tank as a pre-heater, any suggestions on where to get such a tank, Chris? The current arrangement is full stop, but I can just leave the drain stopped up as the pre-heat tank will be in series with the cold water feed. As I can no longer flush the electric tank it may need to be replaced before long...it must be approaching 8 or 9 years old anyway.
  13. cbrodsky

    cbrodsky Member

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    This would seem to be one of the major questions that I need to do some calculations around. The units I have been looking at are 12,500 BTUs. Our basement is about 1,000 SF fully open, and below grade block walls with no insulation. From what I read on here, it is fairly challenging to heat such areas with 30,000 BTU woodstoves as the heat just sinks away quickly, so this is why I'm thinking it may be even harder to cool that space with a much lower BTU compressor system. But that is something I need to size carefully.

    I run my woodstove 24x7 and still need a little extra heat on cold days, so I don't want to duct cold air into the house I actually need a net addition of heat on some days, but probably not more than 10,000 BTUs at any given time, which has me interested in these.

    Agree with comments of others that such an approach really necessitates using radiant whenever possible - then the 110-120 F water output is just about perfect. I would assume that I'd have to go down this path throughout the first floor to make it work, and would use higher temperature resistance-heated water only when using 2nd/3rd BR heat on baseboards, and even there, it would only be low supplemental load to the wood heat that hits most of the house fairly well.

    -Colin
  14. Telco

    Telco New Member

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    Is there any way to insulate the basement? If the basement were insulated on the floor and side walls it would take far less to heat, and that heat would travel to the upper floors taking some of that load off. You might find that the system you have is up to the task without the wood boiler, if you had a warmer buffer zone under the house. Then all you'd need to do is address the domestic hot water.
  15. cbrodsky

    cbrodsky Member

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    Sorry I may not have been clear - our basement is intended to be unheated, and my woodstove is upstairs in the insulated house. The floors are insulated so our heated area is completely isolated from the basement.

    The problem with the heat pump water heaters is that they cool the area they're in as a byproduct of the process, so I would be counting on the basement reequilibrating through the un-insulated sub-grade walls to stay in the low 50s. Opening a basement window to vent the cool air isn't an option because the efficiency drops as you get to sub-freezing air temperatures, and you'll have to either pull in cold replacement air from outside, or warm air you worked to heat in the house.

    I just got thinking about this because I hear how hard it is to heat an uninsulated basement - so now I'm wondering if I can make the same argument the other way in my favor for this application. Particularly since the BTU cooling output would be about a third of a typical woodstove's heating output.

    -Colin
  16. Jerry_NJ

    Jerry_NJ Minister of Fire

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    If your below grade basement walls (NYSoap..) is 5', say, then only about 2' of that is significantly above freezing in the dead of winter. The freeze line in Upstate NY must be at least 3'. So you have little or no wall sourced by the earth at 50 degrees or better in the winter. I'd expect you'd see a noticeable drop in room temp in the basement due to HP water heater. That may not be a problem, just reduce the efficiency of the HP, not freeze the plumbing, I'd guess, I don't have any real empirical numbers - but I do recall some study data in one of the early mentioned links that talked about drop in room temp.
  17. cbrodsky

    cbrodsky Member

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    Good point - I'd guess on average we're about 6 feet below grade. Without any heat or A/C in the basement, we stay around 50-55. Maybe some of that is coming from the house above being warmer, and part could also be the fact that the boiler is on and likely leaks some heat to the basement. Shutting down the boiler maybe would drop us lower if it's actually providing enough heat to overcome conductive losses out the walls.

    I know the freeze lines are supposed to be fairly deep, but then again, I keep fish in a 2 1/2 foot deep pond outside the house and I usually only see this freeze a few inches thick. I was really questioning the need to bury my porch piers over 4 feet deep :)

    -Colin
  18. Redox

    Redox Minister of Fire

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    Wow, must have some serious minerals up there...

    I would consider a 50 gal regular electric water heater as a preheat tank ahead of your regular water heater. Let the desuperheater or heat pump warm up that tank to 100-110 or so and let your regular heater bring it up the rest of the way. That way, if you have a heavy demand, you will still have a full heater available. Quick and dirty, disconnect that pipe from the drain valve and tee it into the cold water inlet. It's almost the same thing, but no chance of clogging.

    There are solar storage tanks available up to about 120 gal, but they are very expensive. 50 gallon heaters are very popular and therefore more price competitive. You don't really need to save up an entire day's worth of hot water as the heat pump will cycle many times a day to keep up. You just need to cover a load of laundry or a shower for maximum effectiveness.

    Chris
  19. Redox

    Redox Minister of Fire

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    Careful with the specifications on a heat pump as there is the heat of absorption or the cooling BTU rating and the heat of rejection or the heating BTU. The difference will be the heat of compression and is basically motor heat. Assuming reasonable efficiencies, that 12,000 BTU unit will absorb about 9000 BTUs of heat from the room and use about 1KW of electricity to do it. The equivalent electric element would need about 3.5 KW to do it and give you a COP of about 3.5. I am basing this on the typical AC compressor rating of 45/130 delta. These are rough numbers and will depend on the compressor and heat exchangers used.

