What is the lowest temperature I can maintain with this heater ? That is, I plan to use it in crawl space where water pump and pressure tank and copper pipes and water heater are located and in winter just want to keep space heated to slightly above freezing. Will this heater work ? Thanks. ms

Hello,

Expansion tanks are used in wet or dry fire suppressions as well as other hydronic (heating/cooling) systems to compensate for temperature and pressure differentials (expansion/contraction) in the piping system and to maintain a constant pressure in the system. As liquids heat and cool they will expand or contract. The pressure switch you describe is usually connected to an air compressor that will cycle on and off as needed to maintain the system pressure.

The expansion tank should never be completely filled because liquids are not compressible and can cause excessive system pressurization and subsequent malfunctions or rupture of components (water hammer) or pump seals. Some expansion are equipped with "site glasses" so the water level in the expansion tank can be observed and monitored.

Deep well.
Pressure valve located on front of above-ground-storage-tank will turn pump on-off depending on amount of water pressure inside tank.
When water is consumed inside home, the pressure valve responds to the lower pressure and closes electrical contacts... sending electricity to water well pump located deep underground.
The pump turns on and lifts water through a pipe into the above ground storage tank.
When the tank is full of water, the pressure valve senses the higher pressure and turns off electricity going to pump
Pump shuts off.
There is a one-way check valve on the water line going down to pump, and this keeps water from flowing back down into well.
The above ground tank maintains water pressure for the house.
When pressure is reduced by taking a shower, the pressure valve turns pump on again, and cycle repeats.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Usually toilets do not require hot water.
Is the plumbing divided into hot and cold water pipes?
If toilets are hooked to hot water, how many flushes and how many gallons per flush per day.
What is the expected consumption per each worker?
What is the latitude from equator, and expected solar input. Further north, and hazy atmosphere reduce input. Beijing has smog I believe.
How often is the solar panel going to cleaned of smog and dirt for maximum collection?
If the area has hard water, then the entire system is prone to sediment build-up and failure unless water is artificially softened.
Solar water heater temperatures can exceed 180 degrees at times. This temperature will damage storage tank that is not made for that temperature.
Most solar water heater systems have pressure regulating valve and expansion tank to accommodate occasional high temperature and pressure.

A good fellow to contact might be at following website:
http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm

Solar collectors are usually connected to a circulating pump, and then the heated water is stored in a tank.
The pump circulates water continually from solar collectors through a storage tank.
This type of system would use ordinary tank-type water heater for storage.
Most of these systems also utilize a back-up heating element inside the tank.

More complicated systems circulate special liquid through spiral tubes inside the water heater.
This type of heater would be an indirect water heater.
Indirect heaters are used for solar collectors, and used with boilers, or any outside heat source.
http://www.bockwaterheaters.com/Products/products_coil_tank_water_heaters.html
http://www.hotwater.com/products/residential/cirrex_solar.html
http://waterheatertimer.org/images/solar-water-heaters-on-roof_lbiRv_5784111-500x282.jpg

Do you mean expansion tank? To relieve pressure?
Expansion tank goes on cold water line before cold enters tank.
See image:
http://waterheatertimer.org/images/water-heater-expansion-tank.jpg

Expansion tanks are installed when TP valve is constantly drizzling hot water due to very hot temperature settings. Or when water pressure is high, etc
It's also smart to install expansion tank so plumbing lasts longer, and older tank is less likely to rupture and cause damage. Inspect tank for leaks each 6 months.
Some regions require expansion tank by local code.
Water heater can be damaged when pressure exceeds 80psi. : Test pressure using pressure gauge at water heater drain valve.

Other resources:
http://waterheatertimer.org/9-ways-to-save-with-water-heater.html
http://waterheatertimer.org/How-to-adjust-temperature-on-water-heater.html
http://waterheatertimer.org/images/BTG-100-Pressure-gauge.jpg

1. COLD MIX, HEATER DEACTIVATES
   If inlet water temperature is over 70°F due to geographic location, water maybe very hot out of the tap. This requires a lot of cold water to be added with it in order to attain a useable hot water temperature. The addition of too much cold water will overpower hot water demand from the tankless water heater. This slows the flow within the tankless water heater, decreasing it below activation point, which shuts off the heater. The end result is nothing but cold water coming out of the outlet. See: TWH E2-04 Overcoming fluctuating temperatures [PDF].
2. FLUCTUATING WATER PRESSURE
   If the water pressure in the home is erratic and the water flow is not consistent while a tap is opened, then the temperature of hot water will fluctuate. The minimum water pressure for the home should be 30psi or greater. For people on well systems the minimum pressure range is 30-50psi. The use of a pressure reducing/regulating valve is an effective way to maintain constant water pressure to the water heater. Watts brand 25AUB- ¾" or N35B-¾" pressure reducing/regulating valves or equivalent is suggested.
3. CHANGING FLOW RATE
   Major changes in flow rate can adversely affect the output water temperature of the heater. Increases from one major fixture running to multiple fixtures running at one time can cause the temperature to fluctuate. Stay within the heater's specifications. See: TWH E2-03 Outlet temperature vs. flow rate [PDF].

From the Bosch website.

Good Luck,
Gary

The problem with all that type of water heater is that once the hot tap is turned on sufficient flow must be maintained through the water heater to keep the main gas burner alight. If the heat adjuster is turned up to high, then the person using the hot water will turn it down to a low rate which means the main burner will turn off and result in cold water. Try setting the heat control to the lowest setting that still gives hot enough water. If you have a thermometer adjust control for a temperature of around 50 to 55 degrees Celcius. You may have to fiddle with fine adjustments to get the temperature and flow rates to maintain a constant output of hot water. You may still have intermittent problems. If the heater only supplies baths or showers you could set the temperatuer even lower (45 to 50) to ensure that the hot tap is used more than the cold which will maintain flow and therefore heat.

Hello,This valve is pressure only.The model XL is pressure & tempature for a hot water storage/heater tank(has the stick).You need to know male or female thread and the relief # setting of the old one.The info is on the tag on the old one.

Maintaining the lowest acceptable setting and leaving it at that temperature is the most economical way to run the unit. If the space is going to be unoccupied for an extended time (several hours), lower temperatures are okay. This minimizes the need to burst the heat to warm the space quicker. As I understand your unit, it minimizes electrical & gas consumption by basing the need on the temperature difference of actual space temperature and setpoint. The larger the gap, the more power and gas is consumed.
In summary, when the space is occupied, maintain a consistent temperature setting. When the space is unoccupied, maintain a lower setpoint.

Hope this helps. Good Luck. :-)

is there a check valvle on hot water side of heater,a flow restriction of some sort, water pressure to the heater, a filter in the line, line pressure in the heater kicks out burner, when you turn on the water, line pressure drops, heater comes on, when you turn on water in other part of house, heater should not fire, so there has to be a check valve somewhere,in heater?

You are talking about a gas fired system I assume. So no, it's not a big deal. Code now states that you need to vent the gas furnace from the attic or a crawl space and not from the living space. They change this code back and forth all the time. It's nothing to worry about if you do not plan to sell the home.