# Geothermal Desuperheater



## mbcijim (Nov 3, 2008)

After doing a search here and googling, I found very limited information.

I just got my ground source geothermal up & running two weeks ago in my new house.  Too my surprise, the geothermal unit (5 tons) has a built in desuperheater (or maybe I bought it and didn't know it?).  

My next surprise was my plumber gave me an 80 gallon electric hot water heater (for 2 people)!  The contract says 52 gallon and he installed the bigger unit for free because our tub is 60 gallons.  He thought he was doing me a favor....  This 80 gallon hot water heater says it cost $550/year to run ($45/month)!  Needless to say I am not happy.  Our old house had 52 gallons and I can't ever remember running out of hot water.  

I have a couple of questions:
1.  Everything I have read (very limited) about desuperheaters says 'free hot water during cooling season (my location May- September) and very low cost hot water during the heating season.  Can anyone verify?  
2.  Any suggestion on measuring my electric use to verify?  I've read about those Kill-O-Watt meters.  Can I install one on my heater to watch my electric use?  
3.  Any general knowledge about these desuperheaters would be a huge bonus.  

One general comment about my GSHP (forced air) seems a lot hotter than my Oil (forced air).  Doesn't really make sense does it?


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## Redox (Nov 4, 2008)

That's a first!  I don't think I've ever heard anyone say their heat pump was warmer than their oil burner.  Wait till winter sets in and get back to us!   ;-) 

Ignore the sticker!  The water heater won't use much more electricity than the old one as long as your consumption habits don't change.  If you suddenly start taking 20 minute showers because you have 80 gallons on hand, welll....

Answers:

1. It's free as long as your A/C is on.  It can provide all your hot water in August, but probably not in May.  During the winter, it's probably about 1/3 the cost of the electric water heater, so the word "very" is probably appropriate.

2.  The Kill a Watt is only good to 1800 watts and 120 VAC.  The heater is probably 5000 watts at 240 VAC and won't work.  Since the element is a known wattage, you could attach an hour meter and multiply it out.  OTOH, if the heater is that big, you might want to try connecting it to 120 volts and plugging in into the KaW.  A 5000 watt element would use about 1250 watts on 120 VAC.  Recovery would take all day, but if it's just the two of you, this shouldn't be a problem.

3.  Desuperheaters provide a low volume of water at very high efficiency.  The big tank is to your benefit to fill in the gaps between production and consumption.  I think your plumber did you a favor!  Enjoy!

Chris


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## Jerry_NJ (Jan 23, 2009)

I finally got my DSH hooked up correctly, after only 15 years of the hot water DSH from my Geothermal HP not very well.  Now with a lot of run time due to the very cold weather in the north east I find the HP provides all the hot water I need.  How do I know?  I shut the electric hot water heaters off.  I have to keep an eye on things though, if my wife is washing dishes and cloths and taking a shower, I run down and kick the electric water heat back on.  I've done this a couple of times in the last few weeks, but returned to running with the electric water heater off.  I also found out that the way I modified my HP hook-up to the electric hot water tank is now what the Mfg recommends. They didn't 15 years ago.  This is the subject of another thread, I can't find at the minute, it has pictures of the hook up modification I made.  I even got a complaint from my wife that the hot water is too hot.  The tank is set at 125 degrees, and the HP will drive it up to 150 degrees when hot water usage is low.

The 80 gallon tank is better, the DSH will not generate a lot of hot water fast, so you need a big reserve.  Your only additional loss is due to the larger surface area of the larger tank.  Still, with modern insulation and the fact that the HP is producing hot water for 1/3 or 1/4th the cost of resistive electricity (and for free when you're throwing heat away in the summer) hot water heating, you're ahead of the game by miles.  Enjoy the savings.


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## mbcijim (Jan 24, 2009)

Jerry,

Very good info, thanks for posting.  I might just try shutting it off myself.  I never imagined I would get that much hw from the de-superheater.

Question for you:
1.  What size unit is your ground source heat pump?
2.  Any idea how many KW/day it uses?  

I'm running two units - cooling size 5 ton & 2 ton in a brand new home.  That plus the hw heater are my major electrical users.  
Now, I didn't realize this when I bought it, but they have electric back-up heat built into them.  RESISTANCE heat.  The 2 ton unit electric resistance heat part never runs.  The 5 ton unit... it runs ALOT, especially when it's cold.  I'm using between 120- 250 KW PER DAY!  Now, today, the resistance heat hardly ran and I used about 60 KW in 12 hours (it's pretty warm).   That's suggesting my two heat pumps are using 80 KW/Day.  Is that possible?  In my old house I would use closer to 30 KW Per Day with oil heat.


