High Limit as sole boiler control- is this a bad thing?

Bucky_2

Hi gang, looking to add full reset via a Tekmar 256 or 260. Investigating my current control- Honeywell L8148A aquastat with Taco SR502 (soon to be SR506) circulator relay. I investigate further, and I see these two devices never 'talk'. The T-T leads on the aquastat relay are tied together with a wire nut. So, the high limit (set at 180) controls the whole show...boiler reaches overtemp, shuts down. Cools down to where the limit switch is OK with firing, and it cranks up again. I gotta think I'm wasting lots of oil just keeping the boiler water at 180. It's a newer ( >1 year install) of a Crown Toboga, done by a heating contractor. If I tie this T-T to the Tekmar for reset, should I add some sort of low-limit device so the boiler water doesn't get TOO cool and startup is problematic? Running DWH, adding a zone for an injection panel serving two radiant zones, adding a zone for secondary heat via hydronic coil in a SpacePak, and the existing zone for the full-sized rads in the main house, all piped P-S (yes, a card-carrying steamfitter is doing all the NBP for me, with design assistance from Dan H (well, his excellent books anyway, but it FEELS like he's talking to me!). And is full reset not a terrible idea in this setup? Thanks.

Ted_4

Similar question

This question has crossed my mind, but for a different reason. Is one high limit really enough for safety? Imagine a situation where the burner relay contacts weld. Even though the aquastat says "enough" the burner keeps running, and... well, you know. Shouldn't we be installing a second line voltage aquastat in series with the burner wiring? Even this isn't fail-safe, but it's alot better than just one control. Or am I just being too cautious here?

Al Letellier

aquastat

Bucky, the TEKMAR control will "talk" to the boiler aquastat. You can program in to the TEKMAR a boiler high limit, low limit and reset. The boiler contacts on the TEKMAR will tie into the TT on the aquastat and operate the boiler safely. Set the aquastat limit to the needed temp for you high temp zones at design, and crank down the low limit to just below the setting on the Tekmar and the TEKMAR will do the rest.

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Al Letellier

second high limit

> This question has crossed my mind, but for a

> different reason. Is one high limit really

> enough for safety? Imagine a situation where the

> burner relay contacts weld. Even though the

> aquastat says "enough" the burner keeps running,

> and... well, you know. Shouldn't we be

> installing a second line voltage aquastat in

> series with the burner wiring? Even this isn't

> fail-safe, but it's alot better than just one

> control. Or am I just being too cautious here?





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Al Letellier

second high limit

Ted, one is enough on a residential boiler. these controls are very reliable and it's extremely rare to see a high limit fail. the boiler safety will discharge if the pressure gets too high. If the boiler is located in an area where safety discharge could result in serious damage, a second high limit may be worth the investment, but there are millions of residential boilers out there with only one safety high limit.

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Roger_5

One high limit

The national codes call for as a minimum one line voltage High limit for Residental installations. Public or State buildings there may be a requirement by local codes for the second high limit and would be specified as a Manual reset type control.