Gas and Oil Home Heating Furnaces - Can't bleed radiators

I have a boiler system with 5 radiators on the main floor and one on the second. This system is in a old house with VERRY low water pressure. there is a backflow valve on the fill line but i cant seem to get any fresh water into the system to bleed the radiators. the backflow valve drain tube drips slowly when both valves are on (there is a shut off on both sides of the valve) but not when the one on the supply side is closed. should i try taking out the backflowvalve and bleeding or... ?

1) Do you have a pressure gauge on the boiler?
2) As an alternative to taking the Back flow valve out, Can you put a hose from a hose tap onto the boiler drain And boogee it that way?
3) You will need to presurise the system enough to raise the level of the water above the highest radiator.
If you have an alitude gauge (marked in Ft) measure the vertical height and fill the system to that measure plus 4ft minimum .If you have p.s.i. gauge allow 2.3 ft for each ft above the gauge level Plus about 2-3 psi minimum.
4) Are you losing water from a leak?
If the pressure rises a lot when the boiler heats up the expansion tank needs attention.
However get the pressure up and come back.
One step at a time.

Yes I have a pressure gauge/temp gauge. there isnt enough water in the system to build any pressure.

I could hookup a hose to the drain, But how would I pressureize this house has Very low water pressure, could i rent a pump?

AS for leaks, there are none someone tried to bleed radiators last year and didn't now what they were doing.

It is very doubtful that your house pressure is below what you need in the heating system. Use a washing machine hose or a double female hose adapter and do like Ed says. You should probably get it fixed right at some point and eliminate the headaches.

At most big box stores (Home Depot, etc) you can buy a pressure guage that screws on to a hose bib (outside faucet), with that you can tell how much pressure you have in your home water system.

1 psi of pressure will push water up 2 feet, or 15 psi will push water up about 30 feet. So, if you measure the pressure, and it is above 15 psi, you should be fine for filling the boiler thru the drain, enough to get water to a second floor radiator.

Is there a pressure control valve in the feed supply to the boiler? If so, you can tighten the screw/stem on the top of the valve and increase the water supply to the boiler.

Some of the old backflow preventers have a basket strainer in them, that often rusts, and prevents water flow to the boiler. There should be a removable plug on the bottom to remove the basket strainer. If you remove it , and it is totally plugged, just take it out for now, until you get water to the boiler and the heating system full of water. It appears you have valves on both sides, so you can valve it off and replace the basket later.

Hope some part of this help, if not, please post back.

If you have p.s.i. gauge allow 2.3 ft for each ft

OOPPs, What I meant was that 2.3 ft for each 1 lb pressure on the gauge.
ie. 2.3 ft of water = 1 psi.

So I took out the backflow preventer and out of curiosity I turned the water on... Dribble, dribble. No water.
Hokked up a hose to the system drain, open the valve and hear water rush int the system. bleed the radiators no problem. 20-25 psi. Should the expansion tank be full or should it be emptied ? before or after bleeding? Thanks

You do not want the expansion tank full, because when water is heated it expands, and if it is full, it cannot expand without rupturing the tank.

Nor, should it be totally empty. Make sure you have water to all radiators (coming out of the bleed valves) and a little water in the expansion tank, you should be good to go. You can tap the side of the tank to determine water level.

There is no need to worry about rupturing the expansion tank. As long as your system has a safety valve (which all do) the worst that will happen is that water will be discharged from the safety valve outlet. The only way to reset the water level in a conventional expansion tank is to drain it COMPLETELY. Isolate it from the system and drain it completely. That usually requires finding a way to get air into the tank to replace the water that is draining. There may be a small air vent on the fitting in the tank that you can open to admit air to the tank. Once it is empty, close the air vent if you had one and close the drain valve and open the isolation valve to allow water from the system back into the tank. All should be well after that.

To determine the amount of water in the tank, it is usually more accurate to lift up on the tank slightly to gauge its weight as tapping on a tank full or empty sounds very similar.

Ken

Water expands at 1/22 times it's volume from cold to hot so for example If you had 22 gals of water in your system when cold you would have 23 galls when hot.
This expansion has to go some were so thats why a method of taking up the expansion is fitted, ie a open feed tank or more commonly now days an expansion tank.
This should be empty when there is NO pressure in the system and then as the system fills it will push up into the tank compressing the air then as the system heats up it will push up more to take up the expansion.
Now provided the tank is not too small the pressure in the system should not rise more than say 5 psi at the most.
Most safetys will blow at 3 bar (45psi)
But if you calculate the head to get the water to the highest rad allowing 2.3 ft = 1psi and add say 5 psi that should be the most pressure your system will need.
little point in running the system with more pressure than needed.
If you have a bladder tank then the pressure in the tank should = the cold pressure in the system. You can measure this with a tyre gauge on the gauge nipple on the tank when the sytem is empty.

Our safety relief valves open at 30 psi. They will usually start to drip around 28 and open fully at 30. Just offering this to prevent a misunderstanding about what pressure to run. You can probably run with 20 psi hot or cold.

We use the European equipment and all the safetys blow at 3 Bar (45psi). But as I said you don't need any more pressure than necessary just enough to get the water above the top rads with a bit to spare.