One of the most important aspects of hot water heat or boiler systems (aka Hydronic or radiant heat) maintenance is understanding the basics of water quality such as air or scale in the system and how it affects or does not affect the system and performance. When you have this knowledge you will be better able to determine which maintenance services are good for your system and which home boiler services are a waste of money. The following are, in my opinion the most important aspects of water quality in a residential hot water system.
Oxygen (aka air). Think of the water quality in your boiler or hot water system like a container of carbonated beverage (i.e. Coke, etc.). When the container is cold and you open it, you’ll get a minor amount of “fizz”. Now what happens when that same container is warm? Or shaken?! See also NASCAR winner podium champagne celebrations. The lesson: Cold water retains more dissolved gas than warm or hot. New water added to the system will have dissolved oxygen in varying amounts. Most boiler feed water is piped from the cold supply of the house rather than the hot (If I have to repair or replace the boiler feed water valves or add a backflow device on this line I will always recommend re-piping to connect it to the hot water supply of the house). Therefore when you add water to the system, you are adding air. Flushing and filling the system? 100% of the water now in the system has dissolved oxygen in it, which will come out of solution over time. Possibly over the course of more than one heating season. That is why you’ll need to bleed the radiators more than once. What is a better solution (pun intended)? Avoid adding new water to your boiler or hot water system! Key point: Old boiler water is good boiler water. We’ll see more on this.
If your water pressure is low, you need to add fresh water, but eventually the system should remain stable or constant. Yes you’ll need to add fresh water to boost pressure and that new water has air dissolved in it, but eventually or over time the amount of water added will be minimal or zero and so will your need to bleed the air out.
What if the pressure drops and you have to add more water? It depends on how much and over what period of time. Bleeding a little air and adding a little water once a season, no big deal. Once a week? You’ve got a problem that should be fixed. One way to get air into an otherwise sealed system? Have a device installed that “automatically” bleeds air out of the system. Two problems with these. One is that they don’t eliminate the possibility of air accumulating at an individual radiator. And two, when they malfunction (e.g. get gummed up with mineral deposits) the allow air to go INTO the system! These are necessary and thus common in commercial steam systems and rare on residential boiler systems. The simple rule is, as any system becomes more complex (more parts) there is a direct increase in the probability of system malfunction (more stuff to break). So not only do you have to pay to have installed it increases your maintenance costs. A guaranteed beneficiary of any system? The person that is paid to install it and maintain it. In my opinion the most effective air removal method is one that is nearly fool proof as it will not break or malfunction; a bleeder key between your thumb and fore finger!
This brings me to my next point. I do not currently understand any reason why a homeowner should care about the ph of the boiler water. Basic chemistry: two chemicals may react with each other. Once a reaction is complete a solution is “neutral”. In general no further reaction can occur if new chemicals are not added to the solution. Do not confuse this with ph as a measure of neutrality. And don’t ask me the ph of boiler water. I don’t know and/or it is irrelevant to a residential boiler in my opinion. Therefor the best practice is to not add water if it is not required. Sometimes for system repair it is necessary to drain the boiler (e.g. replace a drain valve, boiler feed water valve, etc. ). But in my opinion you should never routinely flush the system. This provides no benefits that I am aware of, may only be detrimental to the system and as a result is a waste of money.
Another questionable sales pitch involves scale treatment for a residential boiler. Most commercial boilers (e.g. for a large office building) use steam not water. These systems in turn use a large amount of fresh water as an ongoing part of their day to day operation. So much that they have automated controls for adding water. For this reason water quality, scale buildup, control, removal, etc. are a big deal and a real issues for proper system maintenance of commercial systems. The reason is that part of the chemistry of water is minerals precipitating out of solution (same as air, you heat it up, things come out). First, residential hot water systems are not seeing lots of new water. Second, Minneapolis water has minimal amount of dissolved minerals, that’s why we don’t have softeners. Next, because there is not usually new water being added to the system, whatever minerals in the water have long since been deposited (i.e. stuck) to the walls of the pipes and radiators. Whatever mineral deposits (i.e. “scale”) may occur will only increase if you add new water. The lesson here? First, I believe that any company selling you scale treatment for a residential boiler is providing an unnecessary service in most cases. Repeat after me campers: Old boiler water is good water (oops, that’s still the same lesson)!