We have a Crown gas boiler. The temperature in the control box was changed from 120F to 175F. The radiator gets too hot, makes the floor vibrate, and makes banging noises. Is this normal?

must be your control slide stuck on internally,probably come away from the slide.
replace control box,its an easy fix..in the meantime,turn down the room thermostat to zero,and turn off/down your boiler temperature just for the radiators..your heating should then stay off till repaired.

It sounds like your thermostat could be the problem. Have you noticed the boiler cutting off at all, or does it run continuously until it goes out on low water?

You stated, "when boiler is on", so I am wondering how you get the boiler on. If your boiler is controlled by a thermostat, you need to check it for a cold draft possibly from behind the thermostat in the wall where the wires come through. The thermostat should never be on an outside wall. The thermostat could be defective.

An aquastat can also control your boiler. If you have an aquastat, check the setting, typically 175 - 180 degrees. You should also have a "high limit" which should be set at 210 - 220. Some people run that high side limit higher; not a good idea - defeats the purpose. If the aquastat is set too high, the boiler will operate too hot and the problem you have will occur.

"pressure rises and pressure relief valve opens" This could mean that your pressure switch is prematurely venting. Check boiler temperature to see exactly at what temperature the T&P valve purges. If it blows off below the high side setting, either adjust or replace. The T&P valves are usually not adjustable. That blow off temperature is stamped on the head of the valve. So, if it discharges too soon, replace it.

Have you actually seen evidence thgat the T&P valve is opening? You may have a bad bladder in your expansion tank and your system may have air in it. Your boiler should have 12 psi when cold and 20 - 22 psi when operating. Check your boiler pressure guage.

"this eventually drops below the minimum operating pressure" Technically, there is set point for a NO MINIMUM OPERATING PRESSURE. That is a function of the amount of water in the boiler. If the T&P is blowing off, it will vent water from the boiler and the boiler pressure will go down. There is a minimum operating TEMPERATURE.

Your boiler could be low on water. You will have to manually fill the boiler. Be careful of thermal shock to the boiler. Feel the piping, and fill only when the circulator is running. Make sure the water is not going directly into the boiler. The supply water should go into the circulating supply side which feeds the radiators, radiant floor, or base board circuits.

All this will adversely affect your hot water. The heat exchanger for the potable hot water may have scale build up in it. Using the hot water keeps the boiler running, and a scaled heat exchanger will give you cooler hot water and the boiler keeps running. If this is a problem, you will need to replace the heat exchanger. A better idea is to not use the boiler for your domestic hot water. Why run the boiler in the summer?

You don't have an optimum distribution of steam in your house. Try closing a little bit the valves supplying the heaters in the lower floors. Close a little bit more until you get the desired balance.

There should be a control called an aquastat that regulates the temperature of the boiler. The thermostat will turn on the circulator to start the flow of water through the radiators. If the water coming back from the radiators is very cold, a control will shut off the circulator until the boiler can come up to a predetermined temperature then start the flow of water again. If you have a tankless coil for domestic hot water, you now have a triple aquastat that maintains the temperature for domestic as well as the boiler temp. So yes the boiler thermostat (aquastat) does control the heat of the radiators and domestic hot water. No the pump is regulated by your house thermostat. You should take an amp draw on the boiler and componants to find out what is drawing more electricity than it should and popping your fuse.

Hi the cylinderstat controls the hot water only not the heating the boiler stat controls the water temp the room theromstat controls the rads , if you have one ,

so boiler stat set on 180f or number 5 cylindersat set on 140f to 120f roomstat to your comfort , the boiler stat works as a over heat so if the cylinder stat or roomsat fail it will turn off on the boiler you should beable to check wiring from that .

gas pressure may be increased but most boilers are designed to a limited output 24 28 30 etc

you could have a flow problem due to sludge or scale in the boiler, it could be a pump problem as it may not be circulating enough water around the system. or it could be a faulty thermostat if it is not cutting out soon enough.
1st try turning the boiler stat down to its minimum, if it still over heats then it is probably the stat.
if you have a switch on your pump to increase the speed make sure it is on setting 3. if the pump is old then it may be time for a replacement.

the boiler should modulate to controll the radiator temperature but the central heating will go off whilst you are drawing hot water

If you start by looking on your gas valve, you should find the correct amperage to set the heat anticipator at. I am not sure if you have a forced hot water system or a steam system, but depending on the case, you may also have a second problem, which is easily addressed. If you have forced hot water, the upstairs baseboard/radiators may be airbound and need to be bleed. If you have steam, you may need to slow down the downstairs valves, and open the upstairs valves to get a balance. Hope this helps