Age old question ( I guess)

I have a 3 T water to water heat pump heating about 80 gallons of water in a well insulated buffer tank set to about 105F.    Could set it higher however it appears to do the trick.  The buffer tank has a thermostat on it set to kick on the GHP at 97F (it is adjustable of course).    I have 3 infloor heat zones and 1 hot water tank zone link to the buffer tank.  I do not use the desup heater to heat my DHW.  I have the pre-DHW tank being indirectly heated by calling for water from the Buffer. The indirect DHW tank is set to cut in when the water hits 85 and out when 95. My electric finishing tank brings to 130F.   There is only 2 of us living here

Here is the question.  Now that I have no need to heat my home (I do not use A/C) is it worth it to maintain the buffer tank at that temp or at all?   Should i adjust the tank temp? the indirect tank temp or something like that .  Or do I power down the GHP system, and let the finishing tank do all the heating of the DHW until heating season inevitably returns in SEP/OCT?

Would appreciate your opinions and experiences

Cost of electricity would be the biggest factor.
Not a fan of running a very expensive heat pump to do the job of a far less expensive water heater. For the same reason I'm not a fan of your piping scheme that chooses not to employ DSH.
My 2 cents would have you shut heat pump off in summer and pipe in DSH for winter use.
Good Luck,
Joe

I don't share all of Joe's concern of a few extra compressor cycles by a heat pump to heat domestic water. That said, I don't feel it would be economical to maintain buffer tank temp all summer to pass a bit of heat to a preheat tank which in turn passes a bit of heat to a finishing tank all for just two occupants. The geo unit would likely short cycle, resulting in poor efficiency and excessive wear and tear, and the standby losses from the 3 tank system would be significant compared with hot water needs of two people, and the standby losses could add unwanted heat to the structure during summer months. It is important to me to take a 'whole system' approach to these questions rather than just a look at individual components.

I also agree that foregoing the desuperheater to preheat domestic hot water is a bad choice. The unique aspect of desuperheaters is their ability to harvest heat from refrigerant whose temperature is well above the saturation temperature corresponding with high side pressure. Stated more simply, compressor discharge superheat isn't much heat, but it is 'hot' heat making it especially suitable for warming domestic hot water to a temperature well above that needed by a well designed radiant or forced air geo system.

3 water tanks for 2 folks strikes me as quite a Frankentank arrangement - overly complex. That said, if I had 3 tanks, I'd allow cold water to pass through the radiant buffer tank's indirect coil, thence into a pretank tank dedicated to the desuperheater, and finally into a finishing tank (which I would heat no warmer than 115 F, not 130). Both the buffer and pretank would be inactive all summer if air conditioning isn't an element of the system.

Fred, make sure that you understand that 115F is within the "ideal growth range" for legionella. Your setup is similar to the one that Phil, maker of the WEL uses.
http://www.welserver.com/
http://www.ourcoolhouse.com/monitor/monitor.htm

Excellent info everyone. We bought this house last year. It would be fairly easy to re-pipe. I believe you are saying the DHW system should not call for a re-heat, it should be heated when the HP is running using the DSHW? from my perspective (tho i am a newbie) it does not make sense to have the DHW call for DSHW. It does make sense during heating season. How much DSHW can a system create during a cycle, copious amounts? Right now, when the system is calling for heat to the DHW, it runs for about 30-45 min.

I do keep the finishing tank at 130 to "scare off' legionella. PLUS I like a hot shower! I am also on a well water system (270 ' deep well)

We are "almost" done with shoulder months heating. I will shut down the system this month and give 'er a vacation. She worked very hard this past winter and did all she was suppose to do. What more can I ask. The cheap DHW was a perk compared to the toasty warm floors.

Posted By Fred2010 on 01 Jun 2010 04:57 AM
Excellent info everyone. We bought this house last year. It would be fairly easy to re-pipe. I believe you are saying the DHW system should not call for a re-heat, it should be heated when the HP is running using the DSHW? from my perspective (tho i am a newbie) it does not make sense to have the DHW call for DSHW. It does make sense during heating season. How much DSHW can a system create during a cycle, copious amounts? Right now, when the system is calling for heat to the DHW, it runs for about 30-45 min.

I do keep the finishing tank at 130 to "scare off' legionella. PLUS I like a hot shower! I am also on a well water system (270 ' deep well)

We are "almost" done with shoulder months heating. I will shut down the system this month and give 'er a vacation. She worked very hard this past winter and did all she was suppose to do. What more can I ask. The cheap DHW was a perk compared to the toasty warm floors.

