Heat transfer from roof loop to tank below expectations.

[MikeSolar]

Russ or Rich please explain further.

LAwindsurfer - It sounds like this system is providing hot water for an apartment building. If so, how many units, how many people per unit?

The high temperatures in the solar loop indicate that the heat exchangers may be undersized. It can be hard to balance the flow in a parallel system like this, so one of the tanks or heat exchangers could be starved for flow.

I have used the Rheem tanks and was disappointed in their heat exchange performance. These heat exchangers are simply 1/2" soft copper wrapped around a conventional steel tank. If that copper were submersed in the tank, they would work much better. But Rheem doesn't do that because corrosion is a problem. Most installers probably use two collectors per tank, and you have three. From my own testing, I believe this HX can only handle one 4x10 panel.
Rheem doesn't say. Nor do they provide ANY heat exchange data: http://www.rheem.com/documents/solaraide-he-rheem-solaraide-he-spec-sheet

"QUESTION .. Does the performance of this system seem to be what would be expected? Would a second pump to circulate the water in the storage tanks significantly increase the efficiency of the heat exchanger ..." NO
The problem with those HXs is a design problem with the amount of heat transfer area, and increased water circulation inside the tank cannot improve that.

But your solution isn't that difficult or expensive. Just add a pump: , and a brazed plate stainless steel heat exchanger: http://www.amazon.com/175-000-BTU-30..._sbs_indust_10

But once you start getting most of the available solar energy into the tank, you're going to find that you don't have enough storage unless the people use a lot of hot water during the day. Good design would have 360-500 gallons, but you have only 160 or 240 depending on which Rheems you have.

The 006 is fine for the HX to tanks and I assume the pump to the panels is now a 009, high head and lower flow. He only needs 0.5 gpm/ panel so the 009 will give him the head needed and 3GPm. You are right though, the HX around the tanks are garbage and it should have an external HX. Also, the OP should have at least 2- 200GAL tanks and not mismatched ones.

[Art VanDelay]

Recirc, etc.

Russ,

Could you explain: "No storage for an apartment building" ?

LAW,

You have the optimum control strategy for your recirc loop. As Russ says, the only improvement would be to disable it completely, but that may invite a complaint from the apt. that showers first in the am.

[LAWindsurfer]

Update .. to Heat Transfer ... below expectations

I'm preparing a better explanation of situation and will probably post later today with added data.

Last comment is correct. Installation is for an apartment house with 14 bathrooms and currently 19 people. Sketch was prepared to help service people know which valves to shut off if there was a leak and not necessarily for a design review - sorry. Sufficient attention was not paid to how roof circulation lines were drawn as they entered storage tanks; in fact return line is connected roughly mid tank where it is supposed to be connected and feed the to roof comes off the bottom of the tank. The "drain-back" tank is actually a large section of insulated pipe on the roof; it is sized such that if the pump is not running, all the water from the collectors puddles in that "drain back" tank.

The recirculation line is well insulated. I'm aware that if no water is being demanded, the gas water heater, and not the solar panels, keeps the water in the recirculation line warm. The recirculation line pump is controlled by a clock and thermostat. The pump runs continuously for about an hour in the prime morning an evening use periods. In non-prime hours, the pump only runs when the water in the return line, just before the water heater, drops below the set point - 105 oF. The thermostat controlling the pump has about a 1 oF hysteresis, so it is not necessary for the line to get really hot before the pump shuts off. Hence, during non prime hours, the water in the recirculation line is kept "warm" but not really hot.

More photos later.

Thanks for help and comments.

No storage for an apartment building

With the 100 & 80 gallon tanks & heat exchangers piped in parallel as they are you have no control over the flow - it goes to the path of least resistance

What a 'flow back tank' is I have no idea

The hot water recirc pump is a horrible idea unless you have great insulation on all the piping - mine ha substantial heat loss so is disconnected

The T2 connection should go to the bottom of the tank - not top

For maximum water temp the T^& T+ connections should be reversed. As they are they would give maximum heat collection - take your choice

[russ]

No storage for an apartment building

With the 100 & 80 gallon tanks & heat exchangers piped in parallel as they are you have no control over the flow - it goes to the path of least resistance

What a 'flow back tank' is I have no idea

The hot water recirc pump is a horrible idea unless you have great insulation on all the piping - mine ha substantial heat loss so is disconnected

The T2 connection should go to the bottom of the tank - not top

For maximum water temp the T^& T+ connections should be reversed. As they are they would give maximum heat collection - take your choice

[Art VanDelay]

Any gas usage data before installation?

Like Rich pointed out - if it is piped like the drawing it is a disaster.

