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g/hvac • Posted by u/boiler_bill •

Boiler short cycling after a "high efficiency" control upgrade — outdoor reset done wrong Diagnostics

Small commercial, two 400 MBH condensing boilers in lead-lag. Owner had a controls contractor add outdoor reset last fall. Since then the boilers cycle roughly every four minutes at mild outdoor temps and the gas bill went up, not down.

Reset curve as programmed: 180°F supply at 0°F outdoor, 140°F supply at 60°F outdoor.

Boilers are condensing, so lower supply temp should mean better efficiency, and the reset curve looks reasonable on paper.

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u/admin_jake ADMIN •
✓ Accepted Solution

Widen the differential and fix the lag staging, in that order.

At 4°F differential with a 140°F setpoint you are asking for a cycle every time somebody opens a door. Take it to 10 or 12°F. The building will not notice — the loop mass smooths it out entirely — and your cycle time triples.

Then check the lag boiler enable. If the controls contractor left lag staging on a fixed setpoint while lead runs on reset, the lag boiler is firing on a condition that no longer exists. On a mild day you want one boiler at minimum fire and the other one off and staying off.

Also confirm the return water temp is actually below 130°F. Condensing boilers only condense on the return side. If the loop is primary-secondary with a bypass that was sized for the old non-condensing setpoints, your return could still be hot enough that you are not condensing at all, which would explain the gas bill going the wrong direction.

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✓ Solution
u/boiler_bill OP •

Return is running 148°F. There is a bypass from the old system still in place. So we spent the owner's money to lower the supply temperature and then mixed it right back up before it hit the boiler.

Not condensing. Has not been condensing since the upgrade.

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u/freon_frank •

And that is the whole gas bill. A condensing boiler running non-condensing is just an expensive 84% boiler. The controls upgrade was sold on efficiency that the piping made impossible.

Close the bypass, verify return stays under 130 at design, and the numbers will come back.

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u/bas_brianna •

Worth adding a return water temp trend to the BAS graphics while you are in there. Nobody was watching the one number that determines whether the equipment does what it was bought to do.

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u/bas_brianna •

Reset curve is fine. The problem is what happens to boiler firing rate when supply temp drops and nobody adjusted the minimum.

At 140°F supply the load is small and the boiler's minimum fire is still bigger than the load. So it satisfies, shuts off, the loop cools four degrees, and it fires again. Four minute cycles.

Outdoor reset without a matching change to the differential and the minimum-fire staging is a half-installed feature.

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u/boiler_bill OP •

Differential is set at 4°F, which was the factory default and nobody touched it. What should it be with reset in play?

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