While 130F is hot enough to slowly kill legionella over several days, it's not going to kill it in just a few hours.   A temp 130F makes the legionella nervous, even sick, but not even close to being scared to death. (Instant-kill temps are more like 170-180F.)  If the pre-heat tank is stagnating anywhere between 80-120F,  the system is susceptible.

But if the bulk water in the tank is heating system water, and the potable water is just small volume inside a heat exchanger (sometimes called a "reverse-indirect water heater") you're fine, since 100% of the potable water is being purged  with (nearly) every draw of hot water rather than stagnating long periods at the susceptible temp range.

If there is sufficient hysteresis in the pre-heat tank control, using the geo will be quite efficient, and won't be running some ridiculous number of cycles to wear it out. The whole point of a buffer tank is to provide enough thermal mass to keep the system from efficiency-robbing & wear-inducing short-cycles.  If it was setup properly- use it!

The Legionella threat seems to be more of an issue associated with stagnation rather than moderate hot water temps. If mild hot water temp was truly an issue we'd have to be concerned with the cold side of domestic potable water. In other words, if 140 hot water is safer from Legionella than 120 F hot water, then 70 deg F cold water must be positively deadly...which it isn't, so I don't sweat Legionella too much despite oft-repeated dire predictions of imminent death forthcoming from sub 140 degree hot water.

As a parent of 3 and 5 YOA kids just starting to independently bathe the last thing I want is 140 deg water in the lines going to their bath - they don't really understand the delay associated with hot water delivery, so I really don't want them to be able to inadvertently call down upon themselves water much above 110.

I could use a tempering valve to reduce 140 F water down a bit, but why would I want the expense, complexity and added standby losses associated with such an arrangement?

Posted By engineer on 01 Jun 2010 10:50 PM
The Legionella threat seems to be more of an issue associated with stagnation rather than moderate hot water temps. If mild hot water temp was truly an issue we'd have to be concerned with the cold side of domestic potable water. In other words, if 140 hot water is safer from Legionella than 120 F hot water, then 70 deg F cold water must be positively deadly...which it isn't, so I don't sweat Legionella too much despite oft-repeated dire predictions of imminent death forthcoming from sub 140 degree hot water.

As a parent of 3 and 5 YOA kids just starting to independently bathe the last thing I want is 140 deg water in the lines going to their bath - they don't really understand the delay associated with hot water delivery, so I really don't want them to be able to inadvertently call down upon themselves water much above 110.

I could use a tempering valve to reduce 140 F water down a bit, but why would I want the expense, complexity and added standby losses associated with such an arrangement?

Legionella doesn't thrive in temps much lower than ~80-85F.  It won't die when cooler, but it's reproduction rate is many orders of magnitude lower at 70-75F than when it's 90-115F.   I don't have the exact numbers in front of me, but a legionella colony stagnating at 70F takes something like a year or two to grow  as much as a colony in 110F water grows in just a few hours.  At 80F & up the curve gets very steep for awhile, and the flat peak is in the 100-110F range. 

In many location (such as MA) tempering valves or thermostatic mixing valves are required by code at the output of HW heaters to mitigate scalding risk. (Here we already have the expense & complexity no matter what the heater's temp is set to.)  You can store water at 180F if you like, but the temp going into the distribution plumbing is determined by the valve setting.  115F is hot enough for even the most thick-skinned of hot-showerers, 110-112F is hot enough to fill cast-iron tubs.  Most people prefer to shower in ~104-107F water mixed down to that temp at the shower.  (My DHW is provided by a heating system buffer/reverse-indirect with a hysteresis keeping it between 120-130F, but the mixing valve for the DHW heat exchanger output is set to ~115F.)

Standby loss on buffer tanks isn't nearly as egregious as with standalone fossil-fired units since the insulation has no gaps and there isn't a center-flue heat exchanger to suck the heat out of the tank when it isn't firing.  Insulating the distribution plumbing (especially the near-tank plumbing) is important if you're serious about standby losses, and adding insulation to a buffer tank to take the standby loss from very low to "ridiculously low" is cheap & easy.  Most indirect tanks on fossil-fired boilers are operated between 150-180F, and have 1/2 degree/hr @180F ratings.  A  decent 140F buffer or indirect just isn't very lossy compared to uninsulated 110F DHW distribution runs, (and FAR less lossy than a 110F standalone fossil-fired tank.)

Colonies usually take hold in scale & sediment, which is another reason why tank storage of potable water is associated with a higher hazard than reverse-indirect heating buffers where the potable is only inside of heat exchangers (far less growth medium + far less stagnation time= far less hazard.)  If you keep your tank heater @ 110F, it's probably worth draining & flushing it annually, since 110F is smack in the middle of the zone.  Water softeners to minimize scale is also a good idea.