Boiler Temperature

[Scott McNab]

Hi
I recently installed a Biasi propane fired conventional boiler. It only has a capacity of 4.5 gallons of water, a far cry from what I was used to with my ancient oil boiler that had a high capacity. How it works is, home thermostat calls for heat and fires the burner, when aquastat reaches a certain temperature the circulator pump kicks on. Burner shuts off when thermostat is satisfied. So from what I understand it's set up as a cold start boiler. Question is, what would the best setting be for the circulator pump to come on? Right now it's set to come on at 145 degrees with a 30 differential, so shuts off at 115 degrees. With Slantfin rads, would it be more efficient to set the circulator pump to come on at a higher temperature, say 180 degrees and then shut off at 150 degrees? Thanks for any help in advance. Just trying to save some propane as it's getting down to -25 degrees Celsius at night.

 

[Dana]

It's more efficient to run the boiler at a lower temperature as long as there is enough baseboard even at the lower temperature to emit all of the heat the boiler is putting out. If the burner is turning on and off during an extended call for heat the baseboard isn't emitting as much heat as the boiler is putting in, and the temperature needs to be raised to the point where that doesn't happen.

If it's not cycling during extended calls for heat, what is the boiler's temperture stabilizing at?

Efficiency is a secondary issue here.

While it's more efficient to run it at lower temperatures, even a cold-startable propane boiler will be ruined by constantly operating the burner at entering water temperatures lower than about 125- 130F. If there isn't a stainless flue liner it's better to hold the line at 135-140F. When the boiler's heat exchanger plates are cooler than 130F acidic propane exhaust will condense on the plates. That's fine if it's only a brief amount of time as the boiler is climbing to a much higher temp, re-evaporating the condensate, but if it's during a significant portion of the burn the acids will erode the fire side of the heat exchanger. Similarly, even with dilution air the exhaust in the stack can condense on the flue liner (especially when it's -25C outside), which can destroy the mortar in masonry chimneys, or corrode non-stainless flue liners.

If the boiler is stabilizing during long calls for heat at a temperature where the entering water temperature that is too cool, it can be protected with a bit of bypass loop plumbing, mixing some of the boiler output back into the return water before it enters the boiler. A system bypass is preferable to a boiler bypass, and a thermostatic mixing valve can be employed to fine tune it if desired. But I won't go into detail unless it's really warranted.

It's fine to start the circulator at 145F, but set up the differential to re-fire when the boiler hits 130F, not 115F. If the burn times end up being shorter than 5 minutes under those settings, raise the temperature to allow a bigger temperature swing without dropping below 130F.

A smart heat purging economizer control may ultimately deliver the best efficiency on your system, but I'd need more information about system behavior before making that call. Biasi sells boilers already fitted with those controls, but I don't recall for sure which one they use- I think it's the Hydrolevel 3250 Plus. (There are others.)

 

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[Scott McNab]

It doesn't cycle the burner during a call for heat. Just gets up to temperature where the thermostat is satisfied and shuts off which must mean the rads are getting rid of the heat quick enough. Water temperatures range from I'd say 140 to 180 when the boiler shuts off, depending on the outside temperature. I just figured if the circulator ran longer to say the 115 degree mark with the burner off, it'd continue to produce heat from the rads therefore not fire as often. Or would 145/130 be better for that or another setting do you think?

 

[Scott McNab]

Would just be a lot of starting and stopping for the circulator as the cool water in the rads will rush back when the circulator pump turns on and the aquastat would shut it off at 130 until the boiler reaches 145 again. Continuous cycle until the water stays above 130 and the aquastat leaves the circulator on.

 

[Dana]

It doesn't cycle the burner during a call for heat. Just gets up to temperature where the thermostat is satisfied and shuts off which must mean the rads are getting rid of the heat quick enough. Water temperatures range from I'd say 140 to 180 when the boiler shuts off, depending on the outside temperature. I just figured if the circulator ran longer to say the 115 degree mark with the burner off, it'd continue to produce heat from the rads therefore not fire as often. Or would 145/130 be better for that or another setting do you think?

