Downfiring to prevent short-cycling

Joe Brix

Honeywell L8124L Low set to 140° High set to 180° diff =20
Phase III set to 120° or so. Really haven't thrown a large hot water dump against it yet. Mostly just take showers and 3 loads of wash a week but never multiple showers/dish or clothes washer on at the same time. Seems like the L8124L doesn't do priority, so I guess the boiler can handle zone and indirect call at the same time. Still seems too short a cycle for a single zone, especialy since multiple zones calling at the same time probably only happens near design days.

Joe Brix

Burnham LEDV with Riello BF5

only fires for about 2 minutes to keep a 40 gal Phase III up to temp. It's fired with a .85 nozzle @ 1 GPH. Would using a .75 help stretch out the run times? New boiler this summer, so I don't know if it will run longer with the zones calling. Maybe smaller nozzle and put the tank on priority? Has an L8124L aquastat now.

Bill Nye_2

John?

? is it reaching high limit , or satisfying the aquastat on the indirect? Are you using the EC5000?

What do you have for an aquastat, and what is your high limit setting?

Is this just to "maintain" the indirect, or recover from a hot water demand cycle?

Need more info.

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Unknown

That would help

I've downfired my Peerless and Riello to .65 to make the runtime longer , it seems to work . No problem heating the house last year . No condensation issues either . What are the settings on the aquastat and how is the indirect and the zones wired in now ? Does the indirect bump the temp up in the boiler if it needs to make hot water ? How are the zones controlled by the aquastat ?

Tundra

You should be able to underfire it by one nozzle size without too much problem. make sure your exhaust temp is 325F or so. also set the low limit to 110F and the differential to 25. You do not need this boiler to stay hot. It would be more efficient if you set it up with a cold start control. The LEDV-2 does not meet energy star ratings. You can fire this boiler down to a .50 nozzle but that requires a modification of the burner but it then would meet energy star standards.

Ron Schroeder

The Smart 40 phase III has a less than 7 degree differential aquastat. You may want it to run off the low limit on the boiler, and not have it turn the boiler on directly, just turn its circulator on. This water heater has a huge surface air and you may be satisfied with the recovery. My suggested set up is not optimum, but it may be good enough.

Joe Brix

It's doing that now

The 40 is just hooked up to a Taco relay that just switches the circ on/off for the indirect. So it's driven by the low limit of the boiler's 8124 aquastat. I'd still rather downfire the boiler and put a Tekmar 260 on it. With the indirect on priority, the Tekmar will always fire the boiler right away to 180° for the indirect call, but the 40 will probably reach temp way before the boiler hits 180°.

Does anyone know when an 8124 gets a heat call, will it continue to fire up to high limit even if the heat call is satisfied? If I wire a 260's boiler contacts to the 8124's TT terminals, I want to make sure that the burner will stop when the 260 resets to a lower boiler temp or will the 8124 just keep firing up to it's high limit.

The 260 controls the circulator power leads, but I just wanted to leave the 8124 on the boiler as the high/low limit control.

Unknown

John

My advice would be to not use the L8124 Low Limit control and to replace it with a cold start control such as the L8148A. By maintaining temperature in the boiler such as you are now, you are not taking advantage of the full potential the 260 Tekmar control has to offer regarding temperature reset capabilities. The LEDV water content is only about 6 gallons and the time it takes to rise from the low limit setting you are maintaining will always be minimal just to maintain temperature. Another problem is that if you are now running a low limit of 140F then the burner will always turn on when the boiler temperature drops to 130F when there is no call for heat and will rise by the differential setting of 20F to shut off at 150F. This is certainly going to account for short and inefficient burns.

In answer to your question as to whether the burner will continue to run up to the high limit setting even after the call for heat is done the answer is no. The only time the boiler will operate up to high limit is on a call for heat. Once again my advice is to get rid of the low limit control and utilize the 260 if you wish. This should improve your burn times and do away with all of those nuisance short cycles the boiler is seeing to just sit there and stay hot all of the time.

Glenn Stanton

Manager of Training

Burnham Hydronics

Ron Schroeder

Hi John,

With the tank set for 120 and the boiler set for 140, there is not enough difference in temperature across the heat exchanger in the indirect to transfer all of the BTUs that you are putting into the boiler with the flame so you are getting short cycling. You really need to set up the controls so that the boiler can go up to the high limit to get maximum recovery and minimum cycling inefficiency losses.

Cold start usually will save you oil in the non-heating seasons especially if you can increase the differential on the indirects aquastat. If you increase the differential on the indirect's aquastat, a tempering valve is even more important to prevent scalding.

One of the worst things that you can do to a boiler is to set the low limit on a warm start aquastat too low. Setting the low limit much below 125 degrees puts the inside of the boiler in condensation range of oil for long periods of time causing premature boiler failure. Remember, with honeywell and most other aquastats, the actual burner turn on temperature is 10 degrees below the low limit setting.

On my own personal system, I have my high limit set for 180 degrees with a high limit differential of 20 degrees, my low limit at 125 degrees with a low limit differential of 40 degrees and my indirect aquastat set at 140 degrees with a differential of 40 degrees. The indirect's aquastat has a symetrical differential so the tank goes between 120 and 160 degrees. I disable the low limit durring the summer to allow cold start and I typically only get 5 or 6 fires of the burner per WEEK for domestic hot water.

As a side note, since my house has so much baseboard, I don't need over 125 degree water to heat the house so the indirect aquastat goes to the TT terminals to bring the boiler up to the high limit but he house thermostats only runs the heating circ. (and zone valves) and the boiler stays near the low limit.

Ron