Boiler Efficiency and Understanding Load/Run Times


Old 11-02-20, 04:15 PM
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Join Date: Feb 2019
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Boiler Efficiency and Understanding Load/Run Times

The specs for my system (some frequenters around here probably know my system by heart by now):

- Weil Mclain WTGO-4 w/ tankless coil for DHW.

- Fuel: converted to natural gas in 2018

- Four Zones: (1) Hydroair. Heats all of upstairs but has several vents into downstairs rooms (2) 50ft or so of baseboard convectors - covers rooms downstairs - this is supplemented by zone 1 (3) two kickspace convectors ~10k BTU each, added after garage was converted into a kitchen this is also supplement by zone 1 (4) hydroair. covers a newer extension that is one 300 sqft room

-Aquastat: Hydrolevel 3250 set at Low is 160 - High is180F with a diff of 20F

If all zone simultaneously call for heat, the boiler actually balances output quite well and will run for a long cycle (my initial tests suggest it will burn throughout and meet demand as long as all zones are calling). However, if three or fewer zones are calling, the boiler will quickly reach high temp target and shutdown. Worst case, if only one zone calls, burns can be as short as 3 minutes followed by refires 5-10 minutes after.

1) Is the scenario describe above concerning? If so, what are my options to alleviate the issue short of a full redesign?

2) To lesson the chance of short cycles, I use pre-purge on my aquastat. This uses latent boiler heat to try to satisfy a call before firing. In the least, it still lowers my boiler temp longer before starting a burn, making the burn go longer. I also stopped using the aquastats eco features since it was frequently using a 10F high differential (and its not so useful when using a tankless coil). Instead, I felt it was better to manually set a greater "high differential" to create longer burns then let the aquastat do short burns to target lower temperatures. I figure this will result in fewer, long burns instead of more frequent short burns. Does this make sense? I appreciate any feedback and suggestions.

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