In this Weston home, a single aging boiler was responsible for two jobs at once. It heated the house and it heated a storage tank for all the hot water. When one piece of equipment does both, a long shower or a busy morning can leave the rest of the house waiting.
Pespisa rebuilt the mechanical room around a simpler idea: let each system do one job. A high-efficiency condensing boiler now handles the heat, and a separate pair of tankless units handles the hot water. Both run better for not having to share.
The Challenge: One aging boiler doing two jobs
The home’s heat and its hot water both ran off the same aging setup:
- An older Buderus GB142 condensing boiler handled space heating across the home’s zones.
- Hot water came from a SuperStor indirect water heater, a storage tank that the same boiler kept warm.
- Because the heat and the hot water shared one heat source and one tank, they competed. Back-to-back hot water use could outrun what the tank was able to reheat.
The boiler was getting on in years, and the hot water was capped by the size of the tank. The homeowner wanted something more dependable, and hot water that wouldn’t run out or make them wait for the tank to catch up.
Our Solution: A dedicated condensing boiler for heat, twin tankless for hot water
We put the heat and the hot water on two separate systems, then piped and wired the mechanical room so it would be easy to live with and easy to service.
Heating: one high-efficiency wall-mount boiler
- Lochinvar Knight KHB199N wall-mounted condensing gas boiler
- 95% AFUE, 199,000 BTU input, natural gas
- Hung on the wall to keep the floor clear and the unit easy to reach for service
Hot water: two tankless units, twinned
- Two Rinnai RX199IN condensing tankless water heaters, twinned on a common manifold
- A hot water recirculation loop so hot water shows up at the fixtures without a long wait
- Twinning gives the home plenty of capacity for simultaneous use, and a backup. If one unit ever needs service, the other keeps the hot water running
The parts you don’t see, done right
- High-efficiency ECM circulators on the heating loops
- A Spirovent air separator and a new expansion tank for a quiet, well-behaved system
- A new Caleffi fill valve with backflow preventer for code-compliant make-up water
- Isolation valves and a purge set-up on every zone, so future service is quick and clean
- New vent and intake piping, plus a condensate disposal system with neutralizer for the tankless units
Controls: An outdoor reset sensor lets the boiler adjust its water temperature to the weather. On milder days it runs cooler, which is easier on the fuel bill and more comfortable.
Backup heat: The old Buderus boiler and SuperStor indirect tank were removed and hauled away as part of the job.
Why This Design Was the Right Fit
Swapping the boiler one-for-one would have left the same problem in place. The heat and the hot water would still share one heat source, and the hot water would still be limited by how fast a tank can reheat. Splitting them up is what actually fixed it.
Hanging the boiler on the wall gave back floor space in the mechanical room and made everything easier to get at later.
A dedicated boiler for heat is sized and tuned for heating alone, with nothing else drawing on it.
Two tankless units for hot water give continuous, on-demand hot water with no tank to empty and no wait to reheat, plus the cushion of a second unit.
A recirculation loop means hot water is ready when someone opens the tap, instead of running the faucet until it warms up.
The Result:
The home now runs on two systems that each do one job well: efficient heat that responds to the weather, and hot water that doesn’t run out.
Highlights:
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- 95% AFUE high-efficiency condensing heat
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- Continuous, on-demand hot water from two tankless units, with no tank and no reheat wait
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- A built-in backup, since the second tankless unit covers hot water during any future service
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- Hot water that arrives faster at the tap, thanks to the recirculation loop
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- A cleaner mechanical room that’s set up for easy service, with isolation and purge valves on every zone
Equipment Used for This High-Efficiency Boiler + Twin Tankless Water Heater Installation, Weston, MA
| Project | Boiler replacement plus twin tankless hot water, Weston, MA |
| Boiler | Lochinvar Knight KHB199N, wall-mount condensing |
| Boiler efficiency | 95% AFUE, 199,000 BTU input, natural gas |
| Water heaters | 2 x Rinnai RX199IN condensing tankless, twinned |
| Hot water extras | Recirculation loop; condensate neutralizer |
| Circulation | ECM circulators, Spirovent air separator, new expansion tank |
| Controls | Outdoor reset sensor |
| Replaced | Buderus GB142 boiler and SuperStor indirect tank |
| Completed | April 2026 |
Is one tired boiler running your whole house?
If your heat and your hot water both lean on an aging boiler, or you’re just tired of running out of hot water, Pespisa can design a system that splits the load and takes care of the bottleneck. We’ll look at your mechanical room, walk you through the options, and build around how your home actually uses heat and hot water.