Instant DHW

[Bob Rohr]

Here is a system I have been trying this summer. With 5 collectors in a drainback I can keep about 150 gallons of my system around 160- 170 in the summer, easily. In the winter it will be heated by solar and wood. So far I have not need to supplement the DHW this system produces.

Basically when any hot water faucet is turned on a Sika flow switch turns on a small circ to circulate the DB water thru the A side of the HX. The cold well water flows thru the other side and is heated instantly. It takes about 5 seconds to get up to temperature after the faucet is opened.

The next Caleffi I-dronics 11 will be on DHW, here is a preview of some of the concepts.

The 5X12- 30 HX is exactly what I used on mine and it supplies 3 GPM non stop, easily.

The main goal it to not have to store DHW, and eliminate the possibility of Legionella potential in a DHW tank that falls below 120F.

There are codes in Europe that regulate DHW production. Most folks have tankless. If a tank style DHW is used, it must be elevated every 24 hours to 60C (140F) for 60 minutes.

I notice more and more of these pre-plumber DHW heat exchanger systems arriving over here. Viessmann offers a nicely packaged instant DHW HX box to be added to their boilers.

The solar drawing credit: Appropriate Design

hr

 

[Don2222]

Here is a system I have been trying this summer. With 5 collectors in a drainback I can keep about 150 gallons of my system around 160- 170 in the summer, easily. In the winter it will be heated by solar and wood. So far I have not need to supplement the DHW this system produces.

Basically when any hot water faucet is turned on a Sika flow switch turns on a small circ to circulate the DB water thru the A side of the HX. The cold well water flows thru the other side and is heated instantly. It takes about 5 seconds to get up to temperature after the faucet is opened.

The next Caleffi I-dronics 11 will be on DHW, here is a preview of some of the concepts.

The 5X12- 30 HX is exactly what I used on mine and it supplies 3 GPM non stop, easily.

The main goal it to not have to store DHW, and eliminate the possibility of Legionella potential in a DHW tank that falls below 120F.

There are codes in Europe that regulate DHW production. Most folks have tankless. If a tank style DHW is used, it must be elevated every 24 hours to 60C (140F) for 60 minutes.

I notice more and more of these pre-plumber DHW heat exchanger systems arriving over here. Viessmann offers a nicely packaged instant DHW HX box to be added to their boilers.

The solar drawing credit: Appropriate Design

hr

Does your circulator come on to keep the water warm if the faucet is not turned on for a very long time?

 

[woodsmaster]

Are co-op elec. company allways says turn your water heater to 120 to save money. They never say anything
about turning it up to 140 once in a while ? This seems to be more of a canadian thing than here from what I gather?

 

[woodsmaster]

I do like the instant hot water idea and If I ever change mine that's the way I will go.

 

[BoilerMan]

Bob,
I've read some about drainback and circulated systems, my question, is it better to use the storage water directly through the collectors such as with drainback? Or I was thinking of useing a large indirect (120gal) and having a small closed loop antifreeze circuit go through the collectors and the coil of the indirect. The circulator run from a solar panel (PV). Would there be overheating problems? I do like the plate HX off of storage though, but same question do you have any overheating issues, or do the collectors just maintain the storage in the 150-160 region?
Taylor

 

[wood thing]

Bob,
I've read some about drainback and circulated systems, my question, is it better to use the storage water directly through the collectors such as with drainback? Or I was thinking of useing a large indirect (120gal) and having a small closed loop antifreeze circuit go through the collectors and the coil of the indirect. The circulator run from a solar panel (PV). Would there be overheating problems? I do like the plate HX off of storage though, but same question do you have any overheating issues, or do the collectors just maintain the storage in the 150-160 region?
Taylor

I have the same set up as Taylor S. My indirect is a 40 gal. It does ok but I also am interested in the instant idea. I wonder when hydronics #11 will be out ?

 

[Bob Rohr]

Bob,
I've read some about drainback and circulated systems, my question, is it better to use the storage water directly through the collectors such as with drainback? Or I was thinking of useing a large indirect (120gal) and having a small closed loop antifreeze circuit go through the collectors and the coil of the indirect. The circulator run from a solar panel (PV). Would there be overheating problems? I do like the plate HX off of storage though, but same question do you have any overheating issues, or do the collectors just maintain the storage in the 150-160 region?
Taylor

I like the drainback for it's simplicity. I prefer water as the transfer medium compared to expensive, maintenance prone glycol. Yes the same water circulates thru the collector, storage, wood boiler and radiant heat loops. the only exchanger is for the domestic water.

A closed loop with glycol and a pv powered pump is a good choice. Not many larger sized DC pumps available at an affordable price, so depending on the size of your array that is a concern. Every heat exchanger you add reduces efficiency and costs an extra pump and power to run it.

