I have created a drawing for piping two tank DSH. A drawing that Bergy created has been posted here before. It shows how some manufactures recommend piping DSH with two tanks. I am showing another way to pipe this set up using another manufacturers suggestions. I am looking for opinions on which is better and why. You will notice an expansion tank which some locals require anyway. After all when you heat water it expands and has to go somewhere, and going out the pressure relief valve is not desirable.
I don't like the idea of pulling from the boiler drain valve - possibility of sediment being drawn into DSH coax
With usage the tank will balance out within a certain duty cycle....meaning heating without demand for water...aka recovery. If this were two tanks in series being heated electrically, it would make more sense (not much) than something that takes 6-8 hours to generate DHW. IMHO..not this way...Bergy's is more efficient taking into consideration a slower hot water generation source, and engineer's point about sediment is valid.
gabby - could you elaborate? I'm not sure I understand what you are saying and why. I agree that sediment can be an issue (in some localities more than others) however all tanks and the DSH(HWG) loop should be flushed as a matter of course if only for the sake of peak efficiency. If the HO doesn't do it the pro should during their (bi)annual service contract visit. With the system piped per teetechs drawing above, all this can be accomplished in less than 5 min. using a short hose and a 5 gal bucket giving instant proof of air purge, flow and visual inspection of sediment conditions.
The theory of a hot water tank is that cold water travels down the tube emptying out closer to the heat source. The hot water will rise to the top stratifying between the bottom and the top. The thermostat will sense a lowering of water temperature with cold water input causing the elements (fuel source) to start heating to maintain set point that is dropping with cold water injection, or hot water demand is in progress. The reserve hot water is at the top being used first. There is a balance between the size of the tank, the size of the heat source, and the delta T of the mixture of cold and hot water that accounts for the recovery time of the system. Excluding the sediment issues, the rate of change of the heat pump HW is much slower to reach temperature, much slower to stratify in the buffer tank (why two tanks are needed rather than a one tank system). In this drawing taking the input from the bottom of the buffer tank injects the possibility of contamination from sediment, puts the hottest water at the bottom (cold tube exit), and relies to heavily on a check valve to keep the hottest water out of the cold water supply side. If this had two check valves instead of one I'd feel better about it, but that still doesn't address the sediment issue that engineer pointed out. You want to take the supply of hot water as far away from sediment as possible. Not everyone who owns a hot water tank does anything with them until there is a problem, let alone any preventive maintenance. These are the same people who put Geo systems in and the majority will have them serviced until the warrantee expires.
The drawing is based on Climatemasters recommended piping for DSH, I added the check valve and expansion tank because of some peoples concern about pushing water back up the cold water supply. My problem with the other design is reliance on pushing water up the dip tube (opposite of water flow) to recirculate through the DSH coax. As for the sediment issue I think the purge valve can easily flush the coax. I am curious why the largest GSHP manufacturer would recommend piping this way if it were an inefficient and troublesome design.
I have a dual tank setup with a thermometer about 2/3 of the way up the side of the buffer tank. The buffer tank stratifies well with the DSH water pulled from the cold dip tube and injected in the boiler drain. I believe the DSH flow is low enough, around 1 GPM, that mixing isn't a major factor. We had a couple hours heat use a couple days ago, and the top of the buffer obligingly rose from 75 to 90, though there's no way the entire 80 gallon buffer warmed that much in 2-3 hours of low stage heating
It would be interesting to see what happens to those numbers if you swap the in/out DSH lines. How about a real world test here. Something crossed my mind, what if you pulled the elements and connected DSH lines there?
I have to say the hottest water belongs at the top of tank where is available to pass to the second tank. This occurs by stratification which happens naturally without intervention. If you pipe according to teetechs drawing- -the hottest water is at the top of the tank -the tempered water from the DSH(HWG) goes to lower tank (via dip tube) -the coolest water is at the bottom of the tank available to pass to the DSH(HWG) for best heat transfer efficiency All just the way nature intended. The check valve is unnecessary (although if you do use one make sure you use the expansion tank also). The lower pressure at the suction end of loop induces flow only to the return of the desuperheater loop. I believe the biggest problem with hot water tanks is standby loss. Think of how many hours a day hot water is not being used and how many hours the geo isn't running. Water heater manufacturers are trying to help with better insulation and heat trap nipples. I do not see the benefit or efficiency of removing heat trap nipples so you can pump tempered water to the bottom of the tank (where it doesn't want to be).
Obviously we see it differently. The exit port of the DSH enters the cold water tube---90-120 degrees, dumping the water to the bottom of the tank, not the top. It is the heat source, thus stratifying to warm at the top and hot moving up with demand for hot water. An expansion tank, nor any check valves are necessary if you split the cold supply between the DSH input and the cold water tube of the buffer tank. Both drawings will equalize water temp and functionality over time with no demand for hot water, but the question was which is the preferred method and why. Asked and answered. If you choose to disagree, that's fine. It is not and will not be the way I pipe the system for the reasons noted. If one is in a heat dominate region, the system will run nearly all the time, maybe not so much up North where you have milder summers.