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## Jerry_NJ (Jan 24, 2009)

mbcijim

I assume you mean KWH, killo Watt Hours, and wow, that's a lot.  My house must be a lot smaller, it is 2000 square feet and my total electric usage on a cold month would be no more than 1400 KWH, or under 50 KWH per day.

I have a 4 ton Waterfurnace HP, two speed with about 20,000 BTU (per hour) in stage I and 36,000 BTU in stage II.  It has a two stage resistive auxiliary and emergency heating, each is 5KW.  The aux resistive heat doesn't come on unless the outside is below zero, very rare her in north/central NJ.  But, when I set the heat back by 6 degrees for over night I have to bring the heat up a couple of degrees at a time in the morning.  My unit is 15 years old and it has a "dumb" thermostat, I understand the newer units will "walk" the heat up in the morning on an internal program/clock.  I sometimes let the first stage of 5K resistive come on for 1/2 hour to help speed up the warm up.  This is the tricky part of saving energy/money by setting back at night.  If bringing the temp back up in the morning causes a lot of resistive heating to take place all the savings over night will be spent on resistive heat in the warm-up.

My house is engineered to run HP only down to zero degrees outside and 70 degrees inside.  This seems to be the case.  The engineering data estimates my loss at zero degrees outside is about 35,000 BTU/hour....or my full stage II HP capacity.  At this point hot water is also shut down so that that heat can be added to the house heat.  My DSH is specified to have a small load anyway, only 2 - 3 thousand BUTs per hour.  But as said, this will keep us in hot water when heavily heating.  On light heating periods, say outside temps in the 40s, some resistive water heating may be necessary.  My unit runs with a COP in the 3.5+ range, as high as 4.2 if running in stage I.

Again, 80 KWH per day is a lot, and here in NJ that's about $13 per day, or $400 per month.    If you've got a 3,000+ sq foot home and a couple of teen age children I suppose you could use that much electric power, otherwise something doesn't seem right. Of course a home with poor insulation/window/door seals could also cause that much power usage.


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## mbcijim (Jan 24, 2009)

Jerry_NJ said:
			
		

> mbcijim
> 
> I assume you mean KWH, killo Watt Hours, and wow, that's a lot.  My house must be a lot smaller, it is 2000 square feet and my total electric usage on a cold month would be no more than 1400 KWH, or under 50 KWH per day.
> 
> ...



Yes I mean KWH, etc...

I believe my units are 5 ton and 2 ton.  The house is 3,500 sft but it is all new, blown-in insulation and the expansion foam insulation in the walls.  I started a new thread on this topic, but I'll follow up with you here.  I think my backup auxillary heat in the unit that runs is 20KW.  It runs ALOT.  Especially last weekend when it was so cold.  I have a smart thermostat, but I have it set on 70 HOLD right now, because whenever it brings the temperature up, the auxillary heat would run for an hour.  I had to be using more heat bringing the temperature up than to hold it steady.  

My COP is 3.9.  If we proportion the 7 tons to your 4 tons, I should be using 75% more electric, with maybe a little discount for a slightly better COP.  But that would seem to indicate I should be using (1+ 75%) x 50 KW = 87.5 KW/Day.  I'm not I'm using much more. 

My 'smart' thermostat was saying it is 68 in the house and set at 68, and the auxillary heat would turn on.  That doesn't make sense.    Any suggestions?


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## semipro (Jan 24, 2009)

Sounds like your thermostat isn't working correctly if aux. heat is on and the set temp and indoor temp are the same. 

Some thermostats like my programmable Hunter unit will automatically engage the resistive heat if the set temp is more than 2 degrees higher than the indoor temp.  My previous house had a different thermostat that also had this "feature".  There was an incon on the display that indicated when the resistive ("emergency") heat was on.  If I wanted to raise the temp manually I had to do it in 2 degree increments.  Since we have a GSHP which is way more efficient than electrically resistive heat I disconnected the wire on the back of my thermostat that engages the resistive heat.  I plan to add a switch to be able to enable/disable it that way but as yet have not needed the resistive heat at all.  

Wish I had the desuperheater that was optional when my GSHP system was installed.  The previous owner didn't spring for the extra cost.  I think half our electric bill goes to heating hot water for my 2 teenaged sons.


