Hot water recirculator with Geothermal/Tankless Hot water Supply

[aadelgandio]

I am looking to install a hot water recirculation system to my domestic hotwater system. I have a Tankless hot water system coupled to my geothermal system with a 40 gallon storage tank. I'd like to install a recirculating system such as a Grundfos Comfort system to have instant hot water available at the taps as well as keeping my storage tank filled with hot water. Can I accomplish this with one pump or would I need one circulator pump with a return loop to heat the storage tank and the Grundfos System to have the hot water available at the taps? Any thoughts?

[zacmobile]

page 15 & 16 of this manual should be what you are looking for.

http://centralair.ca/pdf/Hot_Water_D...nual_Rev_C.pdf

[DJV]

Correct me if I am wrong, but the diagrams you directed to were not using the storage tank as a desuperheater. The diagram shows cold water entering the tankless unit and then going to the storage tank?

I am unsure how a circ pump would operate with just the 40 gal storage tank but I assume a desuperheater tank can be piped in and have that water coming from the desuperheater enter the tankless, then proceed to the circ pump and another smaller expansion tank.

[zacmobile]

Good call, it would require some modification: I would use diagram 16 and eliminate the small buffer tank, instead put your geo storage tank in between the rinnai cold water connection & the recirc return tee. I can't see any problems with that, anyone have any opinions?

[OBS]

Combining a tankless water heater with a storage tank make no sense. The only energy benefit for tankless is the absence of a tank. But if you already have tankless, I guess it's what one would refer to as a 'sunk cost'.

As for hw distribution, conventional recirculation pumps are a bad idea. Our local community requires timer based recirc pumps (for water conservation), but this type of system wastes a lot of energy. I'm not talking about pump energy, but conducted heat loss through the pipes. Given the temperature of the circulating water, this is obviously an issue even if hw pipes are inside the envelope and insulated. The problem is, most folks can't predict their hw draws, so they end up setting the pumps for up to two or three hours a day, circulating COP 1.0 hot water to heat to their homes. Double whammy in summer since you now have to remove that heat.

The best solution is an on-demand pump (ChiliPepper, Metlund D'mand, etc.). These activate by push button (multiple buttons can be installed on a single pump), and only run long enough to get hot water to the last fixture on the trunk. And they don't require a dedicated hw return loop (they can use the cold water trunk). The pump costs more than a conventional recirc loop, but that's quickly made up for in energy savings by eliminating wasted heat loss from pipes.

Water and energy conservation experts in my community have finally convinced the city to reverse it's mandate for the stupid recirculation loops and require on-demand. In new construction, structured plumbing and fixture clustering is the best option. I'm on a working group to update the RESNET HERS standard to reflect the efficiency of hw distribution systems. BTW, RESNET recently amended the HERS standard to de-rate tankless water heaters since it's well known that the official EF ratings are bogus.

[docjenser]

Circulation lines are clearly huge energy hogs if run continuously. We have used motion sensors in the bathrooms to turn on the circulation pumps for a short time.

[martyg]

This reply may be a little late for you but I have personal experience with this set up. I had a propane fired demand water heater and a recirculating loop before I installed my GSHP. Initially I didn't try to tie them together but I just couldn't leave well enough alone and so invested a lot of time and about $1,000.00 in parts only to be disappointed. The BIG issue is I don't get enough run time on my heat pump to produce enough hot water to fill a 40 gallon holding tank. And the run time I do get doesn't correspond with hot water demand time.
My original demand water heater/recirculating system was set up per instructions and diagrams from Ranii using a 20 gallon electric water heater, the smallest I could find that had 3/4" inlet and outlets. Originally I didn't include an aquastat, the pump just ran 24-7. My loop and branch hot water pipes are insulated with 1" thick foam. I now have an inline aquastat installed that senses the water temperature in the loop. I managed this by using an aquastat with a remote bulb and a bulb well. I installed a tee in my loop and installed the bulb well in the tee. So my electric heater is set at 120 degrees, the Ranii at 130 degrees and the aquastat at 105 degrees. The pump hardly runs at all but I always have 105 degree water at my disposal. When I do call for hot water, the Rnaii turns on and replenishes the system with hotter water that the electric tank is set to supply and so limits the amount of time I'm using electricity to heat water (the electric heater runs on 110 volts!)
I did couple that existing system to a 40 gallon electric water heater by running my recirculating loop line into the pressure relief valve opening at the top of the tank and the desupperheater loop into the cold water inlet opening in the tank (my desupperheater has a built in circulating pump so I am using two pumps). This works but since I don't get enough run time from the GSHP I end up using the electric tank to make hot water and storing 40 gallons of it at all times; not what I had hoped for.
My HVAC contractor tried jumping my desupperheater control wire so the desupperheater would run any time the system started up and that did supply a lot more hot water but not on a "demand" schedule.
The most encouraging news for this combination is Climate Masters announcement of a new model with "demand" hot water controls built into the system. I'm hoping they will make available a retro-fit for existing systems. That will eliminate the need for the Ranii heater.

