My Aquastat only has a high limit any reason why or should I put a low limit controller on.

beatbox82

Recently had a peerless 3 stage oil fired boiler with a riello f5 burner installed. My installer put in a Honeywell Aquastat with one dial to set the temperature which is set to about 175 but no low limit setting like my old aquastat had.( which I still have and works)

My question. Is it more efficient to have a high and low limit temperature control setting lets say 140 -180. compared to just having just the high limit only.

It just seem like the burner is just running alot more almost like stop and go traffic in your car.

Thanks

EdTheHeaterMan

Does your boiler need to maintain a minimum temperature for any reason? (for example 140°F for domestic hot water coil)

If that burner is not needed to make your domestic hot water (DHW) and there is no other reason to maintain a minimum temperature, then why would you want to waste the energy keeping the heater warmer than the room temperature?

If your boiler is connected to a chimney or a vent of any kind, the hotter the water in the boiler the faster the off-time heat loss out the vent pipe will be.

Think of it like this. 65° basement air passes thru the heater when it is off. it contacts steel or cast iron with 65° boiler water on the other side. how much heat can be transferred? NONE because there is no temperature difference.

Now keep that boiler water at 140° and that same basement air flows across the boiler during the off-cycle time (when your thermostat is not calling for heat) there is a greater difference in temperature, so more energy is waisted out the vent. On a mild day, you could burn fuel just to maintain boiler temperature and never have a real call for heat from the thermostat.

How efficient is that?

STEVEusaPA

I wouldn't do that, per se. I assume you don't have a Thankless Coil for domestic hot water. How are you getting domestic hot water?
I'd replace the well, with the proper Hydro Well, and put in a Hyrdostat.
Then you have low water cut-off, and circulator hold off, and other features.
The circulator will cycle to keep the boiler from condensing based on the setting, and you won't need to keep the boiler warm all year round.
Better would be a mixing valve piped in to protect the boiler. But with your boiler and the well in the block, this would work too. If it were a triple pass, where the well is above the boiler, not so much.

EdTheHeaterMan

Another thought. You said "installed"... Does the new burner have a different firing rate than the old burner? On an oversized boiler, if you only need .5 GPH fuel to maintain heat and you fire it at 1.0 GPH the burner will run 1/2 the time to do the same job. this will be less efficient if the cycles are very short because your efficiency only reaches its peak after it has been operating for a few minutes. It is less efficient on start-up and there is waste during off cycle.

So operating at .5 GPH for 1 hour will actually be more efficient than operating at 1.0 GPH off and on and off and on over the same hour. You may actually burn .52 gallons of oil over the same hour and waste .02 GPH every hour of operation over the heating season..

beatbox82

thank you. just want to add my dhw is on a separate burner (riello)  and 30 gallon bock tank.

i think I understand what your saying. Correct me if im wrong. Without a low limit control my boiler would eventually drift down to room temperature as long as there isnt a call for heat from one of the thermostats in the house. So yes I would agree that would be more efficient.

Another question. Does the nozzle control the fire rate? if not what does?

Jellis

Hey @beatbox82, you are describing a "cold start" boiler by having the Low limit "off"
this is desirable if you do not need to maintain boiler temp for your Domestic hot water such as in your case with your separate water heater.

By allowing your boiler to cool off more in between cycles you achieve longer run times during a call for heat which is more efficient than maintaining your boiler temp at say 140°F
Longer run times are better then short run times... (think highway driving vs city driving)

If the tech changed your aqua stat from a cold start to a non cold start then I would call them and ask them to install the correct control.
you will no doubt use some extra fuel by maintaining the boiler temp full time.

EdTheHeaterMan

beatbox82 said:

thank you. just want to add my dhw is on a separate burner (riello)  and 30 gallon bock tank.

i think I understand what your saying. Correct me if im wrong. Without a low limit control my boiler would eventually drift down to room temperature as long as there isnt a call for heat from one of the thermostats in the house. So yes I would agree that would be more efficient.

Another question. Does the nozzle control the fire rate? if not what does?

The nozzle along with the fuel pump pressure controls the firing rate. For example, a nozzle with a 1.00 stamped on it has a firing rate of 1 gallon per hour at 100 PSI. so for every hour the fuel is flowing through the nozzle you will dispense 1 gallon of fuel.

You can also use a nozzle rated at .85 GPH and set the fuel pressure to 140 PSI and get about the same amount of fuel, 1 Gallon per Hour.

I hope that was not too much information. I guess a simple Yes would have been enough.

beatbox82

actually I learned alot. Thanks and happy holidays

Big Ed_4

With oil and a cold start boiler I would use a low limit to control the circulators . Let the system reach 120 , past the condensation period before letting the circulators turn on .