Letting it pre-purge the boiler of heat down to 130F before firing would be safe if the flue has a stainless liner. If it's a terra cotta liner in a masonry chimney 140F is safer.

When the average water temp (AWT) of the baseboards is 110F (115F out, 105F return) the fin-tube is typically delivering less than 150 BTU/hr, about 1/4 what it's delivering at an AWT of 170F (180F out, 160F return).

Some heat purging economizer controls "learn" the system behavior and start purging the boiler of heat prior to and in anticipation of the end of a call for heat, then at the beginning of a new call for heat purge the boiler down to the user-programmed low limit before firing. If the pre-purge is set to 130F the entering water temp (EWT) at the boiler will be in the condensing range at the very beginning of the burn, but it the boiler temp stabilizes at a much higher temp for the majority of the burn that will be OK. Operated that way the average combustion efficiency of a cast iron propane boiler can run about 87-88% without destroying itself. If it takes more than a couple of minutes for the EWT to rise above 130F at the boiler (the boiler would likely be 145F or higher at that point) it's prudent to raise the pre-purge temp.

The actual dew point of propane exhaust varies a bit with the amount of excess combustion air. Most cast iron boilers with propane burners are set up with sufficient excess air in the mixture that exhaust won't condense at 130F, but at <10% excess combustion the dew point could theoretically be as high as 140F. Burners are almost never tuned that tightly due to the higher emissions of carbon monoxide unless controlled very carefully. Even modulating-condensing propane boilers designed for it are rarely condensing at 135F EWT, but most are at 130F.

 

[Scott McNab]

The masonry chimney has a new aluminum liner. So you think I should start with the circulator to come on at 145 and set the differential to 15? Or would say 160 and differential of 30 be any better? Or wasteful?

 

[Dana]

There's no need to let the boiler hit 160F before drawing heat from it with the circulator, and starting at 145F will be more efficient. The lower the average boiler temperature is during and after the call for heat the less the jacket, distribution, & standby losses are, and the higher the raw combustion efficiency is. Combustion efficiency at an EWT of 160F is about 2% lower than what it is at 135F, but the standby and distribution losses are more than 30% higher when operating near the high-limit temp vs. 130F.

You want the boiler to be heating the HOUSE, not the boiler room. If the boiler room is the warmest room in the house there's some work to be done to correct that, the easiest of which is to lower the boiler's average operating temp.

 

[Scott McNab]

Sounds good. Thanks for your help. I'll change the settings when I'm home and we'll see what happens. I may be asking more questions. Thanks again

 

[Scott McNab]

Hopefully it will help with efficiency changing to 145 and 15 differential as opposed to 30 differential. Any bit helps.

 

[Dana]

To fine tune the recommendations a bit, can you share the model numbers for the boiler, the burner, and the controls?

How many feet of SlantFin is there on the system (broken down by zone if multi-zoned)?

 

[Scott McNab]

Yes I'll get you that information tonight. It branches off into different zones but comes back as one. I believe it's considered one zone. What would make it a single zone or multiple zones?

 

[Dana]

Yes I'll get you that information tonight. It branches off into different zones but comes back as one. I believe it's considered one zone. What would make it a single zone or multiple zones?

The number of thermostats that can independently call heat to different areas is the number of zones. No matter how convoluted the distribution plumbing if it's all slave to one thermostat, it's a single zone system.

 

[Scott McNab]

One thermostat, so single zone.

 

[Scott McNab]

Boiler is Biasi B10 series Model B-4. Gross Input M.B.H 126, Gross Output M.B.H 110. Circulator is Bell and Gossett K20. Aquastat is Honeywell Type L6006C. Burner is LP gas Riello 40 N12OS. 85' feet of Slant Fin 30 baseboard rad.

 

[Dana]

110MBH= 110,000 BTU/hr of output, into 85' of SlantFin Fineline 30 is about 1300 BTU/hr per running foot.