In it's simplistict form it starts like this. Adding heating zones and mixed temperature and DHW adds components and piping. In this case it is a pressurize drainback. the expansion "tank" is the air bubble trapped at the top of the tank.

 

[BoilerMan]

Wouldn't that be technically an open system with air in the tank? Disolved oxygen and all? I do like the simplicity! But I was alswys told air in any closed system will disolve in the liquid and cause corrosion problems, have I been miss led? I would certainly like to do this with the DHW going directly to the collectors via a bronze pump, than piped back to the big indirect w/o and heat exchangers. I'll just keep the bottom coil of my indirect connected to the wood system. And only the BTUs needed to bring it up to 140 (if any) to avoid legionella would need to come from wood or another source. The caleffi Idronics has provided to be excellent sources of information and piping arrangements. I look forward to reading and studying it when it comes out.

 

[maple1]

I am going to leave a couple of stubs for future use on the tops of my stacked horizontal tanks when I put them in. I like the idea of a future simple direct hook up from there to solar panels - but was thinking I'd need a HX and separate (maybe glycol) loop for the panels on the other side.

Is that pressurized drainback setup you're talking about a commercial unit? Would you have a link? I don't have much solar knowledge, but was under the impression drain backs were open systems, therefore require a HX.

 

[Bob Rohr]

Wouldn't that be technically an open system with air in the tank? Disolved oxygen and all? I do like the simplicity! But I was alswys told air in any closed system will disolve in the liquid and cause corrosion problems, have I been miss led? I would certainly like to do this with the DHW going directly to the collectors via a bronze pump, than piped back to the big indirect w/o and heat exchangers. I'll just keep the bottom coil of my indirect connected to the wood system. And only the BTUs needed to bring it up to 140 (if any) to avoid legionella would need to come from wood or another source. The caleffi Idronics has provided to be excellent sources of information and piping arrangements. I look forward to reading and studying it when it comes out.

The water in a closed loop solar is the same as in a hydronic system, it becomes 'dead" water so to speak, once the O2 is depleted. The small amount of O2 will oxidize on the plain steel surfaces and provide a small protection layer. Early hydronic systems had plain steel expansion tanks, no bladder. Typically those tanks hung above the boiler between the floor joists. Those tanks lasted for many years, so say longer than the modern bladder style tanks

One issue with a direct system where the potable water circulates thru the collector is scale build up. As you add new, fresh water you add minerals, calcium, whatever is in your water. A very thin scale deposit causes a large amount of the heat transfer efficiency. So a closed loop with plain water is just fine. No need to use a bronze or stainless pump on a closed system.

Direct systems with tank and collector are very common in warm climates where there is no freeze potential. They are fairly in-expensive and have a life cycle like a ttypical water heater 5- 10 years, then they replace the whole system.

I have plenty of friends with close loop drainbacks and 20 plus years of service. one in upstate NY has 27 years on a plain steel tank in a drainback with air bubble.

This I-dronics shows wood and solar combi systems

www.caleffi.us/en_US/caleffi/Details/Magazines/pdf/idronics_10_us.pdf

Idronics #6 is about Solar Combi systems and this picture is from that issue.

hr

 

[kopeck]

That's interesting.

How many gallons of storage do you usually need for a setup like this? I'm guessing not a whole lot.

K

 

[Bob Rohr]

That's interesting.

How many gallons of storage do you usually need for a setup like this? I'm guessing not a whole lot.

K

1- 1.5 gallons of storage per square foot of collector. I have 5 collectors 32 sq. ft. each, so a 200 gallon storage, with about 15 gallons at the top of the tank for the air bubble.

Since it is a drainback over sizing the array is not a big deal. Right now I have a 120 gallon tank, sitting at 182F today. A thermostatic mix valve assures the faucets don't see over 120F.

It's a little rough looking I'm about ready to upgrade the tank. A homemade insulation jacket for the plate HX, water softener to the left.

 

[BoilerMan]

Can you just turn the pump off and let the array drainback when the tank reaches it's high limit or say 200? Will it hurt the collectors to have full sun and no water in them?

Taylor

 

[Bob Rohr]

Can you just turn the pump off and let the array drainback when the tank reaches it's high limit or say 200? Will it hurt the collectors to have full sun and no water in them?

Taylor

Correct, the pump turns off when the tank, or load hits setpoint. Check with the collector manufacturers I don't know of any flat plate type collectors that cannot be used for drainback. Really it's copper tube in an aluminum frame with glass over it. Fairly simple low tech stuff.

The collectors will need to be mounted so they can drain, a small pitch to one side, or a collector built to draindown.

hr

 

[maple1]

I'm finding this all very interesting.