Three more observations: 1) Insulating both the hot and cold lines to a tank up to and a foot or so beyond the first bend helps with losses from not having heat trap nipples 2) Conventional electric storage water heaters don't have much insulation on the very bottom owing to the fact that under normal operation they don't heat the bottom 4-6" of water, and insulation interferes with physically supporting the tank. So when conventional storage tanks are used with a DSH, there can be substantial losses through the bottom of the tank. I cold feel perceptible warmth on the slab next to my tanks so added a layer of blue board below. This is an aspect where the Marathon tanks are clearly superior. 3) I only got 'round to removing the heat trap nipples a year into my system's operation but found to my surprise that they had made little difference. Apparently the DSH pump was able to overcome the itty-bitty rubber flap in the nipple to the extent necessary to backflow down the cold dip tube. The effect could be greater in systems moving more heat than mine, an 038 running nearly always in low stage
That is exactly what marathon is doing with their new Thermal Storage tanks. The 2 elements are removed and replaced with 1.25" connections. It should simplify installation,But wouldnt you know they are charging more for these tanks, not less. A poster pointed out sediment not being an issue in a particular locale, I think that is a good point, in my area most are on well water with sediment challenges, making pulling from the boiler drain unwise
My installer will be including a buffer tank, but to my surprise he said it alone will be able to supply my DHW by using the top heating element - the lower one will be off - and that I will not need a second hot water heater tank to bring the temperature up to useable ranges. He says he does this all the time. After all I have read here, I am not satisfied that this is the right way to go. For one thing, I thought a buffer tank was necessary to qualify for the tax credit, but I guess actually it would be the desuperheater that's necessary - or was when I first started reading this forum early last spring. Searching for information today I seem to be seeing that the desuperheater part might have been changed from being mandatory to being optional. Either way, I am getting one. However, should I tell him I'd like a second tank, which I will pay for, piped in line? I am going to be asking him to upgrade whatever he's planning to put in to a Marathon with me paying the difference, as I understand they have better insulating values. Plus, I love the fact that they won't corrode. I have call in to Marathon now and hope to hear back from them today. I will be asking about their thermal storage tanks.
Thanks. I think you're right. Engineer told me about the marathon tanks and suggested I get a pair, if I could swing it. So, that's what I've had in mind. My problem has been that most dealers around here haven't heard about them so I am going to have to push a bit to get them ordered. I did speak to a firm that will ship them freight to me, so that's possible. I got their name from the Marathon website.
Know that while the Marathons are nice, they may not be cost effective - don't say I didn't warn you. I'm skeptical of being able to make required hot water using top element alone. Shoulder seasons, spring and fall, generally bring at least one multi-day period during which there is no heating or cooling needed. During that time there will be no DSH recovery. The water heater will be on its own, and if only the top 10-20 gallons in it are warmed, as will be the case if only the upper element is allowed to fire, there will likely not be enough hot water for the house. I can fool around to my heart's content with experiments and setpoints on my house hot water system...until my wife has a less-than-105 degree shower on a cold morning!
I am sure he does do it all the time. and I am sure it will work. I am also sure it will not be as efficient as having a unpowered buffer tank, I am also sure sooner or later someone will get a cold shower
"I'm skeptical of being able to make required hot water using top element alone. Shoulder seasons, spring and fall, generally bring at least one multi-day period during which there is no heating or cooling needed. During that time there will be no DSH recovery. The water heater will be on its own, and if only the top 10-20 gallons in it are warmed, as will be the case if only the upper element is allowed to fire, there will likely not be enough hot water for the house." "I am sure he does do it all the time. and I am sure it will work. I am also sure it will not be as efficient as having a unpowered buffer tank, I am also sure sooner or later someone will get a cold shower." On thinking about it, I'm sure you both are right on that point. "Know that while the Marathons are nice, they may not be cost effective - don't say I didn't warn you." Why do you say that, are they expensive to run? In my area, power is costly at 19 & 1/2 cents a watt. Propane runs between $3.49 and $1.95 a gallon depending on how much is used and as I won't be using much, it will probably be $2.50 or higher. So, I am afraid neither is going to be economical, but I could be wrong. I'm getting rid of my oil tank, and it would cost me $18,000 to bring Natural gas to my house, so both of those are out. My choices seem to be electric or propane. I will be getting a reduced rate on electric once the geo system is running, but that's only during the winter and only after a certain amount of electric usage. I was hoping the Marathon's better insulation might help it be more efficient. On the other hand, the New York Times just had a disturbing article two days ago about plastics and the illnesses like cancer the chemicals in them are causing. Marathons are lined with plastic. The service rep told me it did not contain BHA, which seems to be the chief offender. But, now I am wondering what else might be in their plastic that we don't know about yet. It's bad enough to find out that most of the canned foods on my shelf are probably contaminated by BHA - I'm not sure I want to eat any Progresso soups again after that article, but now I have to be concerned about what's lining my water heater.
Yes. I know it sounds silly, but the Times article was convincing. OTOH, I just spoke to Royce at Marathon and he told me that not only do they not contain BHA, but the state of California has no problem with them. So, I'm satisfied. California seems to test and red tag a lot of things as potentially dangerous. My garden hose, according to it, will do me in if I dare take a drink from it. Now, I just have to find someone who sells the darn things. No one around here seems to know about Marathons, not even my utility company which I contacted to see if by chance they sold them - I've read that some do. Royce gave me a person at Suffolk Rural Electric Coop in New Jersey to contact because it sells them to everyone, even non-members. He also suggested that a 50 gal tank used as a buffer teamed with an 85 gallon tank might be a good plan for me. I've been playing phone tag with SREC for two days, but hope to get through to someone soon. Meanwhile things are moving along here:
