Trouble-shooting DHW systems, part 3

IN LAST MONTH’S article (pg. 34), we began the art of trouble-shooting domestic hot water system complaints commonly aired by unhappy homeowners. Here is the concluding installment in this three-part article.

“The water temperature starts out just fine, but gradually gets cooler and cooler until there is no more hot water.”

This is an easy one. They’re running out of hot water. The question is why.

Is the storage tank correctly sized in relation to the largest dump load? Is the heat exchanger fouled by calcium and other hard water byproducts? Is the boiler correctly sized in relationship to the actual load being carried? Is the DHW system prioritized? Is the pump that moves water between the boiler and the storage tank correctly sized?

It pays to do your homework on this matter prior to specifying and installing the DHW system. The information you need to properly size the DHW system includes needing to know the largest fixed volume load, which is usually the Olympic-swimming-pool-sized one-man, three-woman hot tub the homeowner calls a jetted tub that is sitting in the middle of the master bathroom.

Forget about trying to deduct the volume of water displaced by the occupants. They’re probably going to want to take a hot shower as soon as they get out of the Jacuzzi anyway.

Also be sure a look at what I call “load diversity.” Ask the homeowner if he has a habit of starting the dishwasher and clothes washer and then jumping into the shower. If he says yes, take this into consideration when you are sizing the DHW system.

Follow the tank manufacturer’s information pertaining to pump and boiler sizing. If the manufacturer messed up, then you have a partner in culpability. If you messed up, you’re all alone, pal.

If someone else installed the system, it will pay to do his homework before you recommend a solution. Don’t forget to pay attention to the manufacturer’s recovery rate for double-walled heat exchanger-type storage tanks. It can be a real Btu killer.

Pay particular attention to the tank manufacturer’s warranty as it relates to the use of other heating system sub components.

As an example, I’ve seen systems where the designer/installer decided to use the less expensive non-oxygen barrier tubing in a radiant floor application to save a few bucks. Even though he had to use more expensive non-ferrous components to make it work, he thought he was saving money on the installation.

In order to comply with the full intent of the non-ferrous sub component issue, he specified a particular stainless steel storage tank thinking this would satisfy the need for non-ferrous components. Think again, Non-Barrier Breath. Certain stainless steel tank manufacturers specifically spell out in their warranties that their warranty is voided by the use of non-oxygen barrier tubing. Take the time to read the installation instructions and the warranty information before you make a decision to go one way or another.

“The water starts out really hot and then goes cold almost immediately.”

This complaint is generally associated with semi-instantaneous water heaters (side arms and immersed coils). It usually indicates that the heat exchanger is fouled with calcium and other hard water constituents that like to come out of suspension and attach themselves to a heated or cooled surface. This may require the cleaning up or replacement of the coil.

At the same time, I recommend that you install some means of pre-treatment or avoidance that will ensure satisfactory operation in the future.

Pre-conditioning can include water softeners and magnetic water conditioners from reliable, responsible manufacturers. Please do not call or write me and tell me that magnetic water conditioners do not work. I have been doing this work for more than 28 years now and I have seen them work all over this wonderful country of ours. Conversely, if improperly sized or used on untreatable water (silica-based calcification, a.k.a. water glass), they will not work correctly, just like an improperly sized or misapplied water softener.

Responsible manufacturers include The Superior Water Conditioning Co. (a division of Chemtune Inc.) and MorFlo American Appliance Co. I’m sure others may be out there, and you can use whichever device you want to; just make sure that they will be here tomorrow to stand behind their product. Also make sure they offer a free water analysis to avoid misapplication.

The key to trouble-shooting any hot water heating system is making sure you are familiar with all aspects of its operation and making sure all the key elements are in place and properly sized to ensure proper operation of the system as a whole.

Keeping your customers in hot water is your business. Keeping you out of hot water is mine.

Happy Domestic Hot Water Hydronicing!!

See you next month.

Mark Eatherton is a Denver-based hydronics contractor. He can be reached via e-mail at [email protected] or by phone at 303/778-7772.