moving heat between tanks in two-tank solar hot water

Royboy

the 2-tank design has been my default design for retrofitted solar water heating systems. a solar "pre-heat" tank feeding a conventional water heater, typically the water heater that was already serving the residence.



earlier this year I did a system where the existing water heater (an LP CombiCor) also did the space heating for the house via a flat plate heat exchanger supplying PEX tubing in the joist cavities under hardwood floors. the owner would manually dial up the CombiCor temperature in colder weather to keep up with the heating load. I slightly oversized that system (96 sf for 2 people) and pitched it steeper than usual in order to better contribute to the space heating load. (since our 9000 DD heating season runs into May - the spring months in particular provide an opportunity to cut into the annual heating bill).



one piece of feedback I got from the homeowner was her frustration with hearing the CombiCor periodically cycle on due to tank & stack losses when the solar pre-heat tank was sitting there with a full load of heat. she expected this will really get to her next spring when the CombiCor will be providing space heat, but the only time solar heat will move through to it is when tap water is run. she asked if I could give her a price on creating some sort of recirc system to actively move heat from the solar tank to the CombiCor whenever the solar tank got hotter than the CombiCor. have been pondering how to best accomplish that.



I'm now beginning work on a new-construction system which raises the same concerns with a tw0-tank setup that does both water and space heating. in this case the water heater will be a Phoenix, and it will heat, via a flat plate exchanger, the radiant slab on the ground floor. the rub is that in a standard 2-tank system, the only way the heat gets transfered to the water heater (the second tank) - is via use of hot tap water. with the residents potentially not home all day and the sun shining, it seems worth considering moving the heat by another means so it can do some of the space heating - and keep the solar tank cooler.



so - I've come up with a piping schematic to do this and am wondering what you think of this approach or what other ways you would consider addressing this situation. my approach adds two pipe connections, a small circ pump, a zone valve, a couple check valves, and would be controlled by a differential controller measuring the temps of the two tanks.



I'm going to try to post four jpgs here to show my thoughts (my first attachments at this new site, so wish me luck). hopefully will be self-explanatory.



looking for some feedback on this situation and my approach to it.



Roy



Sunrise Energy Services

Washburn, WI

Jamie_5

a better way?

Rather than move the heat to the less insulated tank for possible use, is it feasible to have the plate exchanger piped into both tanks and have it draw from whichever is hotter?  You would have to have some differential to keep it from trying to switch back and forth constantly, but are there other problems I'm not thinking of?

EricAune

Circ

Why not come out of the solar tank into the drain opening on the standard tank with a small pump like a laing, then return from hot side of standard tank to lower opening on solar tank.



If you have checks on either side of the house consider room for expansion.

Royboy

good thoughts

Jamie - rerouting the flow to the heat exchanger had occurred to me at one point but I never drew it out. now that you bring that up, I'm realizing it would save adding a recirc pump - the pump for the potable side of the heat exchanger would do the work whichever tank was being sourced. would just need to add a 3-way zone valve and probably a check valve. I'll draw it up later - I think its a better way to go.



Eric - my thought in connecting tanks top to top and bottom to bottom was to try to get the conventional tank to stratify if enough heat available. but I think it could be done either way.



not following you when you say "If you have checks on either side of the house consider room for expansion."



Roy

NRT_Rob

but

if you do not transfer heat from hot tank to cold tank, you will always have the "gas tank is firing even though the solar is hot" aggravation from the client. People hate that.



I would stick with a basic differential control and recric. If solar is hotter than water heater storage tank, transfer occurs. cools down the solar tank (more solar transfer, you've effectively increased your available storage volume), provides de facto heating integration if the solar is hot enough to do so, and solves the psychological teeth gnashing of hearing the combicor fire when the solar is hot.



This is why I don't really like two tank designs, typically, but in a retrofit you get what you get for sure.

Royboy

more good points

this is just the sort of feedback I was looking for ...



I'm seeing that there are (at least) two options for how to move usable solar heat from a solar preheat tank without waiting for tap water usage to deliver to the primary water heater. both require a differential controller, zone valve, couple check valves. option 1 requires an additional bronze/stainless pump.



1) recirc between solar tank and water heater. effectively increases solar storage capacity. allows solar heat to help cover standby heat loss from water heater & thus minimizes use of conventional fuel.



2) option to pull space heat direct from solar tank.



Rob - your points have me leaning back towards option 1, despite the cost of the added pump.



also still curious about the single tank approach. I've generally steered towards 2-tank - or one tank feeding tankless in DHW-only systems. main problem I see with single tank is that to have an equivalent solar storage capacity, you generally need a much larger tank, due to the conventional heating of the upper portion of the tank. that then gets quite ungainly/pricey. any further thoughts/comments on one tank vs two?



thanks - Roy





here are the drawings for option 2:

hot_rod

a couple thoughts

I've seen systems with a 3 way zone valve on the DHW output. It pulls the solar tank down first, then pulls from the combi. It won't change the CombiCor standby loss, firing however.



