I have an Astral HX120 Spa heater without any markings to state which is the inlet / outlet water connections. I'm assuming looking at the back of the unit, the left is the inlet. Is this correct?

possible bad temp sensor if using a controller make sure heater all the way up and let controller dictate temp

Hayward H series with ED option have electronic ignition with one or two dial thermostats. Along with thermostats, there are other temperature safety switches to shutdown the heater in case of over temperature, both for water temperature and for interior cabinet temperature.

CAUTION: Always turn off power, gas and the water pump whenever repairing your Hayward H series pool heater.

CAUTION: The Hayward H series pool heater is essentially a gas furnace. Only a qualified technician with proper training and tools should make repairs of your heater.

I'm assuming you are seeking the location for the thermostat sensor rather than the temperature switches, and I'm assuming you have dial-style thermostat(s). Incoming water temperature is measured at the inlet of the water heat exchanger. For most H series models, provision is made for two thermowells. If your unit has dual thermostats, both of these wells will have a bulb-style sensor inserted directly into the thermowells. If only one thermostat, then one of the thermowell ports will be sealed with a plug. The factory orientation of the water inlet and outlet is on the right side of the heater (as you face the front), but the installer has the option to locate the inlet/outlet on the left side with a field modification. Check your particular inlet/outlet orientation for either right or left side.

The heat exchanger thermowells and the thermostat sensors are located directly behind a two metal access plates that surrounds the heat exchanger inlet and outlet ports. Several Phillips-style screws secure the access plates to the heater cabinet. After several years of outside operation, these screws will likely be corroded and need some anti-rust oil to aid in their removal. When you have completed your examination of the thermowells and thermostat bulbs, I recommend applying some grease to the screws before re-assembly.

Once the access plates have been removed, the thermo-wells can be seen on the left side of the heat exchanger, which is the inlet side. Use the arrows on the top of the heat exchanger as a guide for inlet an outlet. The thermostat sensor bulb is directly inserted into the thermowell without any securing mechanism other than a copper or brass friction strip. I have seen some situations where a small amount of fiberglass has been inserted behind the bulb. The bulb should be inserted all the way into the thermowell until it touches the back. The copper or brass strip is used to keep the bulb in contact with the sides of the thermowell.

(For non-dial thermostat systems, the inlet water temperature measurement is made in the same location, but with a different device.)

CAUTION: The thermostat sensor is a liquid-filled thermo-expansion sensor. It relies on the thermo-expansion properties of a non-compressible fluid to operate the thermostat switch. Take care to not crush or kink the capillary tube between the sensor bulb and the dial thermostat switch.

If you remove the sensor bulb, take care to capture the copper/brass strip in order to use during re-insertion.

NOTE: Thermowells are sized to the particular thermostat sensor bulb. If you replace the thermostat sensor be sure to use the same size sensor bulb as the one being removed. If this is not possible, the thermowell may also need to be replaced.

Check your manual for verification of part numbers. The H series dial thermostat part number is CHXTSW1931. The matching thermowell is CHXTBW1930.

Cold inlet and hot outlet are marked on top of water heater.
Usually there is a letter or word embossed into the metal top.

If tank is connected, then turn on hot water tap for a few moment and feel which pipe is hot.

Cold water enters bottom of tank through dip tube located on cold side. If hot and cold are reversed, the water heater will not perform as expected.
http://waterheatertimer.org/Water-heater-dip-tube.html

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

are there shut off vavles you may have overlooked after the the filterpump?

They are soldered on with a propane torch kit..

My water heater has three water connections on the top of it one has a C for cold is that a inlet or outlet ? IT'S THE COLD INLET. The other spot has a H for hot is that a inlet or outlet ? IT'S THE HOT OUTLET. And the other one is not marked what is that one for and is it a inlet or outlet ? please email me at [email protected]. The last one is a pressure release valve that lets off excessive pressure when the temperature goes too high in the unit. When you install the heater, it should be installed into the drain pan that existed before or into a new one. You should also install a drain pipe from the valve to that drain pan, or into the drainage system for the house. This may depend on the codes where you live. Also install a good quality shut off valve on the inlet side for emergencies. A Ball type is good. Hope this helps,
Best Mark
I'm new to this site, so I'm not sure if I can email you directly or not. I believe not. Good Luck.

THERE IS NO HEATER CONTROL VALVE ON THE WATER HOSES. NEW HEATER CORE. INLET LINE right before heater core hot, outlet after core look warm. Burped cooling system 5 times with engine at operating temp. Installed homade burp VALVE right after heater core outlet(high part of hose). Still no heat. Removed water pump, impeller still in good shape not stripped off shaft. Removed heater core hoses flushed water thrue the connections on the block to the water pump INLET and outlet. flushed water thrue heater core and hoses. Going to put it back togeter today. Used known good gauge to check water temp for operating temp in test. 195 f

I assume this is on a swimming pool. It might depend on how it is plumbed in. Quite often, there is a bypass valve allowing the pool water to bypass the heater incase it's being serviced, or a leak etc. You would close inlet and outlet valves to heater and open the bypass. This would isolate the heater. If you have the bypass open while heating, you will reduce (or eliminate) the flow through the heater. This will allow the water in the heater to heat very fast, and lets be glad there's a temp control or your plumbing would melt. These heaters should have full flow through the heater, and you'll see min temp rise per pass.

Water flo through the heater is too fast and the water can't get hot enough. Your by-pass valve is stuck closed and you need to open your water header and check it out. Or you may have an external by pass that need's to be opened 10-20%. This will be a valve between the inlet and outlet water pipes going to the heater. Good luck and HAPPY NEW YEAR.

This heater has a thermal regulator and a high limit that could be the problems. If it were a high limit that was bad, the display would show the error when the heater goes off. In the absence of an error, one would look to the regulator. A bad regulator would cause the water flow to bypass the exchanger and cause a slow heat. The part is easily replaced and runs about $90. It sort of looks like a car thermostat and also has a spring. It is accessed by removing the large black plug near the plumbing inlet/outlet.