If one had two stacked horizontal propane tanks in their current system, with extra fittings & stubs piped into the top of each one for future use (what I'm planning with 330 gal. tanks), they could plumb solar panels directly to those in a pressurized drainback scenario? In doing that, the panels would draw from the top of the bottom tank, and return to the top of the top tank. That is, in non-heating season, and when the panels are doing their thing, it would effectively halve the storage being used from 660 to 330 - much more suitable I would think for a solar DHW application. I just haven't got my head around how the air return tube setup actually funtions and allows the panels to drain back, and how the water & air move back & forth on drain back. Could that air return tube, rather than being plumbed to an air space in the top of the storage tank, be plumbed to the top of a non-diaphragm system expansion tank (110 gal.), the top of which sits lower that the top of the storage tanks? I'm picturing the air space in the expansion tank filling with water over time doing that - but that's without a good understanding of the forces at work here.

 

[Bob Rohr]

I'm finding this all very interesting.

If one had two stacked horizontal propane tanks in their current system, with extra fittings & stubs piped into the top of each one for future use (what I'm planning with 330 gal. tanks), they could plumb solar panels directly to those in a pressurized drainback scenario? In doing that, the panels would draw from the top of the bottom tank, and return to the top of the top tank. That is, in non-heating season, and when the panels are doing their thing, it would effectively halve the storage being used from 660 to 330 - much more suitable I would think for a solar DHW application. I just haven't got my head around how the air return tube setup actually funtions and allows the panels to drain back, and how the water & air move back & forth on drain back. Could that air return tube, rather than being plumbed to an air space in the top of the storage tank, be plumbed to the top of a non-diaphragm system expansion tank (110 gal.), the top of which sits lower that the top of the storage tanks? I'm picturing the air space in the expansion tank filling with water over time doing that - but that's without a good understanding of the forces at work here.

The air space you allow in the top of the upper tank becomes the expansion space for the entire system, solar, wood boiler, piping, etc. So leave the equivelent of what you have in the size of your current expansion tank. So fill the upper tank to within maybe 12" of the top. A sight tube made from clear pex tube, shows where that level is.

I never use an air tube, the pipe coming down from the collector stops just into the tank so it has access to that air bubble. When the pump stops, air rises up and breaks the siphon.

like turning a gas can over and trying to pour gas without allowing air up into the tank, same concept. So 280 gallons into the tank allows about 50 gallon of expansion "tank" trapped inside. You may need to add some air over the years as some will re-absorb into the fluid. A simple air hose fitting works fine for adding air.

 

[maple1]

In my case, I'm pretty limited on storage space - so would not really be wanting to give any of it up to the air bubble. Plus, my heat loads are going to be pulled from the top of storage, where the air bubble would now reside. After seeing that air tube in a couple of the diagrams, that's why I was wondering about being able to utilize a non-diaphragm expansion tank if ones system had one, and if the height of the top of expansion relative to the height of top of storage would be a determining factor in that. Maybe one could shoehorn/plumb in another small tank on top of storage for the 'air bubble' section? Guess I should do more reading - those idronics references are loaded with info.

 

[Bob Rohr]

In my case, I'm pretty limited on storage space - so would not really be wanting to give any of it up to the air bubble. Plus, my heat loads are going to be pulled from the top of storage, where the air bubble would now reside. After seeing that air tube in a couple of the diagrams, that's why I was wondering about being able to utilize a non-diaphragm expansion tank if ones system had one, and if the height of the top of expansion relative to the height of top of storage would be a determining factor in that. Maybe one could shoehorn/plumb in another small tank on top of storage for the 'air bubble' section? Guess I should do more reading - those idronics references are loaded with info.

Can you locate that expansion tank above the top storage. it would act as the drainback tank and the expansion air space.

 

[maple1]

I've gone thru things in my head over & over the past few months, and have no place above to fit an expansion tank - although I might end up with a foot or so of headroom between the top of my insulated tanks & the bottom of my floor joists. That's why I was wondering about putting something small up there for a drainback tank, along with my main expansion tank on the floor under the stairs.

I'm pretty sure it wouldn't go over too well if I proceeded to try to put the expansion tank in our living room or a bed room.

Then again, if I did that, I'd likely end up living here all by myself & have all kinds of room for all kinds of things...

 

[mmudd]

Great post HR. I have a small 64 sq ft draindown solar system (homemade collector) in operation for past two years which works good in summer. Very trouble free. I heat DHW with wood boiler in winter/off season. Collector area to storage volume is important. I have 120ft 1/2" copper coil immersed in open storage which contains potable water which circulates thru DHW heater with aqua stat and small circulator. I have not had my electric water heater on for about a year and a half. One thing I noticed is that since I have turned off the elec water heater, I don't get scale build up in bottom of the water heater like I did with electric turned on.