Really anytime you send warmed water to the Combi, you lower the load it has to cover. A btu is a btu regardless of how or when they show up at the second tank.



I like the concept Dan Foley used. Install a dual coil solar tank with copper or stainless coils.



Use the solar coil at the bottom connected to the array, as you normally would. Then take the incoming cold water through the upper coil before it goes to the Combi. just a preheat for DHW. Then you have the entire tank capacity to be used for hydronic side loads. Use a 3 way zone valve, Caleffi I solar plus perhaps, to select from the solar tank for heating first, then the Combicor. This gives the solar a more heating load bias.



Of course none of these will eliminate the standby loss of the Combi, it's going to fire from time to time just to maintain setpoint.



I-dronics 6 Solar Combi Systems is up on the website, it has a bunch of unique piping concepts, head over and download it.



hr

Royboy

hr

nice to see you here. hope life is treating you well. I'm starting to get reconnected to this new site & format



I-dronics 6 looks great on initial inspection, will need to spend some time with it to do it justice. you guys are really providing great education with that series. thank you! do you know when it will be out in paper? still my preferred way to take in info-dense reading.



so Dan Foley's approach is to add another heat exchanger to increase the options. did he do this in a system with a single appliance doing water & space heating? do you have a schematic or a link to his posting that?at this point I'm still feeling like option 1 will best address the situation, but welcome further thoughts. in my mind, this solar linked into a single appliance doing water & low-temp radiant space heating is a pretty optimal system.



in the job I'm starting, the solar tank (& drainback tank) will be on second floor, right over the Phoenix. there are plans for a direct convection loop off the solar tank to a woodstove firebox heat exchanger in the living room. that should result in close to 100% annual hot water from renewables, and help move the woodstove heat around in space & time to boot. with two winter sources of heat into the solar tank, it seems extra good to be able to move that heat actively to the phoenix. and with the drainback tank up so high, I'll only have about 10' of lift for the drainback pump to contend with :-)



Roy

hot_rod

here

One appliance for DHW and heat makes sense BUT not if the back up source is within the same tank. Having the solar tank warmed by gas, electric, oil, etc will limit some solar harvest potential.

Two tanks, with the first one being only solar powered will do the best. Now that first tank could be a large drainback with an upper coil as mentioned above, but it should never recieve heat from another source.



Here are a couple concepts from Caleffi Idronics 6 one with internal, one with external HX for DHW preheat, but using the tank volume for heating loads.



hr

Royboy

never?

"Now that first tank could be a large drainback with an upper coil as mentioned above, but it should never recieve heat from another source."



as in my planned first tank receiving heat from a woodstove in winter?

hot_rod

my system wood/ solar

Here is what I have for this year. The drainback tank is an old ErgoMax that I welded some additional ports into. The tank on the right is a HTP Solar Phoenix with external Taco Radiant HX MixBlock to run my radiant. A caleffi Drainback Control runs the solar pump and also the DHW circ on the second function as a thermostat.



A second differential control runs the pump P2 to move from the 500 gallon buffer through the HX above the ErgoMax. Anytime the sensor in the Ergomax is warmer than the sensor in the bottom of the 500 gallon buffer it fires. I'll add a Caleffi mix station on the buffer to pull it down as low as 100F, maybe lower. That will be run on OD reset so as the pull that buffer down as low as possible. That control will also run P1 as a variable speed function delta t to provide return temperature protection for the wood boiler.



Last year the wood stove fed to the lower coil in the Phoenix, but since the Phoenix supplies DHW I could only pull the buffer down to 130F. This years arrangement allows better use, wider delta t, of the buffer. At least that is what I have in mind.



Now I would like to find a single control to run it all properly.



I think one thing you will lose by shuffling from solar to DHW tank is the solar tank will be left at the lowest temperature useable temperature in the second tank, maybe 100- 120F? You would rather pull the solar tank down to 55F, 60, 65?? or as low as the incoming water temperature in the winter.



The colder that solar tank is when the sun comes up, the higher the efficiency of the collector array. presenting 120F to the collector on a winters day may not see it contribute much, if any. So if the goal is to utilize the solar as wisely as possible find a way to pull it down as low as possible for heating, then even lower with the DHW pre-heat also.



hr

TonyS

Hot Rod

The blue arrow on your solar hx is going one way but the arrows and pumps on the blue line to your 500 gal tank are going the opposite. What am I missing ?

Royboy

golly there are a lot of ways

to configure this stuff. my brain, usually fairly functional, has trouble getting around it all ...



didn't see your schematic, hr, in I-dronics ... ;-) ...



