Reverse indirect with geo for DHW preheat?

Gordan

So, as it turns out, most hydronic guys are not geo guys and vice versa. Since the system I'm gunning for is both, using a geo loop to heat radiant floors/ceilings, I'm finding that I keep running into what appear to be orthodoxies and dogmas that give rise to compromises that shouldn't be necessary.



For instance: why would anyone put a desuperheater on a hydronic heat pump? It only runs when there's a call for heat (or cooling), its capacity is a fraction of the full pump capacity, and it steals some of that capacity from space heating regardless of whether it's needed on the DHW side or not. (This last one could be just my misunderstanding of the principle, so I'd appreciate a correction if one is warranted.) Since hydronic ground source heat pumps have buffers on the load side anyway, wouldn't it be better to use a reverse indirect for both buffering and DHW preheat? That way you could also handle most of your DHW heating using geothermal even during the shoulder seasons, and you'd have better DHW recovery to boot.



"Free hot water during cooling season" sounds very nice, but how accurate is it? I would think that it would depend on two things: very large tanks (or very light and spread-out usage) and a sizable cooling load (only about 10% of which is available as waste heat for DHW preheat.) In my case, my cooling load is tiny. In my case and perhaps most other cases, DHW usage is nil during most of the day and high in the mornings and evenings, so I have to question the assumptions of the desup model.



You could even get free cooling out of the scheme I've outlined, if you buffer on the source side as well. Pump heat from the chilled glycol tank until it reaches the setpoint; then, if there's still DHW demand, circulate through the ground loop. The chilled buffered liquid is available for circulation to hydronic coils. The controls and piping for this scenario don't seem complex.



Anyway, I'm sure I'm missing the big picture, so I'd greatly appreciate it if somebody would kindly pop my bubble. :-)

Gordan

Nothing? Surely someone here has some experience with this...

I saw M.E.'s article on the "biggest, greenest" job his company did and it actually did mention using a reverse indirect with ground source heat pumps but no details. Is it really that much of a "boutique" application to use a heat pump in this way? Why aren't people insisting on getting the most use out of their investment in these efficient but super-expensive beasts?

Mark Eatherton

Cost efficiency = .....

CHEAP contractors. THAT is the only reason why they don't do it. They want the sale, but the know the consumer is already in pocket shock over the installed price of the system, and they don't want the DHW reverse indirect to be a deal buster, so they don't even offer it up.



I look at it differently. I say if you are going to make that kind of investment, you should get the MAXIMUM bang for your buck.IMHO, I agree with your assessment on the desuperheater, and have never included it in any systems I have done. Full condensing DHW preheat, or nothing at all.



You are also correct that residentially, the demand for DHW does not match the availability, hence an additional storage tank is needed to hold the hot water until the load comes around.In the case of retrofit, ALL possible conservation efforts MUST be performed prior to the application of GEO, or you are wasting some very precious, expensive BTU's.You are also correct in your assessment of the two trades. I have stopped doing GSHP systems and am waiting for a good variable speed compressor to come to the market before I dive back in. HTH, and thanks for reading the article.ME

Gordan

Mark, many thanks for your reply!

Could I please bother you for some more details on your typical installs? Did you use a separate DHW preheat tank and space heating buffer tank? (With dedicated circulators? With a 3-way diverter valve?) Or did you use the reverse indirect for both functions? If the latter, did you allow outdoor reset to determine the degree of preheat? (That would seem to make sense to me, personally.) Also, did you allow DHW demand to totally cool down the tank, or did you divert around it if it cooled down below a certain temperature?



I absolutely hear you with the conservation efforts, and it is my intent to insulate wherever feasible and minimize infiltration, and am also looking at a GFX unit for DHW pre-preheat.

Mark Eatherton

All in one..

Buffer tank AND preheat for DHW. As for doing the outdoor reset thing, you can, or you can se it to operate at 115 degrees F constant, then do a variable speed injection into the space heating loop.As for prioritization, I would not worry about it. The DHW loads will not last that long, and by the time you realize that it is off, its already back on again.What method will you be using for releasing the low grade heat to the dwelling?ME

TonyS

CLIMATE MASTER

I have a Climatemaster geothermal tied into my radiant heat system. I originally ordered it with the desuperheater but later decided to skip using it for cooling. Chilled water is a pain and I decided to install mini splits. The unit comes with a brass pump on a seperate circuit that would circulate into a tempering tank for the tankless water heater. It would just save the heat that normally would get put in the creek during cooling, it seems like a good idea.  But like you my cooling load is nothing. My geo unit maxes out at 120 and thats not hot enough. Some of the newer units use a scroll compressor and can get 140, but at those temps the COP really drop off. I use it strictly for heating the radiant  floor and use an outdoor reset to get the lowest possible water temp on that day. COPs drop significantly with increases in water temp so keep it as low as you can. I have been really impressed with it overalll. My heating bill is less than the cable bill.

