Piping design when two parallel storage tanks used to preheat water.

[LAWindsurfer]

A solar hot water heating system installed about a year ago in a ten unit apartment building has not yielded the anticipated reduction in energy consumption. The system has three tanks, two with heat exchangers to preheat the water, and a 100 gallon, 199,000 BTU/hour gas-fired tank to boost the temperature as required. Two preheat tanks were installed in parallel to insure an adequate flow rate; space constraints prevented the installation of a single large tank. The 1.5 inch diameter outputs of each pre-heat tank are combined to feed the 2.0 inch diameter input of the gas-fired temperature boost tank before the water is fed to the building.

The solar panel fluid is circulated in a separate closed system with a drain-back tank. Currently, the solar panel fluid is similarly divided and flows in parallel through both of the pre-heat tanks. Is that the optimum configuration? It seems to me that channeling the solar panel fluid through pre-heat tank heat exchangers connected in series would result in more efficient operation, while the pre-heat tanks remain connected in parallel for the delivery of water to the gas-fired temperature booster tank.

[Naptown]

Can you sketch how things are piped?
First things first did you meter the hot water to see what the actual use is?
Solar Hot water is a use it or lose it system. You could be producing more than required or the amount of water being used is lower than anticipated.
Post the sketch with how things are piped.
Generally heat exchangers plumbed in parallel reverse will give the best output. In series one tank gets more heat than the other. If the tanks are not plumbed properly there may be more flow through one than the other which would also diminish the output.
Pictures work too if that's easier

[Art VanDelay]

Warranty?

It actually sounds like the design and piping are probably OK.

What does the original installer tell you?

[LAWindsurfer]

Diagram of two parallel storage tanks attached.

Diagram attached

What is meant by "parallel reverse?"

Effectiveness of solar hot water heating is being judged by comparing cu.ft. of gas consumption (with correction for BTU content) for three years prior to installation with gas consumption over most recent year.

[russ]

Without knowing the actual water consumption all is a shot in the dark. A water meter in the cold water line to the system would tell you that. The gas consumption may well be telling you that one tenant likes hot water.

Balancing flow to the two tanks with the exchangers - how is it done?

Any idea of water temperature out of each tank with exchangers?

[tandrews]

Looks like there might not be much differential temp recorded between outlet of panels and the comparative temp for pump control.
If your cold differential measure is indeed where it appears, you may not be cycling through the entire panels contents.
I would expect it placed prior to the pump on the heat exchanger output feed to panels.

[Naptown]

Diagram attached

What is meant by "parallel reverse?"

Effectiveness of solar hot water heating is being judged by comparing cu.ft. of gas consumption (with correction for BTU content) for three years prior to installation with gas consumption over most recent year.

Look carefully at how this is piped if indeed it is piped this way most of the heat is being put into the tank on the left and most of the water is being drawn out of the tank on the right.

The tanks and heat exchanger should be plumbed like this

[LucMan]

The parallel piping may give higher efficiencies if you are happy with just preheating.
I like series piping, that way at least 1 tank may reach usable temperature before it feeds through the gas fired heater. Refer to page 20 for collector piping in the attached PDF. A flow meter would help to know how many GPD you are dealing with. How many sq ft of panels, how many gal. are the preheat tanks.

[Naptown]

The parallel piping may give higher efficiencies if you are happy with just preheating.
I like series piping, that way at least 1 tank may reach usable temperature before it feeds through the gas fired heater. Refer to page 20 for collector piping in the attached PDF. A flow meter would help to know how many GPD you are dealing with. How many sq ft of panels, how many gal. are the preheat tanks.

isn't efficiency in so far as gaining the most BTU per day what it is all about

[LAWindsurfer]

Impact of solar on gas consumption took water use into account.

Attached chart shows that water use and gas consumption does not appear correlated. Solar installed December 2010. Unknown if gas consumption goes up in the winter because people take hotter showers or because of increased losses in (insulated) hot water circulation line. Hot water circulation line is heated by gas since it circulates the water in the gas fired water tank. Hot water circulation pump is controlled by sensor on return line except during morning and evening "rush" hours. Pump runs when water at return point drops below 110 oF and turns on when temp rises above 112 oF.

