Check the heating elements in the water heater. These may be bad and need replaced.
air lock in hot water pipe
It is a device that opens on its own if a certain condition is reached. Let's say you are designing a water heater. You know if everything goes wrong the unit could reach 210 degrees F, or to have 150psi pressure inside it. You also know if either of those things happen the water heater could explode...so you put a safety valve on it that opens if either condition is reached.
Sounds like the trap in the sink is getting the water pulled out of it. It probably wasn't vented properly.
As water drains, it displaces air in the pipes. If the vent or vents in your house are clogged up, the air must find another way out, which could be another drain in your house.
One of the valves that you closed while hooking up the hot water to the sink was probably an old gate valve... It broke and did not fully re-open.
Is it water or coolant? If it's water, you probably have a leaky windshield seal. If it's coolant you could have a heater control valve leak.
Could be a plugged heater core. Check the engine temp guage. if it is getting warm, your heater core could be plugged or restricted.
If you are not getting heat out of the heater, there are only a few things that could be wrong. The first is low coolant. You can't get heat without coolant running through the heater. If it is low, sometimes it has to be bled of air bubbles. If both heater hose running to the heater are hot, you have circulation. If not the heater could be plugged or not getting the coolant to circulate. There is also a damper in the heater system to stop cold air from bypassing the heater and that needs to be checked.
water pump could be going out, could be low on coolant (leak in system), a stuck open thermostat, a plugged up heater core, temperature blend door may be faulty,
check your thermostat or your heater core may be getting ready to go out
Blown intake manifold gasket or blown head gasket, figure on $600 to $650 to get intake gasket fixed and if a head gasket your looking at least $1,000.
You haven't said what's wrong with it.
the heater core could be stopped up, or could be low of antifreeze, or the thermostat could be stuck open
Low coolant? Wrong thermostat temperature range? Heater core plugged,restricted or airbound? Water pump not circulating coolant? Temperature control cable not adjusted properly?
The cutoff valve from the hot water heater may have been closed.
figured it out, heater coil blew out bypassed and yea don't need heater on Guam neyways
could be thermostat or heater fan if you can hear all the speeds on the blower motor working then it could be your thermostat but your heater core might be plugged as well