Gordan

Radiant floors and ceilings

Heavy extruded plates between the joists for the (all-hardwood) floors. The supply temp at design conditions is supposed to be 102 deg. F for the majority of the house (and 117 for the library which has cathedral ceilings - unless supplemental heat is used to bring that down, which could easily be done with the same hydronic coil I use for cooling.) I was originally going to use a mod-con as the source, but the above supply temps (combined with the 30% tax rebate) have nearly convinced me that geo is the way to go, as I'd be getting it for the same money.

Gordan

Duplicate deleted.

Gordan

Unless you get all the bells and whistles with your cable...

...that's very encouraging! Thanks for your reply - aren't you also in PA, maybe Reading area? I'm in Chester County, would love to compare notes.

SpeyFitter

Clarification?

I work for a company that does a lot of geothermal installations (hydron modules) and we use the Desuperheaters regularly. We often use boilers to indirectly heat the hot water tanks, as well as the desuperheaters to add additonal heat to them with a desuperheater tee. The boilers also provide back up in very high demand situations or in the event of heat pump failure.

Can you guys elaborate for me on what a geo-reverse indirect is. I was trying to picture what you were saying but if someone could clarify?  Are you talking about using an indirect tank as a buffer tank and then have your domestic water go throught the coil to pick up a little residual heat before going into the hot water tank?

Thanks,

Scott

Gordan

Kind of like that, yes.

A reverse indirect has the boiler water in the tank, acting as the heat storage, and the DHW passing through the coil. Typically, it will have multiple coils in order to maximize heat transfer and minimize pressure drop. There's no DHW storage with it, and hence it would seem to step around legionella issues even at heat pump storage temps. You would still use it primarily for preheat, but now you would have the full condensing output of the (water-to-water) heat pump available for DHW heating, and you're decoupled from the space heating demand. You'd have a separate DHW heating tank that would be set to a safe storage temp, and a 3-way mixing valve that mixes to the desired DHW temp between the "preheat tank" and storage tank. (The storage tank would be fed from the preheat tank.) This would make sure that you're getting the most bang for your geo buck when it comes to DHW preheating. (You'd also need a separate anti-scald device, as the mixing valves aren't meant for safety critical operation.)



Clearly, this only applies to heat pumps that have water on the load side.



I've been thinking about a scenario that would involve boiler backup, not electric backup. From what I've seen, boiler backup is typically implemented on an either-or basis - once the boiler kicks in, the heat pump is turned off. What about this instead:



- (non-condensing) boiler has its own buffer tank and is on an aquastat with a wide differential, to minimize cycling

- heat pump has its own buffer tank and is on outdoor reset (with a high limit, to keep the heat pump in the efficient range)

- all load circuit supplies are fed from both buffer tanks through 3-way mixing valves

- the DHW circuit is to a (smallish) reverse indirect tank (this is NOT a preheat tank; it basically acts as a high efficiency and low pressure drop water-to-water heat exchanger) and uses a thermostatic 3-way mixing valve on supply, set to whatever the final DHW temp should be (plus 10 degrees due to the heat exchange factor); the circulator on this circuit is controlled by the aquastat on the reverse indirect tank (this one with a narrow differential)

- the space heating circuit(s) is/are on 3-way mixing valve(s) (depending on how many different mix temps you need, hopefully just one) with the mixed temperature controlled by outdoor reset; the circulators on space heating circuits are on thermostat control (or diff-P control with TRVs on emitters)

- all returns feed the geo buffer tank (bypass the tank if the return temp is higher than max desired ELT for the geo heat pump - or, better yet, if the return temp is warmer than the geo buffer temp)

- geo buffer tank (or bypass from space heating and/or DHW returns) feeds the boiler buffer tank



The benefits of this approach, as far as I can figure out, would be:



- heat pump supplies at least some heat as long as the return water temps are low enough, even if the boiler has to take a portion of the load - maximal use of low-grade, efficient heat

- the use of the reverse indirect (as opposed to regular indirect) allows the tank temp and volume to be minimal (assuring maximal use of the heat pump) without sacrificing recovery (as the high-grade heat source and storage is still available to recharge the reverse indirect whenever needed, and to the extent needed)

- boiler is available as either full (in case of heat pump failure) or proportional DHW and space heating backup without impacting geo efficiency, and only supplies the BTUs that the geo can't - efficient use of high-grade heat

- both the boiler circuit and the heat pump circuit are hydraulically separated from the load circuits by their respective double-tapped buffer tanks

- given the above benefits, the circuits and controls are really quite simple! In fact, they would seem minimal.





Does this seem like it would work, and work well?

TonyS

No problem Goran

This will be my 2nd season on the system. Anytime you want to check it out, your more than welcome. Just email me