I spend time on a number of hydronic chat rooms and tech support websites. Like anything on the www, you get a wide variety of people asking questions about sizing, piping and calculating hydronic jobs.
Sadly, the thread or question often involves a botched or compromised system.
Many times, the questioner is looking for a quick answer. However, it doesn’t save time to skip steps. That is especially true, unfortunately, when it comes to answers to mechanical questions. It can be an expensive lesson learned.
As boring and redundant as it may sound, the very first step should always be to determine the actual load, as accurately as possible. The load should include the heat as well as domestic hot water (DHW), if the boiler or heat source supplies both.
Quick and easy
The good news is that, in our tech-heavy society, numbers crunching is quick and easy.
There are a number of load calculation programs available online or at the app store. And most of the radiant and component manufacturers offer programs. Note below are some of the ones I have used or recommended. For a few well-invested bucks you can buy a Hydronic Design program that will perform load calculations and much more. HDS is my go-to.
Yes, the calculation will be a bit of an estimate, not knowing how the building was or will be constructed. Also, changing weather conditions will affect the infiltration number you selected. Homeowner temperature set point may vary over the years. Older folks like me tend to raise the room temperature as we age. But performing a load calculation puts you on firm setting should your job ever be challenged for performance, or lack of.
Some load calculation sheets have assumption reports, which indicate that the data you input is in agreement with the building designer, architect or homeowner. Some designers insist this assumption report be signed by the parties involved to assure all are on the same page. If insulation, windows or construction methods change, that could compromise the design. The assumption report provides documentation to back up your design numbers.
Try out some of the programs, and ask for help with inputs if you need to. And, of course, practice. The load calculation is essential for an original design as well as troubleshooting, or confirming, an “as built” system.
Some installers add an additional step, like a blower door test, to best nail down an infiltration number, and or identify as construction or insulation detail that was compromised. An infiltration test will add hours to the job, but the payback can be huge. Infrared cameras, which are very affordable these days, can also help with identifying problems and troubleshooting under-performers.
BTU or hour meters can be used to confirm installation details and troubleshoot systems. Many of the new boilers have the ability to log some data. The data can be used for fuel usage calculations, or to determine short cycling conditions.
One other very important number is the amount of DHW the customer desires or expects. With small and efficient homes, you may come across almost unbelievably low heat load numbers. Homes with loads in the single digits for BTU/square foot are becoming more common. This could lead to a condition where the correct size boiler or heat source for the heat load may not be adequate for acceptable DHW supply and production. The low-end burner size on gas-fired water heaters is around 35,000 BTU/hour. The required BTU input would be the same for an indirect.
Currently, there are not many choices in boilers below the 50,000 BTU/hour input range. I’d like to see selections with max output around 30,000 to 35,000, with 5:1 turndown to a few thousand BTU/hour.
This heat to DHW imbalance does present a quandary for combi boilers supplying both heat and DHW. Having tried a few different small-end combis, I feel around 110,000 - 120,000 is the low end for generating instantaneous DHW from a boiler equipped with a plate heat exchanger (HX), for example.
A very general rule of thumb is 50,000 BTU/hour per 1 gpm of DHW generation. So, a 110,000 BTU input boiler can supply around 2 gpm of DHW. Some homeowners and families can live with that, others not so well. This is reminder of why a thorough needs assessment is so important. Remember to ask good questions about the needs and wants of the people who will be living or working in the building.
What tends to happen is a combi boiler gets oversized to, say, 140,000 or 199,000 just to supply adequate 3-4 gpm of DHW for a family. A savvy designer and installer will learn the downfalls and experience a boiler that ends up oversized for the heating load, just to accommodate the required DHW. Symptoms include short cycling, excessive component wear, possibly excessive fuel consumption.
So, balance both these load numbers to get the best answer and product selection. There comes a time when separate components for hydronic heat and DHW makes the best sense. Adding a tank-style combi heat source or a buffer tank can also be a solution.
For those who take the time to crunch load numbers, we salute you. For those on the fence or ignoring this first step, explore your options and add this important function to the bid proposal to show your customers you have their home’s best interest at heart. The win is for you, your customer, the manufacturers and distribution chain when you accurately size components to the job.