Bleeding radiators is quite simple, and can usually be done by homeowners. All that's required is a radiator key, a towel and/or bowl, and a bit of time.
Why Bleed Radiators
Even with closed systems, air can still find its way into the system. When it does, it will collect at the highest points available (the top of radiators). Trapped air can cause radiators to heat less effectively, so removing it will help reduce your heating costs.
How to Bleed Radiators
To bleed the radiator, you'll need a special tool called a "radiator key". A radiator key is a small key like object that will fit on the bleed valve of the radiators, and is used to open/closed the valve.
The first step in bleeding the radiators, is to turn off the system. Next, you'll have to locate the bleed valve on each radiator. This valve should be near the top of the radiator, and will look something like this.
Insert the key into the valve, and hold the towel and/or bowl below the valve. Slowly turn the key anticlockwise, until you hear a hissing noise or water starts to drip from the valve. Once water starts to drip, close the valve by turning the key clockwise.
WARNING: Never open the bleed valve too fast, too far, or completely remove the valve stem.
CAUTION: Water dripping from bleed valve may be hot. Contact with aforementioned water could lead to injury.
Repeat this procedure for each radiator in the system.
Repressurize the System
After bleeding the radiators (or even before bleeding the radiators), you may notice the pressure in the system is too low. The typical rule of thumb is that for a 2 story home, the pressure should be between 12-15 lbs. or 25 - 30 ft. If you notice the pressure is a bit low, you'll want to top the system off.
There should be a supply pipe connected to the boiler, with a valve that is fully closed. This pipe should come from the cold water supply system of the house. Once you've located the supply and valve. Open the valve slowly, keeping an eye on the gauges, close the valve again when the gauge reaches the optimal level.
What is the pressure on your boiler's pressure gauge (often a combined temperature/pressure gauge)?
Several things could be wrong - replacing parts willy-nilly gets expensive.
If the pressure is below the relief valve setting, the relief valve may be faulty. If the pressure is at or above the relief valve setting, the pressure reducer may be faulty. If the temperature is too high, the relief valve may be a combined temperature/pressure relief which is actuating properly due to temperature extremes. The reason for any such temperature extremes should be corrected.
If you don't really have a solid understanding of what you are doing, a licensed boiler service technician can be well worth the cost of the service call.
Best Answer
As Steven mentioned how do you know you've fixed it? Also, do you want to circulate sealing compound through your boiler?And damp under a house can cause a lot of problems. If you want to know the exact location of the leak and under the floor is totally inaccessible you could consider using a thermal imaging contractor to find the leak to localize damage to the flooring and [this site] (http://www.ekoplastik.com/?download=ekoplastik_ppr_mont_en.pdf) has information on repairing of PP-R pipe in page 27.