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## Jerry_NJ (Jan 24, 2009)

Completely agree, you thermostat is not working, or the "control board" isn't.  My thermostat is just like Semipro describes, even being a 15 year old model. The owner manual also shows what wires to connect/disconnect to enable/disable resistive/aux/emergency heating.  I haven't done that, but I have gone to the air handler (inside unit with the compressor inside, along with the blower, and two 5KW resistive heating elements) and disconnected the relay that engages the second 5KW (total of 10KW, about 32 cents an hour to run the full 10K resistive) so I allow resistive aux to a maximum of 5KW to run, and it does briefly if I raise the temperature more that 2 degrees following the nigh set-back.  We also run at 66 degrees for daytime temperatures, so that too accounts for some of the "lower" electric use/cost. 

When I referred to a "smart" thermostat I had in mind what my heating contractor said about he current design which allows one to set the warm up to work automatically in 2 degree increments, and to start, say, an hour before one rolls out of bed.  In my case, and we're a two story home, I get up a bit early and go down stairs to bring up the thermostat 2 degrees and sometimes go back to bed for 30 minutes.  In any case it gets the warm up started before my wife gets out from under the comforter.  In the original design we had a two zone system with an A-to-A HP.  Then we could start the upstairs at the bed side, still had to get out of bed.  The geothermal (gshp) contractor say no two zone, we need to circulate are throughout the whole house to work efficiently.  I've been ok with that given the high efficiency of the HP.  Compared to oil heat at $4 a gallon (yes it was less this year) the HP saves me at least $2K per year, maybe more if I count the higher efficiency I get for the A/C unit too, 17 EER even if the outside temp is 100 degrees.  As a two speed A/C compressor too, has an EER of 20 when it can run in low speed.

The DSH may not be able to keep up with two teenagers, but it would help reduce the cost of hot water in any case, and what I really love is when the A/C is running I'm throwing heat away anyway, so the heat going into the hot water tank is FREE hot water.


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## semipro (Jan 24, 2009)

We never used the AC last year much to my wife's dismay.  If we did have the DSH I'd be more inclined to run AC though knowing the heat was being pumped from my house into my DHW. 

That same relay that Jerry disconnected on his HVAC went bad on ours and was stuck in the closed position.  I noticed our electric meter spinning like a circular saw when nothing seemed to be on.  Then I noticed heat exiting our registers even when the system was off.  One of our resistive elements was running constantly.  Kind of scary actually from a safety standpoint, and yeah, our electric bill was the highest ever.


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## Jerry_NJ (Jan 24, 2009)

Yes scary, I believe the resistive heating elements are designed to have forced air flow over them, not just convection currents.  I'm surprised the element didn't burn out/open.

Just for the record, the electric usage numbers I've posed, e.g., about 50KWH per day for cold January/February months, is from usage before my new Quadrafire Wood burning Insert.  So, the usage is when I was 95% heat from the HP, just a casual fire in the fireplace (an old, but efficient slammer insert) with maybe 4 evening fires all winter.  This winter I'm burning about 2 cords of firewood and expect to see a sizable reduction in heating from electricity.  This is driven more by the enjoyment of a wood fire, than $$$ as I have to buy at least half of the wood I burn the cost of wood heat isn't much less than my gshp cost of heat.


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## woodgeek (Jan 24, 2009)

My ‘smart’ thermostat was saying it is 68 in the house and set at 68, and the auxillary heat would turn on.  That doesn’t make sense.  Any suggestions?

I've got a 'smart tstat' (visionpro8000) on my air source HP.  It is designed to do exactly what you describe in situations where it senses that the primary source is not meeting demand.  The old staged stats would kick in the aux only after allowing a couple degree droop (uncomfortable).  A microprocessor can figure out that your primary is going full out, and still cycle the aux to maintain a single set point.  That droopless control is a major point of 'smart'. 

Sounds to me like you have a serious malfunction in your primary (GSHP) and the tstat is doing the best it can in face of that.  I know it stinks, but it seems that a lot of new (complex) systems run into problems after running a few weeks.  Have a tech out, and have him work out the kinks, whatever they are.


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## Redox (Jan 28, 2009)

That can be a problem with any heat pump.  You don't realize anything is wrong until the electric bill shows up!  Most GSHPs are sized so that they don't need any resistance heat at all.  The resistance is mainly for a backup in case the heat pump malfunctions.  Most of the better ones will have a "lockout" light to show a problem, but most of the time, it's watching the "aux" light that shows up a problem.

I have tried a number of stats over the years and never been really happy with any of them.  I ended up putting a switch on the backup heat and only turn it on in really cold weather.  An outdoor thermostat would do the same thing, but I want final control over when the backup runs.

Chris


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