It seems to me that the reverse parallel connection would only make a big difference if the plumbing between the tanks obstructed the solar water flow by an amount comparable to the obstruction represented by the heat exchanger coils.

I'm in the process of instrumenting the configuration with max/min thermometers to see if all of the pre-heated water is used up during the night.

[LAWindsurfer]

Impact of solar on gas consumption took water use into account.

Attached chart shows that water use and gas consumption does not appear correlated. SOLAR installed in December 2010. Unknown if gas consumption goes up in the winter because people take hotter showers or because of increased losses in (insulated) hot water circulation line. Hot water circulation line is heated by gas since it circulates the water in the gas fired water tank. Hot water circulation pump is controlled by sensor on return line except during morning and evening "rush" hours. Pump runs when water at return point drops below 110 oF and turns on when temp rises above 112 oF.

It seems to me that the reverse parallel connection would only make a big difference if the plumbing between the tanks obstructed the solar water flow by an amount comparable to the obstruction represented by the heat exchanger coils.

I'm in the process of instrumenting the configuration with max/min thermometers to see if all of the pre-heated water is used up during the night.

[Naptown]

Attached chart shows that water use and gas consumption does not appear correlated. Solar installed December 2010. Unknown if gas consumption goes up in the winter because people take hotter showers or because of increased losses in (insulated) hot water circulation line. Hot water circulation line is heated by gas since it circulates the water in the gas fired water tank. Hot water circulation pump is controlled by sensor on return line except during morning and evening "rush" hours. Pump runs when water at return point drops below 110 oF and turns on when temp rises above 112 oF.

It seems to me that the reverse parallel connection would only make a big difference if the plumbing between the tanks obstructed the solar water flow by an amount comparable to the obstruction represented by the heat exchanger coils.

I'm in the process of instrumenting the configuration with max/min thermometers to see if all of the pre-heated water is used up during the night.

remember that the flow will take the path least resistance. In the diagram you sent the path of least resistance for the solar loop is through the left solar tank and the water loop is the tank on the right.
You have an unbalanced situation on both the solar and water sides.
By your chart it also appears that water use in the building increased about the same time the solar went in.
If you really want to instrument put a revenue grade BTU meter on each solar tank. That will bring the imbalance to light quickly.
As far as using more gas in the winter yes people will tend to take longer hotter showers then. But also remember that during the winter the outside temps are cooler than in summer. Water is stored overhead in tanks so the incoming water temp will be higher during the summer than in winter.

[LAWindsurfer]

summer and winter gas variation & plumbing path lengths

Good point about inlet water temperature being colder in winter. I had not thought of that.

As for imbalance between tanks on left and right, sketch did not attempt to communicate plumbing path lengths. I was only seeking to show connections and flow directions.

Thanks for suggestions and comments.

[hlukeh]

Best way to hook up two hot water tanks with a solar pre-heat tank

I'm trying to figure out the best way to hook up two hot water tanks with a solar pre-heat tank and a main tank that the domestic hot water is drawing off. My setup currently has a geothermal pre-heat on a single tank, which is a poor setup because I've been monitoring the heat pump and water heater run times and the water heater runs just as often whether the heat pump is running or not.

I wanted to look at the attachments in the posts in this thread but since my post count it 0, it won't let me.

I came up with a design for two tanks that I thought I'd go with, and it's attached, but after I look at what you guys have all suggested, maybe I'll change it.

The only designs I have found so far are of the solar energy heating a pre-heat tank, and then the main tank drawing from that. I don't like that design because as the heat is lost from the main tank, the elements will turn on even if the pre-heat tank is way hotter.

water heater setup.JPG

Water heater design.pdf

Luke

[Mike90250]

I think I'd only use the solar in the preheat tank, and not circulate between both tanks.