Do the Math, Trust the Math
March 1, 2016 | By Steve Goldie
What is the down side of oversizing equipment?
After over 30 years in this industry I would love to be able to state that I have seen it all and that nothing surprises me anymore. As a matter of fact “nothing surprises me anymore” is a phrase I likely repeat several times a week in the office. However, as a good friend likes to remind me, “Just because you say it, doesn’t make it so” and sadly in this case that is true.
The truth is that I am continually surprised by the creative and ingenious ways people come up with to get things wrong. My favourite example is where a contractor, upon realizing there was no convenient drain nearby, decided to get rid of the condensate from an AC unit by placing an electric hotplate under the drain. That one certainly gets the nod for creativity and hopefully it will remain a one-of-a- kind work of art. Unfortunately, many of the troublesome issues that I see are repeated far too often.
Considering that I spend a lot of my workday troubleshooting, I should be grateful for the missteps that keep me employed. I wish I could say that the majority of the issues I see are the work of unlicensed contractors or moonlighting off duty firefighters, but sadly this is not the case. The hotplate guy was a licensed contractor with many years of experience. The majority of the mistakes I see are the work of licensed tradespeople who really should know better. The truth is, most contractors probably do know better and sometimes we need gentle reminders. So let’s have a look at a few of the most commonly repeated errors in hydronics.
BIGGER IS NOT BETTER
Everyone in our industry seems to agree and acknowledge that bigger is not better. And yet, barely a day goes by where I do not see a job or a design with seriously oversized boilers. I understand why it happens, nobody wants to be caught short in anything and we certainly do not want to get a call on the coldest night of the year from a frozen customer. I am not advocating cutting the sizing so tight that this is ever a real possibility. I am advocating that you do, or have someone do, a proper heat loss calculation, and having done it, trust it. Keep in mind that whatever heat loss calculation or program employed will already have a built-in buffer or margin for error so there is no need to take the final size and bump it up “just to be safe.” Do the math, trust the math and size your equipment accordingly.
What is the down side of oversizing? A boiler plant that is just too big will short cycle resulting in lower than expected efficiency and premature failure of components. Modulation helps but there are still limits, especially when the boiler plant is upsized by 50 per cent or more. I regularly see boiler plants that are 100 to 200 per cent oversized and have seen oversizing by as much as 400 per cent.
In custom residential homes, sizing can be complicated by large snowmelt loads or multi-head shower systems that use as much hot water as a car wash. Still, there is no excuse for not doing the math and sizing accordingly. If the snowmelt and/or DHW load is significantly higher than the home’s heat load, then a single large boiler is not the best solution. You really need to go with a multi-boiler plant (for more on this, see Mike Miller’s article on p150) or consider separate boiler plants altogether, each sized appropriately for the application.
If a boiler is sized too big it just makes hot water faster than the heating load can take it away. This problem is often exacerbated by another problem I see far too often; undersized distribution piping. The boiler plant makes the heat, the piping is the roadway for the heat to travel to its ultimate destination, be it a radiator, a heated floor, an indirect water heater and so on.
If the roadway is not sized large enough to handle the traffic there are bottlenecks and things jam up. If the roadway is undersized piping we cannot move the heat where we want it to go fast enough, the back up results in a short cycling boiler that constantly turns on and off on its high limit. When I get a request to visit a job where there is a complaint of not enough heat, the first thing I will look at is the distribution piping rather than the size of the boiler plant. More often than not, the boiler is more than large enough but the distribution piping is inadequate, resulting in a “traffic jam” of heat trying to get to where it is supposed to be. Throw in lots of bends and a bullheaded tee or two and things really slow down.
As with properly sizing a boiler, properly sizing distribution piping comes down to the math. Do the math, trust the math and you should not have any issues. Maximum pipe flow charts are readily available so if you are not sure, check first. Size the pipe based on how many Btus a line is designed to deliver. For example, the maximum flow rate for 1.5″ steel pipe is 24 gallons per minute. With a 20 degree Delta T, one gallon carries 10000 Btus, so the maximum amount of heat you can deliver with 1.5″ pipe at a 20 degree Delta T is 240000 Btus. Do not make the mistake of sizing your pipe based on the connection size of the equipment.
Boilers, heat exchangers and indirect water tanks are three examples of equipment that often have pipe connections that are sized smaller than the maximum Btu load requires. A pinch point at a connection will create a slight pressure drop but will not necessarily reduce their actual flow rate the same way a reduction in the accumulated length of pipe will.
I have lost count of how many times I have been called on to explain why an indirect hot water tank is not producing the hot water recovery published in the manufacturer’s literature. Invariably the reason is that the contractor sized the boiler supply/return piping to match the ¾” connection size on the tank. The ¾” pipe was just not big enough to supply enough flow to deliver the maximum Btus the boiler was producing. Once the piping was changed to 1″ or 1.25″ depending on the boiler size, the hot water production improved significantly.
Bladder and diaphragm style expansion tanks are one piece of equipment where oversizing will not cause negative issues (although oversizing may seem to be the cause), other than costing more than the appropriately-sized model. It should still be sized according to the system it is being installed on. The issue with respect to expansion tanks is that the pre-charge pressure really matters. It should always be verified and adjusted to match the job.
The pre-charge for a residential tank should typically be 12 psi, which not coincidentally matches the cold fill pressure for three storeys or less applications. The pre-charge pressure prevents system water from entering the expansion tank until the water begins expanding due to heating. If the pre-charge is too low, this initial inflow will partially, if not completely reduce the tank’s volume, reducing or completely eliminating the tank’s ability to accept the additional water volume when it is heated. This can make it appear that the expansion tank was undersized, where in reality the lack of pre-charge has effectively rendered it useless.
In applications for buildings higher than three storeys, the pre-charge must be adjusted to match the cold fill pressure required; the higher the building, the higher the pressure. Whatever the application, the important thing is to verify that the pre-charge pressure of the expansion tank matches the cold fill, or cold static pressure of the system. It can save you a lot of time and money, not to mention saving all those perfectly good relief valves that got changed before someone figured out they were just doing their job because the expansion tank could not do its job.
This is not an exhaustive list of the issues we see out there but it does represent a large percentage of troubleshooting calls I deal with. Our entire industry is not rife with troublesome and poor installations; there are a lot of great contractors out there doing excellent, trouble free jobs.
As a matter of fact, if you are reading this magazine you are probably one of those good ones. I bet you actually read the installation manuals of the equipment you install. Perhaps I am a bit jaded because I get all the calls when things go wrong. Nobody seems to call me when everything works perfectly well. Who knows, if we can just get the rest of the installers out there to read the manuals and trust the math, maybe I can start looking for something else to do.
Steve Goldie learned his trade from his father while working as plumber in the family business. After 21 years in the field, he joined the wholesale side of the business in 2002. He is frequently called on to troubleshoot systems and to share his expertise with contractors. He can be reached at firstname.lastname@example.org.