Is a buffer tank always better than a single tank?

At what point does the overall house system become efficient enough and the water usage low enough that a buffer tank no longer makes sense? Or for that matter, when does a DSH no longer make sense?

For example, would you recommend a buffer tank for a new 2,100 sqft house in central Texas with a 3 ton Tranquility 27 closed loop (3 - 300ft wells) geothermal system?

More info…
Thermostat settings: 78F in summer and 72F in winter. 2 zones - bedroom zone and living room zone.

Metal standing seam roof on 3/4 in. strapping set at 45 degrees for passive venting.

Low solar gain in summer due to siting, roof overhangs and window placement.

Tight thermal envelope w/ an avg. 5 in. of open cell insulation in 2x6 24in OC walls and roof deck + 1/2" SIS styrofoam insulation/sheathing on the walls and 3in. of closed cell foam under the floors in the pier and beam living room zone.

All HAVC ducts in the bedroom zone are within the thermal envelope. The floor register ducts in the living room zone are in the above ground "utility basement". The Tranquility 27 unit and water heater(s) are in this "basement".

Avg. 50 gal/day of TOTAL water usage (hot and cold) by two retired adults. No kids and no overnight guests in the main house.

Water supply is exclusively rain water stored in tanks in the "utility basement" with a temperature range of 50-90 degrees (winter - summer).

Someday House

p.s. Glad I finally found this forum.

 

You need a buffering tank

to take advantage of the de-superheater system.

Do the math, but if you bought the de-superheater put in the tank.

Mark

 

IMO a DSH never makes sense without a buffer tank. It's neither or both. A DSH alone (that is plumbed only to a fired tank heater) will recover so little heat under nearly all circumstances so as to be nearly useless. That said, doing both may represent an investment of $1500+ installed. That investment would offset 1/2 - 2/3 of the household's hot water use.

Absent data specific to a particular household (such as clocking existing water heater elements) I figure about $150 per person per year for electric resistance water heating for southern households. That's at my area average electric cost of around $0.13 / kwh. Adjust that for your area's cost.

So $1500 might buy you $200 in annual savings, and you'd still need a finishing water heater, tank or tankless.

Given your plans for a utility basement, southern location, and small household, I suggest careful consideration of a 50 gallon GE heat pump water heater. Installed it should cost maybe $1500. In a warm room it heats water for 1/3 cost of conventional resistance, so might cost $100-150 for two people. It also provides useful cooling and dehumidification of whatever room it occupies.

Caveats:

1) Best in warm climates
2) Needs to be in at least a small (500-750 cu ft) room, not stuffed in a closet. If in a closet, active mechanical ventilation may be needed to reduce chance of cooling surrounding air to below dewpoint (risk of mold)
3) It does make some noise, a bit more than a typical fridge, not as much as a small window air conditioner or portable dehumidifier.

 

Thanks for your replies.

Mark: As you say we already have the DSH, so we have hooked it up. See below.

Curt: I wish I had thought of installing a 50 gallon GE heat pump water heater. But I had already drunk the geothermal DSH kool-aid that promised essentially 'free' hot water during the long cooling season here in Texas. And I didn't check into how the DSH actually works until I had already hooked it up.

I really wish I had found this forum and asked this question a week or two ago. But I didn't. So here's the current situation….

Someday House, as described in my 1st post, is already built, but not complete. The Climate Master Tranquility 27 3 ton unit has been running since last Dec. while we are finishing off the inside (e.g. painting, installing cabinets, etc.). We love it!

Last week the plumbers completed their trim out and installed a single Marathon 50 gal water heater plumbed to the HWG on the T27 with the draw from the boiler drain port and return to the cold water port. The Marathon is not yet powered up, since we are waiting on the electricians to complete their trim out. We turned on the HWG pump full time yesterday. So in essence the Marathon is currently acting as a "buffer tank". Since we don't need hot water anytime soon, I'll leave the Marathon un-powered to determine just how hot the T27's HWG can get this tank in the middle of summer in Texas all by it's lonesome.

A few weeks ago we had daily temps above 100F and the T27 ran most of the day. But the weather this week dropped back down into the upper 90's, so the T27 has gone back to running mostly in stage 1 and mostly in the afternoon. EWT is currently around 88F with the return water to the wells around 92F. Compressor discharge temp in stage 1 is 120-130F depending on where I measure it. Water exiting the DSH is only 101-105F. So frankly I have my doubts about just how hot that 50 gal. Marathon tank will get and how long it will take to get there.

