Geothermal Desuperheater for DWH plumbing

[LD1]

Got the geothermal unit finally yesterday. Got things plumbed up and getting wiring now. Got some questions regarding heating DHW with the desuperheater.

I was thinking this unit hand an internal pump and temp sensor to know when to circulate the DHW through the unit to heat it. Found out it does not. Basically just a heat exchanger in the unit.

Talked to the tech and he said an additional pump is required to circulate from the hot water heater through the unit. And to tie it into the same contactor that controls the compressor and loop pumps. Ok, no big deal.

But What I am wanting is to do a two tank setup in series. Where the DSH heats a pre-heat tank that the incoming water supply is hooked to and that is what in turn fills the second tank that will have the heating elements. Goal is to keep the pre-heat tank nice and warm, that way when it fills the second tank, the elements have to do little at all. Only maintain. Everyone here showers in the evening, cooking, laundry, dishes, etc all typically done in the evening. I know a DSH is slow, but it will have nearly 24 hours to do its thing and catch back up.

My concerns are:
1. What about temp control? Dont want the first tank getting too hot and popping the T&P on occasion. The tech said as slow and as little heat as you get out of a DSH, that it just doesnt happen. Well, what if I leave the AC set to keep the house 70-75 in the summer and we go on vacation for a week?
2. Is a circulating pump even necessary? Or will convection (or whatever its called) be enough. Where the heated water naturally wants to rise, and in turn, returns to the top of the tank as it sucks cold water off the bottom?

Those with a 2-tank setup and DSH.....can you share (possibly some pics) of how yours is plumbed. Thanks

 

[davesl708]

My system is set up like you want it. The DHW tank stays around 95F. Feeds the hot water tank. Works great.
I don't think you desuperheater will get hotter than your hot water tank.

I will try and get some pics of the setup.

 

[davesl708]

View attachment 464060

The tank on the right is the DHW recycle tank. On the left is the hot water tank for the house.
On the DHW tank the pipe on the right is the feed from the cold water.
The water cycles through the drain at the bottom of the tank.
They use my old gas HW tank as the recycle tank.

 

[LD1]

So you are plumbed like this:
https://www.geoexchange.org/forum/a...ric_piping_for_geothermal_desuperhea-jpg.990/

Does your geo unit have a built in pump for the desuperheater, or did you have to buy one separate to circulate the water through the DSH?

If yours in internal, I bet that controls your temp and wont allow it to overheat. If yours is an external pump that you had to buy separate, what did you get? and did you put on any kind of additional temp controls?

 

[davesl708]

Mine is internal. Yes piped like your drawing.
There are no additional controls.

I have the Waterfurnace Envision model.

The DHW feature is optional. There is a switch on the front of the machine to turn it on and off. When off the water in the recycle tank stays about 85F.
That is enough for two but turn on the extra when the kids are here. Never have a shortage of hot water.

 

[LD1]

I never have a shortage with my 40 gallon heater, with the wife and two kids.

Just the thought of pumping in ~30 gallon of 40 degree water every evening and allowing it to be heated by the electric elements.

Simple BTU formula says 1 pound, 1 degree = 1 BTU. So I am asking for 30 gallons x 8#/gallon = 240lbs of water to be heated up 85 degrees to 125. which is 30,000BTU.

1 watt = 3.41BTU

~8.8 Kwh/day....264 kwh/month.

If I could get 85 out of a preheat tank, that lessens what the elements need to do. Now they only need to heat 240# of water up by 40 degrees.

9600 btu = 2.8kwh/day = 84kwh/month.

Bout a $20 savings at my rates per month.

But off track. My concern is overheating the tank. Everything I am reading about units with the built in pump, also monitor temp at the desuperheater. And will shut off the circulating pump if temps get too hot. Also read some that will shut off the DSH pump if it is costing too much preformance out of the geo as a whole. IE: not satisfying the heating demand of the house.

Read some horror stories bout desuperheaters causing the unit to run for extended periods, and in some cases actually making the hot water colder. Just want to avoid all of this.

Still wondering if a pump is even necessary. As water will naturally try to equalize. Get water hot in the desuperheater and it will want to naturally find its way to the colder tank, and thus sucking in more cold water to repeat the cycle.

 

[davesl708]

My house is total electric after I switched to Geo. Huge savings. Yes the cost of heating hot water is much cheaper now.
Never have any issues with heating and cooling.

 

[LD1]

Looks like a Bard GVDM-26 is exactly what I need to hook up the desuper heater http://www.bardhvac.com/digcat/volume_3_cd/install_pdf_file/2100-517.pdf

Anyone have any experience with one? Looks better than just a simple pump, as this has temp controls.

Excerpt from the above PDF:

Also connected to this board is a temperature overlimit
device which shuts down the GVDM-26 once inlet water
has exceeded 125° so the water cannot create a scald
condition.
There are also two (2) thermistor sensors connected to
the control board. These thermistors are measuring and
controlling to ensure there is a positive heat differential
across the water being circulated. When operating in
Part Load Condition, there are certain conditions
(Ground Loop Temperatures versus Hot Water
Temperatures) that potential exists where heat could
transfer from the hot water into the refrigeration system
instead of the refrigeration system into the hot water.
Through the control board logic, these thermistors
ensure there is at least 2° positive differential between
entering/leaving water temperatures and will shut down
the pump accordingly.

 

[LD1]

Gave the unit a test run today. Kinda sucks I aint got a t-stat yet, but wire-nut the right few wires together.

Low stage pulls 16 amps and at 70 degree return air, was giving 95 degree air out. So @ 240v = 3840 watts
High stage pulls 23 amps and at 70 degree return air was giving 100 degree air out So @ 240v = 5520 watts

Now I just need to find the CFM specs of the unit to calculate BTU's, and thus the efficiency (how many BTU's am I getting per watt).

I know on high stage, the fan speeds up. So 5 degree warmer air, and moving more air = :thumbsup:

Now just have to figure out the desuperheater part, wait on my t-stat to arrive, then the nightmare.......ductwork. As my house has NONE

 

[dodge man]

My system is also setup like you want and I don't think there is anyway it would ever get to hot in the first tank. I've never checked my temp but I don't think it ever gets hotter than maybe 100 degrees F.