I hope this helped.
Marty Graff
(just a dumb builder)

[OBS]

Marty, I don't know how much you pay for propane, but I believe your electric rates are sufficiently low that an electric water heater would be significantly less expensive to operate than propane. Let's say you pay 9 cents per kWh, then it costs $2.64/therm to heat with electricity (multiply electric rate by 29.3 to convert to therms). An electric storage tank is about 92% efficient due to jacket losses, so it costs about $2.87 per therm delivered to the tank.

A non-condensing on-demand gas heater has a seasonal efficiency of roughly 80%, but when operating in tandem with a storage tank, it's going to be closer to 70% (but not as low as a conventional gas storage heater since flue is separated from tank).

Given these assumptions, the break-even point between electricity an propane would be $2.00 per gallon ($2.87 x 0.70). You can change the numbers to match your prices, but I believe your electric provider (Progress) has lower than average rates, so you're probably going to want to disable the propane water heater.

A couple of additional comments...

Don't worry about the fact that your heat pump doesn't produce hot water on demand. That's why desuperheaters require a buffer tank. Whatever heat is transferred to the buffer tank during heating and cooling cycles will (mostly) still be there when there's a hot water call, and will reduce the amount of energy your electric heater has to add.

With the new R410a refrigerant, a desuperheater won't produce as much hot water in cooling mode as older units with R22.

Finally, as mentioned earlier, circulation dhw loops are a huge waste of energy. You should really rethink that. I recommend installing one of those 10 minute wind-up wall timers (Intermatic) in a convenient location to activate your recirculation pump on demand.

[martyg]

Hey OBS,
Everything you say is correct and I have thought it all through looking for the "best" solution. I have an additional issue that drives the propane option and that is that my back up generator is only 11K and while it can supply essential house consumption and run the CM27 it can't do that and power an electric water heater.
I mentioned that we jumped the DSH control to turn on whenever the HP was on and set the dip switches to 150 degrees to get as much hot water as possible during run times but that brought up more issues, like the water is too hot. Then I installed a mixer valve to blend cold water back into the loop so we didn't get scalded. Other issues were that as you correctly point out the loop and recirculating system is an energy and heat waster. So I found I could heat up 40 gallons of water but then over the next few hours I cooled it down in the loop.
All of that was going on before I installed the aquastat and that feature is critical with a recirculating system. I've also had to do some work balancing the temperature settings with all the various pieces of this puzzle partly because of calibration of each componet and partly because of heat loss. For example I have the Ranii set at 130 degrees but the water temperature at the drain valve directly connected to the unit measures 117. And even though the electric water heater was set to 120 the water in the loop ( I have a drain I use for bleeding air) was never warmer than 108. My house is twenty years old and since I built it myself many of my interior walls are insulated making it difficult to fish in control wires for individual locations (to turn the recirculating pump on and off).
All that said I really don't need a DSH on demand but what would be very nice is a dual control so a water heater (the buffer tank) cold call for the system to start up and make hot water (maintain a constant temperature setting) independently of heating or cooling demand. CM's demand DSH just takes that a idea a giant step further.
By the way propane cost varies around here but if you have a tank supplied by one supplier no one else will fill it. I've done price checks now and then over the last twenty years and found my supplier (Ameigas) to be competitively priced but last year my daughter paid $2.70/gal for her propane and Amerigas wanted $3.60 (for a pre-paid 1200 gallon purchase!)
If or when CM produces a retro-fit for my HP I will install it and disconnect the Ranii.
Marty

[OBS]

You're making things too complicated. Sounds like you're more into controls than energy efficiency and saving. Since you're handy with controls, I'm sure you're aware of X10 and other wireless control solutions. The thing I like about the wind-up timer is that you don't have to remember to turn it off. At a minimum, you should be using a timer to limit the pump's operating hours (do you allow it to run all night?).

If or when CM produces a retro-fit for my HP I will install it and disconnect the Ranii.

You already have a DSH, what kind of retrofit are you talking about?

I don't understand why you don't keep the Rinnai turned off except for extended power outages. At those prices, your electric water heater costs much less to operate!