FWIW my reasoning for not installing a buffer tank went like this. With very low hot water usage there is likely to be more standby heat loss in the finish tank than can be compensated by a buffer tank supplying the finish tank with hot water during the brief times of hot water usage. In other words, the finish tank will frequently need to turn on it's upper element to maintain 120 degree water, even though the buffer tank is ready and willing to supply it with hot water if and when hot water is actually used. On the other hand, by plumbing the HWG into the "finish" tank, the HWG is at least trying to keep the finish tank at 120 degrees and reduce the standby heat loss. Note: our "cold" water supply in the summer is already at 85-90F since it is sitting in tanks in the utility basement.

It seems to me that the buffer tank configuration is only obvious when daily hot water usage is high in total volume and/or concentrated in time. In these cases, having a buffer tank to replenish the hot water just used, with even 100ish F water, will keep the finish tank from working as hard to recover. But in situations where the total daily hot water usage is low (10-20 gal) and distributed through out the day, having a large buffer of even 120F water available is not so obvious unless there is almost zero standby heat loss in the finish tank.

Time and data will tell. If it turns out that I am wrong, it will be fairly easy to add another water heater to the system. We could keep the Marathon as a buffer tank and add an on-demand finish water heater. Or we could just add the finish tank that I had originally spec'd but decided to postpone until we understood the DSH better.

Thanks again for your thoughtful responses -- Someday House

 

Yes a buffer is always better. There are a few applications where a DSH can work without a buffer and many where it could cost you money to go without (raise your water heating bill).

 

Dito everything above, never go without one!

 

If daily hot water use is truly only 10-20 gallons and you are in a southern climate where "cold" water eneters at an average of 70+, you might have been better off forgoing all the fancy water heating technologies and just going with a $200 box stock 30 gallon storage electric resistance water heater.

The math: Daily heating 15 gallons of water from 70 to 120 requires about 6250 Btu or about 1.83 kWh. Lets call it 2 kWh so as to account for standby losses. That works out to just $80 per year at national average electricity cost.

A DSH or HPWH might cut that by $50 per year, but the cost of those systems amounts to a piss-poor investment to realize $50 annual savings.

BTW I draw from cold water inlet and return via the boiler drain. That way I avoid drawing any scale or sediment from the very bottom of the tank into the DSH pump and heat exchanger.

Gieven the already-installed Marathon 50 and DSH you may be able to achieve satisfactory operation by configuring the Marathon to serve as both buffer and finishing tank. Do this by setting the lower heating element theromostat at lowest possible setting (or disconnect it). That way the bottom 2/3 of the tank serves as an unfired accumulator for the DSH.

The top 1/3 of the tank will be warmed by the upper element only as needed to make 120 setpoint (BTW 115 suffices for most southern houses - try it). That configuration will likely support 10-20 gallons daily hot water usage,

This time of year the 3T Tranq 27 will likely supply nearly all your water heating energy.

My 3T WF Envision (locked in low stage) is holding its 80 gallon buffer at around 105. That's with 70*F EWT - open loop. Since I'm home alone this week and am content with a 95*F shower, I have the downstream 80 gallon HPWH shut off. It is holding 95 or so. The loss of 10 degrees (105 to 95) comes from the fact that home alone I probably empty an 80 gallon tank only every 2-3 days, which allows plenty of time for standby loss into ambient 75*F mechanical room.

When wife and 3 kids return I'll need to restart the HPWH. It burns about 1.5 kWh / day holding the 80 gal finishing tank at 110 with 105 supply and standby loss.

 

Thanks again for your info packed reply.

That is exactly how I was planning to configure the Marathon. We chose the 50 gal instead of a smaller model (e.g 40 gal) so it might be big enough to act as both the buffer and finish tanks as long as we maintain a low daily hot water usage.

BTW we've been using rain water for all of our domestic water usage for the last 8 yrs. This required a number of adjustments in our thinking and behavior.

1st - rain water is slightly acidic. So there is almost zero scale or mineral build-up on plumbing fixtures, etc. But it also requires that the plumbing contain minimal metal due to the potential for leaching. Hence the use of PEX pipe/fittings and the Marathon WH.

2nd - we do our own water treatment - spun fitter -> charcoal filter -> UV light. So there is no chlorine and almost zero sediment in our water.

3rd - our rain water is stored in above ground tanks (we have 30,000 gals of storage capacity). So the "cold" water supply temp. from these outside tanks varies considerably. We will have an additional 4k gal of storage in Someday House's enclosed utility basement to significantly reduce the temperature range after it is fully enclosed. We insulated and closed off a small part of the utility basement last winter (for paint and tool storage) which hasn't gotten below 65F or above 84F. The temp. generally stays within a few degrees of the temp in the conditioned area above. We hope to get similar results in the whole utility basement once it is fully closed in.