    How much will this drop the temperature in your already chilly basement? I suspect that it is proportional to the temperature rise if you were to add heat, but there will be differences. I'll let others debate this, but if you did a heat loss calculation for the basement, you would be in the ballpark. If 12000 BTUs raises the temp from 55 to 65, then the same amount of cooling would drop the room from 55 to 45. That 50-55 degree number is typical in basements as the earth will moderate at this temperature below 4-6 feet in practically all areas of the country. This is why GSHPs work in the first place!

    I'm not sure why you would want to use electrically heated water in a baseboard, but it is possible. I have never seen a heat pump used on radiant flooring, but it is also possible, though I have never found anything designed for this. I have been considering a radiant floor in our walk out family room and I think a heat pump would be a great way to take the chill off in the spring and fall when the stove isn't lit.

    Chris
  20. Jerry_NJ

    Jerry_NJ Minister of Fire

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    Chris,
    Thanks, I hadn't given a thought to another hot water tank in series with my existing electric water heater, but sure that would work. What I figured would be available was a smaller insulated tank of say 10 gallons that had an upper inlet and lower outlet that would connect to the HP loop, then the tank would have a lower input (could be a tub inside the tank as is the case on water heaters) for connection to the cold water feed, and an upper outlet that would feed into the existing water heater. As to why the existing "T" in the drain of my hot water heater plugs up I don't know we don't have real "Hard" water, no stains in the sink/toilet/etc and soap lathers well, still the HP (Waterfurnace) supplies "T" that was installed in the existing water heater plugged up soon after installation, then a couple of years later I replaced the water heater, made sure the "T" was clear and reinstalled. It plugged up rather soon too...don't remember how long it took, a few months perhaps.

    Now if we were to use another water heater tank we'd be short one tap (I/O) as I see it, we'd have the known three, not the needed four. The three being: cold in, hot out and drain. As I think the HP heat exchanger isn't free flowing enough to handle the full flow of the hot water demand, it has to "set" on the side and pump heat into the HWT as a parallel function. Hope this makes sense, the point is I don't see how I can use a second standard HWT as the parallel pre-heater using the HP waste heat (A/C assumed for this test).
  21. Redox

    Redox Minister of Fire

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    I don't think it really makes a difference. The drain tap on the heater goes to the bottom of the tank just like the cold inlet. The only difference is that there will be less of a pressure change if you are running water through the heater for the house. I have seen gas water heaters with extra taps for space heating, but that is a different application and I'm not sure it really means much anyway. I'd try connecting the inlet of the desuperheater to the cold inlet of the existing water heater and see how it works. It HAS to work better than that drain that keeps plugging up, right?

    My suggestion on adding another tank in front of the heater is to maximize efficiency on the desuperheater. Having the electric element come on when the desuperheater might be able to take care of the high demand isn't as efficient. I would also go as large as possible within reason as the preheat tank is going to even out the difference between demand and production. Just a guess, but the average shower is going to drain that 10 gallon heater pretty quick and force you onto electric resistance sooner. The A/C will be able to warm it up fairly quickly, but you will already be out of the shower. More storage for the "free" heat is a good thing! Besides, price out that 10 gallon heater; I doubt it's 1/5th the cost of a 50 gallon heater.

    Chris
  22. Jerry_NJ

    Jerry_NJ Minister of Fire

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    Thanks, still missing something here on the plumbing and fluid dynamics subject. I believe the desuperheater has to have the ability to circulate water when there is no demand for water so that it can heat the stored water during slack usage periods. The plumbing you suggest would, as I read your suggestion, pass water through the desuperheater only when there was a demand for hot water. The desuperheater has to have a "parallel" in/out path to the storage tank, and at the bottom would be best as it would draw the cooler water into the desuper.. and if the hot output was also low, no big problem, the heat would rise. That's the idea behind the factory method of putting a concentric pipe inside a 1/2" pipe that can be stubbed into the drain hole... with a "T" to allow draining, that's what I know know to be clogged.

    My 10 gallon ideas was just a "number", thinking there may be some special small tank designed to work with a HP desuperheater. I haven't asked Waterfurnace, I may do that.
  23. Redox

    Redox Minister of Fire

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    The simplest and most straightforward way to do this is to tee into the lines going in and out of the water heater. Cold goes into the tank as well as the DSH and hot leaves the tank and DSH, assuming the DSH has a circulator of its own. When you are not actually using hot water, the circulator pulls water back up the dip tube on the cold inlet and returns it warmer to the top of the tank on the hot outlet. I have no idea how you would do this without a circulator; maybe some kind of sidearm arrangement? Could you post a pic of this concentric fitting? I haven't seen anything like you describe. Maybe this is why it has been clogging up?

    Chris
  24. Jerry_NJ

    Jerry_NJ Minister of Fire

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    Yes, a "T" could work, the DSH has a low capacity circulation pump that runs, when enabled, whenever the compressor is running. I'll take some pics of the outside of the existing connection. The pipes are 1/2", so the pipes have the capacity and I assume the design takes into account it is working as a circulatory in a closed storage system that is under pressure, i.e., the well pump/tank pressure.
  25. Jerry_NJ

    Jerry_NJ Minister of Fire

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    Loc:
    New Jersey USA
    Here are a couple of pictures of the connection between my electric hot water heater and my geothermal heat pump. The two insulated pipe lines going to the right go the the HP and the line to the left is the drain line, a third use for the old drain connect on the water heater. I suspect the drain is just the input side of the lines going to the HP.

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