(and actually, looking closer, I got a bit confused. looks like the line labeled "to solar HX" is going the other way according to arrows and pumps)



********



"I think one thing you will lose by shuffling from solar to DHW tank is the solar tank will be left at the lowest temperature useable temperature in the second tank, maybe 100- 120F? You would rather pull the solar tank down to 55F, 60, 65?? or as low as the incoming water temperature in the winter. "



I think that the shuffling I'm contemplating between the solar tank and water heater should, in theory, only help solar efficiency by reducing the temp of the solar tank, though I can imagine that stratification & sensor location could complicate things at times. and any hot water usage will still drop the solar tank temp due to incoming well water temps.



and I do see that hooking a woodstove convection loop up to the solar tank will definitely cut into solar collection efficiency when the stove's being used. I suppose that a solar tank feeding a wood fired tank feeding a Phoenix would be better, but I think I'll take the hit and keep it at 2 tanks ...



I was really hoping for the most solar space heating help during the winter when the house will be vacant at times and there will be no wood fires. then the Phoenix can be set low (goes down to 70, I read) and solar will be entirely delivered by shuffle to the low temp Phoenix. and in the spring, when we actually get some serious sun while its still heating season in March, April, May and when I, at least, find that I have less desire to light fires than in January.



even though my head has trouble getting around it all, I enjoy the attempt ...

hot_rod

this drawing

is how Dan Foley installed a solar component. The upper coil in the solar tank pre-heats the cold before it flows to the indirect. The tank "capacity" is used as hydronic. This allows more solar contribution to the hydronic, and just some DHW preheat. with this system you could pull the solar tank down to the lowest possible useable temperature to the heating load. On a radiant system, maybe 95- 100F.



DHW use would pull the tank even lower even after the hydronic portion switched to the boiler, since it always flows through that upper coil.



In your drawing if you shuffle from the solar to the second heater it seems you could only shuffle out of the solar tank down to 120- 130F whatever the temperature you use the DHW?



Good catch on my drawing , that pump is shown flowing the wrong way.



hr

Royboy

yes, true

"In your drawing if you shuffle from the solar to the second heater it seems you could only shuffle out of the solar tank down to 120- 130F whatever the temperature you use the DHW?"



yes ... and this would only occur when the solar tank is hotter than 120-130F so still is a plus for overall performance (as I see it) ... the only way to get the solar tank down further is the "normal mode" of bringing in cold tap water when hot water is drawn at the tap. shuffle mode provides an additional solar heat extraction method with the only downside I see, besides the cost of the added hardware, being the possible disruption of stratification in the solar tank



ps I'm having trouble viewing your Foley schematic - comes up too small for me to read and if I magnify its blurry. can you post it larger?



thanks - Roy

Bob Gagnon plumbing and heating

Three Tank System

I think this simple design will collect a lot more energy. These large tanks can be constructed inexpensively with homemade coils, and a non-pressurized rubber lining. You will need another tank, either a SuperStor or tankless heater to keep safe hot water temperatures, and for long periods of no sun. This method will provide you with 100% of your domestic for much of the year, and it will preheat your domestic for the rest of the year. You simply run your delta tee controller from your collector array to the large tank during the heating season, and you run the delta tee controller from the array to the smaller domestic water tank during the warmer weather. In the winter with your radiant heating on, the collector's circulator will kick on when collector's temps reach 80 degrees, keeping both tanks at about 80 degrees, to provide radiant heating and domestic pre-heat. The radiant panels in your house will act as storage. You will collect a lot more energy at the lower temperatures. In the summer the temperature in your smaller tank will become hotter and hotter, because as you tank reaches 100 degrees the collector circulator won't kick on until it's 105 degrees in the collector. When the tank is 110 degrees the circ won't kick on until it's 115 up in the collectors, when it's 140 in the tank the circ won't kick on until it's 145 up in the collector and so on. The incrementaly hotter water will drop off BTU'S a second time into the bigger tank for a domestic, double pre-heat. You will sacrifice some BTU collection at these higher temps but unless you have a low temperature load, like a heated pool, you should do it this way. Having a double pre heat will give you 100% of your domestic needs for much of the year by sending water hot enough to your SuperStor, so it dosen't have to come on, you also won't need to have a heat dump, you simply save a lot more BTU'S to use when the sun's not shining.



Thanks, Bob Gagnon

NRT_Rob

my personal favorite

is cold solar tank. perhaps with a top coil for heating system extraction, perhaps not. solar coil in bottom.



electric on demand, or gas on demand rated for "boost" applications (I've seen an "eternal" hybrid heater that fits the bill nicely) for DHW. Even if electricity is expensive, it's reduced usage in the first place, so no problem.



then you never heat the tank, you maximize your storage, and every BTU you get offsets a BTU used (minus tank loss), whether it's "hot enough" or not.