Draw down boiler? ? ?

[Ray]

We have a boiler/radiator system in our six-unit apartment building. We have been flushing it -- I think it might be called "draw down" -- once a week.

But I get mixed advice on how often we should do this, and how long we should let it flush each time.

Any guidance welcome.

[Edwin Pawlowski]

There is only one accurate method. Get a meter that measure Total Dissolved Solids and do a test. A meter is in the $275+ range. The one I use is on the bottom of page 590 at

Number 1128T16

Two things you want to do. One is to flush the bottom of the boiler to remove sediment. The second is to maintain a level of dissolved solids that is less prone to sludging, yet is not loaded with oxygen.

When blowing down a steam boiler, the general consensus is to open the valve three times. The first flush gets rid of what is near the valve, but if you just let it run, much of the deposited solids just stay in place. Closing the valve quickly stirs up some of the sludge so it can be sucked out.

Some boilers require chemical treatment to keep them in good condition. It is not possible to give a guideline on how often as that will vary depending on operating time, the condition of the water supply (hard, soft,, types of minerals, etc) Tb be 100% certain, you need a boiler chemical guy that can do the test and give you guidelines.

[Ray]

Thanks -- that's very helpful.

[beecrofter]

The idea is so that you don't sludge up the system and/or stop the low water cutout from shutting things down if water level drops. On a boiler that size I bet you would pull about 2 gallons for a decent blowdown. During the heating season if it was my boiler I would look twice a week and put an x on the calandar so it doesn't get forgotten.

[Clot]

Living the other side of The Pond and reading this site quite frequently, I;m under the impression that central heating systems on the Left Side are usually different to those we use.

Do you mean this sort of system powered by gas, oil or solid fuel - coal/ lumber?

If so, then frequent bleeding could assist rusting! It would draw in fresh oxygenated water. What is common for such a system here is to add an inhibitor to a new system to prevent corrosion and sludge build up and then drain every few years. We can purchase fluids here to help flush the system for this process, then wash through with fresh water, then refill adding inhibitor, (the cost of the process from recollection is circa $40, excluding my time of about two to three hours).

My current system that I have had from new in a new build house has been up for almost 25 years and I've flushed it just 3 times in that time - if my memory serves me well! No leaks, no problems apart from assisting the circulatory pump to wake up in The Fall some years.

[ransley]

I have a 60 yr old steam system, flushing it weekly meaning a few gallons, is fine, oxygen boils out almost 100 % on the next cycle

[Clot]

Ah! Difference in language across the water, I guess. Not familiar with a steam system. does this circulate through the rads?

[Edwin Pawlowski]

That is the beauty of steam. Water needs a pump to push it and if you have a large building, you need a large pump with high pressure. With steam, the pressure of the steam will move it up and the condensate is either gravity returned or pushed by the steam pressure. Some have a single pipe that handles both steam and condensate.

Steam under pressure carries a lot of energy with it and it can go long distances with minimal loss.

[ransley]

If its a hot water, not steam system, you dont flush it or you bring in oxygen.

[Clot]

Thanks for the info. Yes, I can appreciate those virtues ( and can think of possible negatives!).