DHW system layout

[SolarAndWood]

I have read the dhw threads and have a few questions about my layout.

My hot water demand is at the opposite end of the house from the future boiler room and where I would put the future solar collectors. I want to replace my older propane water heater now and am wondering where I should locate the replacement and what the replacement should be.

Should I locate a 100 gallon tank near the boiler room or near the demand?

My current plan is to do 500 gallons pressurized storage with the boiler and the 105 gallon Marathon electric heater with the extra coil. When I add collectors, will I want to integrate them with the pressurized storage, directly to the water heater or to a third tank directly adjacent to the collectors as a pre-heater? They will be 30' almost directly above the boiler room.

If a third tank, I assume this would then heat the water heater directly and I would need two coils? If I go with the third tank, I also assume that I can do a smaller water heater?

Lastly, I assume that if I eventually add enough collectors to heat a small pool or hot tub in the summer, I will connect to the 500 gallon tank anyway?

 

[Nofossil]

That's a whole lot of good questions. I'll chime in my my thoughts:

1) You want your DHW tank as close as you can possibly get it to the point of most frequent hot water use, usually the kitchen. You don't want to waste hot water purging the lines between the DHW tank and the tap every time you use hot water. Mine is directly underneath the kitchen sink and dishwasher.

2) It takes a lot of solar panels to move 500 gallons very much. You're much better off heating a smaller tank (or two) with the solar panels, and again it probably makes sense to have that tank near the point of use. One simple approach is to connect a small flat plate exchanger on the floor next to the tank to be heated and allow the tank side to thermosiphon.

3) Using a preheat tank (second tank in series ahead of primary DHW) could be an option, and you could draw heat from that tank for other purposes without creating the dreaded 'cold shower' phenomenon.

 

[in hot water]

Depending on how much collector ares you plan I would also suggest covering the DHW load first with 60- 80 gallons of capacity, depending on your needs. Then switch the solar over to the larger tank and load. This is easily accomplished with a 3 way motorized zone valve and a solar controller with multiple tank logic.

 

[SolarAndWood]

Ok, so I can locate the tank within 20 ft of all of my demand in the house but it will be 60 ft from the boiler storage and then about 90 from the solar collectors. Is the suggestion then to:

1. locate a large water heater with 2 coils near the demand
2. locate a solar controller adjacent to the 500 gallon tank
3. run a loop from the controller to the water heater
4. run a loop from the 500 gallon tank to the water heater

Or because of the 90' run, should there be a pre-heat tank adjacent to the collectors?

 

[timberr]

I would re-think the size of your hot water tank. I have 500 gal. of storage and a 60 gal. Superstore indirect hot water heater, and I am always amazed how much storage BTU's it can take to heat my DHW after my wife takes a long shower.

 

[Nofossil]

An alternative to complex controls could be to plumb two DHW tanks in series, with the second one (the one closest to the faucets) heated with electric or whatever backup heat source you choose.

For the solar panel, draw water from the cold inlet of the first tank. pass it through the panels (or panel HX) and return the hot water to the outlet of the second tank. In that way, you'll always be heating the most critical tank first, then gradually push the thermocline back through first tank until they're both hot top-to-bottom. If you only have a little heat, it will go where it does the most good. No fancy controls or expensive valves - just a differential controller to run the solar circ when the panel outlet is hot enough to be useful.

 

[markmudd]

You could put a circulator loop in your DHW system running from tank to the point of use if practical

 

[SolarAndWood]

For the solar panel, draw water from the cold inlet of the first tank. pass it through the panels (or panel HX) and return the hot water to the outlet of the second tank.

So the reason for two tanks is the second is always kept at temp by the backup and/or boiler while the first would only have free heat pushed back by the solar panels? I assume this means running domestic water through the solar panels and draining/isolating them before freeze is an issue? In this scenario, I replace my current 50 gallon propane heater with a 50 gallon electric with a single loop for the boiler?

Of course, as long as my cost of wood is <$15/cord, the solar panels never pay for themselves and I should just put the money in the boiler/storage and keep it simple.

 

[kabbott]

I Think the idea is to heat both tanks with solar, hottest water to the second tank(the 1 with backup elements) and then to the first tank(a preheat tank)
This would serve to 1: lower your solar return temps for better efficiency and 2: when not using solar you will only have standby loss from the 1 tank with backup.

I agree on keeping the tank as close to the use as possible. I moved my tank for this reason. If you have to run water and wait for it to become hot it is the same as running
the hot water down the drain. Some use recirculating pumps but they will waste some energy and hot water as well.

You mention hot tub/pool... If you had enough solar output you could heat a tub/pool with the return water(between the DHW tank and solar panels). I do not yet have solar
DHW but I have some experience with heating a pool with solar and you can get an amazing amount from solar when heating 80-90 degree water as opposed to 130-140 degree
DHW.

 

[in hot water]

how many square feet of collector are you considering? Winter month solar available really drops in your area. Unless you are planning 100 sq. ft. or more you probably will not be doing much, if any of the heating loads. Determine how much DHW you need or want and design your solar, tank capacity, etc around that.

hr

 

[SolarAndWood]

I am currently thinking just enough collector area that I don't have to burn just for hot water. So, that would be mid-May to Mid-September. I have 1000 sq ft of solar south unobstructed 7/12 roof to play with.

 

[Nofossil]

For the solar panel, draw water from the cold inlet of the first tank. pass it through the panels (or panel HX) and return the hot water to the outlet of the second tank.

So the reason for two tanks is the second is always kept at temp by the backup and/or boiler while the first would only have free heat pushed back by the solar panels? I assume this means running domestic water through the solar panels and draining/isolating them before freeze is an issue? In this scenario, I replace my current 50 gallon propane heater with a 50 gallon electric with a single loop for the boiler?

Of course, as long as my cost of wood is <$15/cord, the solar panels never pay for themselves and I should just put the money in the boiler/storage and keep it simple.

Exactly. You don't have to circulate potable water through the panels, though - the same approach works fine with a HX in between - just need another circulator. I'm with you - makes no sense to deal with all the cold weather issues to try and get a few BTUs from your panels when you're burning wood anyway. I set my panels up for optimum summer operation and drain them during the winter. Their purpose is to heat water in the summer so that I don't have to burn wood just for DHW. I end up burning 20 gallons or so per year to provide DHW during cloudy periods in the early spring and late fall when the sun angles are bad but it's not cold enough to fire the wood boiler.

 

[SolarAndWood]

Thanks, I like it. I can get rid of the propane burner now which will make my current use of grill tanks more convenient. Then, add the boiler and summer solar in later to reduce the electric consumption.

Any brands other than Marathon I should be looking at?