That's about twice as much boiler output as the baseboards can actually emit, which means during any extended call for heat it will always going to cycle between it's high limit and refire/low-limit point. The burner does not stay burning until the thermostat is satisfied- it can't unless the calls for heat are pretty short. It's possible to down-fire the Riello 40 N120S to half that, but it wouldn't necessarily work all that well with the B-4 if set that low. I couldn't find a manual for the N120S, but it's probably similar to the G120, which can be set up as low as 38,000 BTU/hr. (See page 11.)

The system would be better served with the B3 (if they even make a propane version) which would still oversized for the radiation, but could still be set up to not-cycle if the high limit is set to 210F. Ideally you'd be looking at a boiler output of no more than about 50-55,000 BTU/hr with 85' of baseboard, but 60,000 BTU/hr wouldn't be terrible.

Is the L6006C controlling just the circulator? If yes, what is controlling or the burner? Figure 11 on page 26 shows an Aquasmart controlling the Reillo burner in a gas version of the boiler as well as a dumb 4006A aquastat high limit control option, but no L6006C.

Set the high limit to something like 160-170F with 30F of differential on refire. The bigger the differential the longer the burns and few the number of burn cycles. Replacing/enhancing the aquastat controls with a heat purging economizer control would allow for even bigger differentials, maximizing use of the available thermal mass in the systems, without increasing the average operating temperature of the boiler.

 

[Scott McNab]

The burns are 20 minutes average. Once the thermostat is satisfied then it doesn't fire for roughly another 20-30 minutes when really cold and the cycle repeats. Yes the L6006C is only controlling the circulator. The home thermostat controls the burner. At first it was set up where the aquastat controlled the burner and the thermostat controlled the circulator. It was cycling on and off all the time so got it wired different. So set the circulator pump to come on at 160 with a 30 differential? Got the boiler used so it has no controls with it.

 

[Dana]

The wall thermostat cannot control the burner directly. It never works that way.

It may take 20 minutes to satisfy the thermostat, but the burner MUST cycle on/off during that period if it's dumping 110,000 BTU/hr into fin tube that's taking heat out at no more than 50,000 BTU/hr. There isn't enough thermal mass in the system to allow a 20 minute burn with 60,000 BTU/hr (600 BTU/minute) of "extra" heat going into the system:

The fin-tube + distribution plumbing probably adds at most about another 4 gallons of water to the 4.5 gallons of boiler water, and with another 2 gallon worth of "water equivalent" thermal mass from the cast iron we have about 100 lbs of water-equivalent thermal mass. At 600 BTU/ minute of excess that water temp is rising about 600BTU/100lbs = 6 degrees per minute. In 20 minutes that's a temperature rise of 120F. If starting at 130F it would hit 200F in under 10 minutes, since it's only emitting half as much at 130F AWT as at 180F AWT so early on there is even more heat per minute going into the system than is being emitted than later in the burn.

There is some control on the boiler that sets a not-to-exceed high limit to stop the flame, and a low-limit at which it refires if there's still a call for heat. The thermostat only asks for the heat- the boiler controls operate the burner in a way to deliver that heat safely.

 

[Scott McNab]

That's how it works. Only other control is a high limit set at 200 to kill the burner if it gets that hot. The burner heats the 4.5 gallons quickly but once the circulator starts it slows down considerably. High limit would kick the burner off but it has its own differential. Thermostat ultimately controls the burner when operating within normal range and not overheating

 

[Scott McNab]

As it's a low mass boiler, once the burner is off and circulator running, the water in the system gets cool quite quickly. I actually liked my high mass oil boiler better. Water stayed hot for a long time even when circulating. So far this boiler has been a disappointment.

 

[Dana]

That's how it works. Only other control is a high limit set at 200 to kill the burner if it gets that hot. The burner heats the 4.5 gallons quickly but once the circulator starts it slows down considerably. High limit would kick the burner off but it has its own differential. Thermostat ultimately controls the burner when operating within normal range and not overheating

When heating the 4.5 gallons there is no significant heat being emitted so that should go quickly, but once the circulator starts heat is also being taken out and the rate of temperature rise slows substantially.

The only way for it to not cycle on/off over a 20 minute call for heat is if the burner tech downfired the burner to match the radiation output when the thing was installed. If it's not cycling it's absolutely not delivering 110MBH into 85' of baseboard.