4th - and most importantly, when we began living with our only water supply stored in tanks, and very measurable, we started thinking about water usage more seriously. Of course we never "leave the tap running". But we also try to always use water for at least two things, like washing vegetables into a SS bowl that is then emptied onto plants. Even the water that does "go down the drain" ends up in our aerobic septic system where it will be used again for irrigation.

Someday House

 

How so?

We have a WF Envision 049 hooked up to our hot water heater. We do not have a buffer tank. How might this raise our water heating bill? I am wondering about this because our electricity usage seems to be a little higher than expected.

Thanks.

 

thomasjeremyl, what temperature are your upper and lower heating elements set at?

 

Upper at 125*F and lower at 100*F. I just changed it the other day. I noticed they both were set on 125*F. The installation manual recommended setting the lower at 100*F.

 

SH - sounds like you "get it" Great job on the use of local water.

TJ - you ask how a single tank system can RAISE water heating costs...

Easy - look at the numbers I supplied above. My DSH holds its buffer at 100-110 this time of year which is cool beans since it feeds a finishing tank.

What if my DSH tank was also my finishing tank, set for the typical 120*F? If that were the case, the 5500 Watt resistance elements would fire pretty much whenever the heatpump operates - heat would flow INTO rather than out of the DSH heat exchanger.

Refrigerant pressures and compressor current would both rise. Heat generated by electric resistance would be dissipated into departing loop water.

All those outcomes are universally bad, causing increased energy use by both the water heater AND the geo heat pump.

Chances are fair to good you would be well served to disable your DSH until you are able to deploy a DSH preheat tank.

I hope that helps you understand the thermodynamics in play.

 

Curt, just asking - would this be the case with an Envision system? From our installation manual:

" Note: Under certain conditions, Envision dual capacity units operate with very low refrigerant discharge temperatures, producing little or no water heating capability. This scenario occurs when the unit is operating with cold entering source water (loop or well). Allowing the desuperheater pump to operate during these conditions actually removes heat from the DHW circulating through the unit. To overcome this, Envision unit microprocessors have been programmed to disengage the desuperheater pump during such conditions. (During low capacity cooling operation, the pump will operate only if the DHW temperature entering the unit is less than the liquid line temperature plus 35º F. During high capacity cooling operation, the pump will operate only if the DHW temperature is less than the liquid line temperature plus 60º F.) Using a preheat tank, as shown in Figure 12, will maximize desuperheater capabilities."

Honestly, I don't follow the "plus 35º F" part.

How would the buffer tank temperature fair in other climates where first or second stage in cooling is used with a 50 gallon tank? Doc, do you have any WEL data?

 

I'm aware of but haven't instrumented a system where that control strategy seems to have been in play. With our 70+ EWT, we pretty much always have useful refrigerant superheat since we alwys use a buffer.

That said, I believe the intent of the 35 / 60 strategy is to infer the compressor discharge refrigerant temperature from a combination of current operating stage and liquid line temperature. Why not measure the compressor discharge gas temperature directly and compare that to the incoming DHW? Cost of one more temperature sensor (A 10k thermistor) is negligible.

Several factors influence compressor gas discharge temperature that I am skeptical that liquid line temperature plus a constant adequately predicts the value.


At any rate, the best advice is found in the last line...use a buffer tank.

 

Buffalo GeoThermal Heating
<br />
We have 15 systems online. Most with a buffer tank. I do see much better response time with a 50 gallon tank, resulting in higher supply temps to the second tank, in heat dominated climate. Plus 35F is a safety margin on how manufacturer's figure out the refrigerant temp in the desuperheater. They don't measure it directly, but indirectly through the EWT (old version) or liquid line. I don't get it either why they don't measure discharge temp directly.

 

The system temps listed above have held pretty constant over the last 2 weeks. I've been running the DSH full time with the 50 gal Marathon acting as an un-powered buffer tank. Even with outside temps of over 100F every afternoon in the courtyard, we are seeing temps of only 88-90F in the not-yet-fully-enclosed utility basement where the T27 and Marathon are. The DSH outlet temp is still running in the 101-105 range. And the Marathon inlet is at around 105F. Note: we have NOT used any hot water for these 2 weeks. In other words, despite a very warm/hot Texas summer with the T27 holding the house at a very comfortable 78F, the DSH didn't get the 50 gals of water stored in a pretty well insulated buffer tank any higher than 105F in 2 weeks of trying.

Next week the electricians will complete their trim out. So my question is.... What should I do when the Marathon is hooked up to power?

A. Keep trying and do as the manual says - set the Marathon's upper element to 120F and the lower element to 100F (or OFF)?

