When the heater is on the radiaters the hot water will not come when we turn the hot tap on

SOURCE: No water flow from hot water tap with a rinnai tankless water hea

Its A Possibility The Left Over Water In The Hot Is Frozen. If Your Line Is Exposed Outside Somewhere It Can Easily Freeze Up. Turn On The Hot Water Everywhere In The House And If Accessible Check The Lines To See Where The Warmth Ends. You Can Leave One Off Over Night In Hopes That It Will Eventually Defrost.

SOURCE: no water coming out of carver cascade water heater

is the cold side hooked to the cold side if not you will be feeding the unit backwars making it to where the cold is just comming across the top of the tank, check the dip tube also, this makes the cold water go to the bottom and push the hot out of its outlet,, how many faucets are involved, if just one might want to check the cartridge to make sure it is working properly

SOURCE: no stop tap. cold water coming through cold taps

your pipes are clogged, what if you were to not turn on the cold and turn on the hot does any water come out at all, with just the hot side turned on,, if not there could be a valve that has been turned off, or the pipes are clogged

SOURCE: baxi duotec is heating radiators but not tap

Sludge / furring possible in primary part but not if the ch & flow through hot tap are ok More likely it's the diverter valve. The rubber diaphragm inside is perished / holed. Replace

SOURCE: turned off the hot water

It sounds like two problems.
Problem 1) Clog inside faucet or clog inside pipes at faucet
Remove supply tubes leading to faucet and see if problem is inside pipes or inside faucet.
My bet is clog inside faucet.

If you have galvanized pipe that meets copper pipe, this can be trouble spot.
Article on clogged water pipes at Inspectapedia:
http://www.inspectapedia.com/water/Water_Pipe_Clog_Repair_Guide.htm

Article on clogged pipes:
http://www.waterheaterrescue.com/pages/WHRpages/English/Troubleshooting/Tanklets/backflushing.html

Problem 2) Water heater leak ... you say tank valve is leaking? Is that cold-water shut-off valve? Or tank drain valve? Or TP valve?

If cold water shut-off is leaking, then replace shut off.

If tank drain valve is leaking, then check if plastic valve is cracked. If drain valve is cracked, then replace with a brass valve from Home Depot. If plastic valve breaks off, then use hammer and screwdriver to gently chip out the broken plastic valve. New valve threads need teflon tape to seal pipe. If valve is just dripping, then put garden-hose cap over end of valve and screw down tight.

If TP valve is leaking, then replace with same temperature and pressure rating. New TP valve threads need teflon tape to seal pipe.

Problem 3) If pressure is low all over house:
Buy pressure gauge at hardware store that screws onto hose connection.

Test pressure on outdoor spigot.
Open faucet and check if pressure drops. This says if problem is inside pipes that enter house.

Test pressure on water heater drain valve.
This says if pressure problem is before -or- after water heater.

If problem is before the water heater, then shut-off valve located on cold water line is suspect.
If problem is after water heater, it could be in hot water outflow pipe on top of water heater, or a nearby elbow.
Remove hot water line leading from water heater and check pressure. This says if problem is where hot water leaves tank.
Read articles above about clogs inside water lines.