Russ or Rich please explain further.

LAwindsurfer - It sounds like this system is providing hot water for an apartment building. If so, how many units, how many people per unit?

The high temperatures in the solar loop indicate that the heat exchangers may be undersized. It can be hard to balance the flow in a parallel system like this, so one of the tanks or heat exchangers could be starved for flow.

I have used the Rheem tanks and was disappointed in their heat exchange performance. These heat exchangers are simply 1/2" soft copper wrapped around a conventional steel tank. If that copper were submersed in the tank, they would work much better. But Rheem doesn't do that because corrosion is a problem. Most installers probably use two collectors per tank, and you have three. From my own testing, I believe this HX can only handle one 4x10 panel.
Rheem doesn't say. Nor do they provide ANY heat exchange data: http://www.rheem.com/documents/solaraide-he-rheem-solaraide-he-spec-sheet

"QUESTION .. Does the performance of this system seem to be what would be expected? Would a second pump to circulate the water in the storage tanks significantly increase the efficiency of the heat exchanger ..." NO
The problem with those HXs is a design problem with the amount of heat transfer area, and increased water circulation inside the tank cannot improve that.

But your solution isn't that difficult or expensive. Just add a pump: , and a brazed plate stainless steel heat exchanger: http://www.amazon.com/175-000-BTU-30..._sbs_indust_10

But once you start getting most of the available solar energy into the tank, you're going to find that you don't have enough storage unless the people use a lot of hot water during the day. Good design would have 360-500 gallons, but you have only 160 or 240 depending on which Rheems you have.

[russ]

Like Rich pointed out - if it is piped like the drawing it is a disaster.

[Naptown]

I hope that system is not plumbed in the way it is shown in the drawing.
Can you get photo's of the installation?

[MikeSolar]

hi there,

First, the inputs to the HX from the solar (hottest water) should be at the top port of the HX. The coldest water in the tank should go to the coil. Also, it appears that a cold water valve is open bypassing the solar tank. This must be closed. water must ONLY from through the solar tanks before getting to the back up tank.
Next, please post the height of the building, pipe diameter, controller type and pump specs. There are many things that can cause the issue because as an average family in your location, 2 - 3m2 panels (64 ft2) should do 75%+ of your hot water with a 300L (80-100USG tank)

[LAWindsurfer]

The solar hot water system shown in the attached figure 2 has not been yielding the gas savings that I expected. The system consists of six, EC40 roof-mounted solar panels, two storage tanks plumbed in parallel with the output of those feeding a gas-fired water heater. The system uses heat exchangers that are built into the Rheem storage tanks. It is a one pump system with a flow back tank for freeze protection.

It seems to me that natural gas consumption for boosting water temperature to 120 oF could be reduced by improving the heat transfer between the circulating collector fluid and the water in the storage tanks. Early in the afternoon on a typical bright sunny day, the temperature of the circulating collector fluid is in the 170 oF to 190 oF range, while the temperature of the water in the storage tanks is typically about 110 oF . The temperature of the inlet water from the street is approximately 70 oF. If hot water consumption is low, the temperature of the water in the storage tanks will occasionally reach the 140 oF range after many hours. If a washing machine is being run, the solar system doesn't seem to be able to keep up with the demand even on a bright sunny day. More specifically, when the temperature at the bottom of the storage tanks is about 105 oF, the difference between the temperature of the collector fluid coming down from the roof and the temperature of the collector fluid being fed back to the roof is about 8 oF at most. As the temperature of the water at the bottom of the storage tank rises, the temperature drop of the collector fluid during its pass through the heat exchange decreases, and it most often is only about 5 oF. I expected a greater drop in the circulating collector fluid as it passes through the heat exchangers.

My rough computations indicate that with 400 BTU/sq.ft./hour of sunny southern California day sun energy and 240 sq.ft. of panel, approximately 96,000 btu / hr would be collected with an 100% efficient system. The storage tanks hold 180 gal. of water requiring 8.3 BTU/gal/oF for heat rise. Hence 1500 BTU is needed to raise tank temperature 1 oF. If system was 100% efficient, system could raise temperature of 180 gallons by 64 oF in one hour. My observation is that under the best of conditions, low water consumption on a bright sunny day, the tank water goes up about five degrees / hour at most. It is hard to get precise measure because I don't know how much water tenants in nearly empty building are consuming during the observation period.

QUESTION .. Does the performance of this system seem to be what would be expected? Would a second pump to circulate the water in the storage tanks significantly increase the efficiency of the heat exchanger even though it would result in the temperature of the exiting water being lower because the hottest water was no longer being allowed to rise to the top of the tank?