Hydronic Water Temperature

ScottMcNab

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.

Jamie Hall

Did you put an outdoor reset on it? And if not, why not? With one, it will be able to select a temperature which will keep the circulator on most of the time and give you nice even heat...

ScottMcNab

No outdoor reset. I'll have to get a more experienced hydronics guy to do that for me.

Ironman

Agree about getting ODR.

I'd leave the aquastat set where it is now. Running it higher would not be more efficient and could cause othe issues.

ScottMcNab

Thanks for your thoughts. Anybody else have any ideas to help with efficiency before I pay for an outdoor reset in the spring.

EBEBRATT-Ed

The most important question is once the pump starts does it continue running for the complete call for heat or is the pump cycling on the aquastat?

If cycling is the issue with low water content you may need a buffer tank

ScottMcNab

When it's cold out, it runs for the complete call for heat. On warmer days when the cool water that has been sitting in the rads cycles back and turns the pump off until the boiler heats up until 145 again. Presumably condensate protection. I'm just trying to figure out if I set the pump on to say 180 would that help keep the boiler off longer between burns. Maximum differential is 30 on the aquastat that controls the circulator pump.

sputnik99

I have a similar setup where the my boiler Honeywell aquastat (low @ 125F and high @180F) starts the circulator pump at 140F and stops once the return temperature falls below 115F. I have oversized cast iron rads and even on the coldest days a maximum supply temp of 140F is enough to warm the house. I raised the low to 140 and 150 before and found that it didn't really change the run-time or cycle time as per my Ecobee thermostat. YMMV though since you have baseboards.

EBEBRATT-Ed

What I would do @Scott McNabb & @sputnik99 is install a three way valve on your boiler for low return temperature protection.

Having the circulators cycle is wasting fuel. A three way valve with a sensor on the boiler return will modulate the return water temperature to the boiler protect the boiler and will allow the circ pump to run without stopping during a call for heat.

Gordy

ODR only gets you so far with efficiency on a conventional boiler, do as @EBEBRATT-Ed protect the boiler from low return water temps especially if you decide to install the outdoor reset.

ScottMcNab

Thanks for the help. I'll get a valve installed in the spring. In the meantime, would it be beneficial or wasteful to turn the aquastat up that controls the circulator? Seems to me it'd be better for the circulator to come on early and start getting the warm/hot water moving.

mikeg2015

I would leave it. It’s a good compromise for economy and boiler protection that ensures descent run times and prevents short cycling. Atmospheric CI boilers have a draft hood for this reason. The and stack temps stay well above flue gas dew point even if return water drops under 130f. They aren’t even counterflow. Bigger issue is the flue liners in cool masonry chimneys or longer runs of b vent.

ScottMcNab

Burn time is 20 minutes, off for 20-30 minutes and the cycle repeats. It's also -15 degrees Celsius. Those run and idle times sound ok? Just trying to figure it all out.

Gordy

Sounds fine to me. For not knowing heatload, boiler size, and emitters.

ScottMcNab

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.

Gordy

At 140 it’s about 330 btus a foot for baseboard

Gordy

So part of your short cycling is lack of enough emitter, and part may be over sized boiler. They go hand in hand. The emitter is in charge.

So at 140 AWT your absorbing about 28k of the boilers 110k output. Which reflects in your cycle times of on, and off.

ScottMcNab

Ah ok makes sense. Anything that can be done to help with the short cycling other than a different boiler?

ScottMcNab

You'd think the boiler would heat up a lot quicker than 20 minutes if it wasn't emitting enough.

Gordy

Are you sure about the total length of emitter? You only count what has fins on the tube not the length including enclosure.

Is this zoned?

What is the room setpoint?

How often did you observe run times with burner on? At what outdoor temp?

Honestly it’s quite simple. The warmer the AWT the faster the room will reach setpoint, and the shorter time the boiler will run its burner cycle. The lower the AWT the longer it will take for the room to reach set point, and the burner will run longer between cycles.

It takes so many btus at a given temp differential to meet that load. It’s how fast or slow it’s met with the water temp.

Outdoor reset is the luxury of a control deciding what water temp works best for any given outdoor temperature automatically. This saves you from going down, and changing the aquastat setting every time the outdoor temp changes. However with a CI boiler you can’t go as low as a mod/con due to condensation when the HX gets below the fuels dew point. This starts to deteriorate the ci HX.

Cycling is referred to here by many as boiler burner running less than 10minutes. However longer is better.

ScottMcNab

Yeah it's 85' of tube with fins. Single zone and room set point is 18 celcius/65 Fahrenheit. Observed run times for a couple months at various temperatures, the burner runs for 15-20 minutes no matter the outside temperature.

Gordy

If your supply temp is 145, and return is 115 your average water temp is 130 through the baseboard. So I over stated the output of the baseboard. More like 260 btus a foot at your average water temp. Also with a 30 degree differential you must be moving water on the low side so that output may be a little less than the 1 gpm the chart states.

If your happy with the output of the baseboard as far as meeting the setpoint then leave it alone, or experiment with a higher aquastat setting to see how much it effects cycle times.

The only issue I see is the low return temps of sub 130 to the boiler. So I would either raise your aquastat setting so you get 130 or more to the boiler by the end of the heat call, or some return boiler protection installed as stated above.

In the end a btu is a btu. Need one make one.

Gordy

Also that boilers net output is 84 k with gas. So numbers are in line with boiler output, and emitter output, and run times.

You really need to do boiler protection IF you are going to use those low water temps, or it will have a slow death from condensation corrosion.

ScottMcNab

Alright. I'll have to get someone in to look at the system who knows a thing or two. It's very difficult to find anybody in my area that deals with boilers.

Gordy

Can you post a picture of the boile s piping?

ScottMcNab

Anything else you'd like to see, let me know

ScottMcNab

If I have ice on the chimney cap, does that mean my boiler is condensing when it shouldn't be?