Ah, the age-old question of which came first. If you watch “Loudermilk” on Netflix, you know the answer!

I was recently contacted by a confused homeowner who has a failing boiler. Several mechanical contractors provided estimates for its replacement along with an indirect hot water storage tank. The boiler and indirect tank sizes are all over the map and, as you might expect, the homeowner doesn’t have one clue about what to believe or do, because not one of the contractors could justify why they chose the sizing! Had just one of them been able to articulate why they chose the combined boiler/indirect tank sizing, the homeowner would have gladly signed on the dotted line.

What comes first?

The egg

From my viewpoint, it’s one of four approaches for sizing the boiler and indirect hot water tank:

• Do an actual heat loss calculation using a recognized program the AHJ (Authority Having Jurisdiction) will recognize and approve. Best CYA (Cover Your Ass-Sets) if the homeowner tries to dodge paying by claiming the system doesn’t heat properly. During your initial interview, always ask if there are any rooms that don’t heat to their satisfaction. Put their answer down in writing and include that on your proposal.
• Connected load survey: measure all the heat emitters and then determine the Btu output total and don’t forget to add the pick-up factor for all the distribution piping heat losses. You can pretty much rest assured the boiler will be oversized! While modcons somewhat cover this issue because they can reduce their input based on outdoor air temps, the programmed outdoor reset curve, and indoor feedback based upon the difference in supply/return water temperatures, an oversized modco0n uses more energy than necessary.
• The old reliable: match the tag. Forget about any changes in the building envelope since the old boiler was installed 50-years ago, take the Btu rating off the tag and propose one that matches, or exceeds, the existing size.
• Size the boiler to whichever size indirect water heater you feel they need. Flying by the seat of your pants can get your arse kicked! Instead, determine peak load by asking who uses what when and for how long. You can safely bet 85% of the total flow, during winter’s coldest weather, will be from the hot water tank.

The chicken

If the owners really want a separate boiler/storage tank, then there are two basic options:

1. Oversize the boiler to reduce the gallon size of the indirect tank.
2. Oversize the indirect tank so you can install a right-sized boiler.

In my home, the boiler is sized for our heat loss while the indirect hot water tank is oversized because we treat it as a thermal battery. That also gives the modcon a longer run cycle, which enhances operating efficiency and reduces wear and tear. When the kids (now adults with families) come home for visits, everyone can shower at the same time (three showers), clothes can be in the washer and the dishwasher can be on too.

By calculating heat loss and/or gain, you determine the exact heating or cooling capacity required to efficiently maintain indoor comfort. Oversized systems can short cycle (turn on and off frequently), leading to inefficient operation and higher energy costs. Undersized systems will struggle to maintain desired temperatures, leading to discomfort.

You want to eat like a king or queen? It’s so simple even I could do it: simply ask them what the issues are, and they will gladly tell you. And because none of the other contractors bothered to ask, you become a trusted advisor so that when you offer the solutions, price no longer is the primary issue. So go ahead and get you some fine French Coq au vin (money!):

I was surprised none suggested installing a combi-boiler. Here again, does size matter? Sure it does, but muted because of the modulation and high efficiencies. Don’t be afraid to inject new ideas into the customer’s head. They, in this case, are focused on a separate boiler and indirect hot water storage tank. If you present them with an option, like a combi-boiler, then juice that up by promoting the benefits and features a combi unit offers, such as freed up floor space; whisper quiet; perhaps a higher turn-down ratio; better warranty (if true); and any other reason why you believe one product is better than the other for them.

Benefits of doing an actual heat loss calculation

1. Right-sizing the system: By calculating heat loss and/or gain, you determine the exact heating or cooling capacity required to efficiently maintain indoor comfort. Oversized systems can short cycle (turn on and off frequently), leading to inefficient operation and higher energy costs. Undersized systems will struggle to maintain desired temperatures, leading to discomfort.
2. Energy efficiency: Matching the system capacity to the actual heating or cooling needs of the building ensures optimal energy efficiency. A system that is correctly sized operates more efficiently, reducing energy consumption and operating costs over time.
3. Comfort and performance: Properly sized heating and cooling systems provide consistent indoor comfort. They are able to achieve and maintain desired temperatures without fluctuations, drafts, or inadequate cooling/heating.
4. Long-term cost savings: Investing in a system that is appropriately sized based on heat loss/gain calculations can lead to significant long-term savings. Lower energy bills due to improved efficiency and reduced wear and tear on the equipment contribute to overall cost-effectiveness.
5. Electrical consumption: ECM circulators; low-watt zone valves; and intelligent thermostats can offer additional long-term reductions in energy consumption, which allows the customer to keep more of their hard-earned money. If you take the time to educate your customers on the electrical consumption side, you will separate yourself from the competition and win more bids.
6. Building integrity: A heat loss/gain calculation takes into account factors such as insulation, building orientation, windows, and local climate. This ensures that the heating or cooling system integrates well with the building's design and construction, maximizing its effectiveness.

Conducting a heat loss/gain calculation before replacing heating or cooling systems is a proactive step towards ensuring comfort, efficiency and cost-effectiveness in the long term. It helps in selecting the right equipment size and type that best meets the specific needs of the building and its occupants.