B. Bite the bullet -- turn the T27's DSH pump OFF and isolate it from the water heater.

Someday House

 

Good question

The difference between setting the lower element to 100 or turning it entirely off is huge

1) If you set it for 100, the DSH will contribute very little heat since most of the time (90+%) it will be working on 100*F water and as has been demonstrated, will add almost no heat to such water.

2) If you turn the lower element off, the bottom 2/3 of the tank will act like a separate buffer tank and the DSH will add quite a bit of heat. That's great until the first mild day when the DSH adds less heat and two people shower in fairly quick succession. Person #2 will be bummed out by a cold shower.

That's because the upper element acts upon a fairly small volume of water, on the order of just 15 gallons. If 15 gallons of 120* water is removed by a 6-8 minute 2 GPM shower and is replaced by 15 gallons of, say, 80*F water from below on a cooler, low-DSH contribution day, the upper element will take quite awhile to recover.

 

Thanks again for your thoughts and advice.

Since we are finishing Someday House ourselves, it will be several months before we will move in. So we have time to continue experimenting by test driving the new dishwasher, washer, etc. and monitoring the amp draw on the Marathon…

Setup 1 - with the DSH pump running, set the top element on the Marathon to 115F and turn the lower element OFF.

Setup 2 - with the DSH pump OFF and the T27 isolated from the plumbing, set the top element on the Marathon to 115F and the lower element to 100F.

I'll let the hot water system reach a steady state for each setup before taking any amp measurements. This isn't a burden because the rainwater in our storage tank is siting at just under 80F. So we really don't need to use any hot water.

It will be interesting to see just how much less juice the Marathon uses in Setup 1 when doing laundry and/or washing dishes.

I have a Klein 200 clamping meter. So it is fairly easy to measure the Marathon's instantaneous amp draw. Does anyone have a clever idea of how to cheaply/easily monitor the Marathon's amp draw over time?

Someday House

 

Ah - you have hit upon a core home energy question...

How much energy does a water heater really use?

Federal Dept of Energy model and figures suggest a storage electric resistance water heater annually consumes approxiamately $500 of electricity at national average usage and rates.

However, their model is seriously flawed - electricity rates, household size, usage, temperature setpoints and incoming water temperature vary widely.

Fortunately, water heater energy usage is farily easy to capture. Assuming supply voltage to the heater remains fairly constant, power input is also fairly constant. The remaining variable is element duration of operation.

I use <$20 Readington LCD hourmeters connected across water heater elements to clock usage.

Your measurement of amp draw is useful and should confirm water heater's nameplate watt rating, adjusted for voltage. What you really need is hours of element operation under your several scenarios.

 

What a timely thread! I currently have an envision 049 installed, and have for about two years. Also have a single 90 gallon marathon tank installed, as was spec'd by my contractor, along with DSH on the WF. I'm also in mid-argument with my contractor, who swears up and down that WF suggests a single tank arrangement, like I have, and this is the reason they sold it to me this way.

We're in mid-argument because I noticed "hotter" water in the off, in between seasons. It took a bit for me to realize what was going on, and how my tank water is being cooled during the regular heating and cooling seasons, since the tank thermostats are never/rarely satisfied (both set at 125 F) as long as the DSH is pumping ~95 degree water. The contractor stands behind WF's claims that a single tank can and should work, unless 1.) EWT from the loop field is too cold to be warmed sufficiently by the refrigerant. 2.) exact words from my contractor - "WaterFurnace’s stance on the de-superheater installation is a pre-heat tank is onlyan absolute necessity when the existing hot water supply is being provided by afossil fuel (oil, propane or natural gas) hot water tank. "

I could sort of understand point #1 after the operation of the DSH pump was explained to me, but I was still suspicious. Point #2 makes no sense to me at all.

When they were here last, about a month ago, my EWT was 74 degrees from our horizontal loop field (seems pretty warm to me, I might add, for Western PA in mid June). So this completely blows point number 1 above, out of the water...no pun intended, haha.

Since they were here last - I've had the chance to run side by side comparison tests, with the DSH turned on and off, during the recent weeks of extremely hot weather. My tests showed my water being cooled by as much as 10-15 degrees, from my tank tstat settings, with the DSH turned on. 24 hours later, run the same test with DSH turned off - water temps are right where they should be with regard to tstat settings.

Not only am I not getting the "free summer hot water" that was sold to me, for a premium price, I'm having to pay for extra electricity for the tank that never turns off, when the heat pump gets into running often in the hot/cold weather.

So if you're considering trying a single tank - you might think twice based on my 1st hand results. I have no doubts that a DSH would work perfectly with a buffer tank installed....If my contractor would only admit it.

I'm going to be contacting WF soon to validate the 2 points above. If anyone from WF monitors these threads, please send me a private message. Thanks.

Good luck to you.
Mike