My 10 year old 80 gallon amtrol hot water exchange unit has an internal leak between the heating coils and the hot water system. anyway to fix or just replace?

is it an electric or gas or oil fired. Er3 means it is not getting up to temperature within the required time. Electrical - element bad. for oil or gas, no ignition or fuel supply.

A stuck divert-er valve yes replace head, and know this will not cause leaks leaks,leaks are caused by o seals worn and getting old ..and olives on connections replace the above as needed some times a quick tighten with rench will do it leaks that is

Hiliary
sounds like the primary coil in side is cracked and is mixing hot water.. with the cold water in tank need to check and replace as necessary

There is an inlet screen on demand water heaters. Look for that and clean it. The tubes in the heat exchanger could be clogged. The manual will tell how to clean it. This is very involved and uses acid. It can be dangerous, so I recommend you have it done.

HI sorry its shot, they go every now &then.You can run it like you say temporary BUT the boiler needs low pressure to force heat @10 or 12 p.s.i. This pressure will leak out of the hole and you eventaully loose heat & get noise.If you got a pinhole leak in tank.With the main off.Slow it down with duct tape or a rag if you can and leave water on.

Hello 23+yrs is execptional service! Yes it is leaking into the boiler.Time to shop for a new indirect hot water heater.Forget a coil.Thanks

From what you are saying it sounds like an issue with either a circulator pump or a zone valve. It depends on what type of system you have ... if you have a circulator pump that only runs the Amtrol then that is probably bad. If you have a zone valve system then it is probably the zone valve if you still have heat for the rest of the house. If you have no space heating then it is probably the circulator pump.

Er3 means that the water is being heated for more than 1 hour without reaching the set temperature.
Need new aquastat.

Since you mentioned 'both temperatures' it seems your unit is like most all of them, a lower and upper coil.
If your hot water supply runs out quickly, this normally means that the lower coil has failed. Since heat rises, the upper coil will never supply a tankful of hot water, just hot to start, and then as the flow continues, it doesn't have time to do more than warm the water flowing into the tank.
Every home should now own a simple digital multimeter; they cost less than $20 US everywhere and one can find them in auto parts stores, electronic supply companies and tool companies.
If you have a Harbor Freight tool store in your area, you can get them for under $10 US any time and sometimes even less:
Digital meter   
You need to be absolutely certain that the voltage to the water heater is shut down. Before you measure anything else, set the meter to its highest AC setting and check across the coils and then from either contact to the metal housing. If it is dead everywhere, then set the meter to the lowest Ohms scale (normally 200). The resistance of the coil should be nearly the same reading as when you simply short the meter's probe tips together. If it measures 'OL' then the coil is open and must be replaced.