How to increase my delta T

JPL

I just installed a hydronic heating system (cast iron boiler, old style column radiators) with two zones - first floor and second. I started with a room-by-room heat load model (used CoolCalc) and placed radiators according to the model output. Second floor works great. The first floor, however, seems to be need some more heat. In a recent cold spell I could maintain a maximum 70 deg delta T (70 deg inside, 0 deg outside). I'd like to increase that to 80 (70 inside, -10 outside) to accommodate for the occasional bitter cold here in Chicago. I read John Siegenthaler's book cover to cover before I started, so I at least know the basics. My question is this:

Just thinking about the first floor as one big area, I am maintaining the 70 deg delta with a total radiator output of about 37K BTUH. If I wanted to kick that up to 80 deg delta, i.e., about a 14% increase, is it as simple as adding 14% more radiator output to the first floor (i.e. get it up to about 42K BTUH)?

(Note: the temp throughout the first floor is relatively consistent, so I am not getting hot and cold spots. I don't think balancing is the issue).

Thanks for any advice!!!

hot_rod

what temperature water are you sending to the radiators? Increase that to increase the radiators output.

EdTheHeaterMan

YES, You can get 14% more radiators in each room. That would be a project.

You could increase the radiator output by increasing the water temperature in the radiators by 14% … much easier in my opinion. What is your high limit set for on your boiler? If it is 180° then set it to 190° on colder days. Are you using any type of outdoor reset to reduce the radiators temperature on milder days? If yes, then adjust the reset curve.

JPL

Thanks for the feedback! Hi limit is set to 180 and low to 155. Never thought of raising hi limit to 190. Not that I would go higher than 190, but is there a practical limit at which point you risk damage to the boiler? Theoretically I assume that's when the water starts to boil, but also assume you have to leave some safety of margin. Just curious. Thanks.

EdTheHeaterMan

https://forum.heatinghelp.com/discussion/comment/1837813#Comment_1837813

you can operate your boiler at 210° safely. When you start to get above 215° there is the possibility of flashing to steam whenever the pressure in the boiler drops to zero on the gauge. Personally, I would use 200° as a maximum for undersized radiators. But that’s just me.

That way, if the control is off by 10°, then you’re not really getting any higher than 210°. And if there is a temperature overrun once it reaches the high limit, you still have a pretty good cushion because water at 12 PSI will not boil until it above 240°F

JPL

Thanks, Ed. You've been a big help. Much appreciated!

GGross

did your boiler keep up with the thermostat setpoint during the cold spell? and if not, was your boiler running the entire time?

JPL

When the outside temp was 0 deg F, I could maintain 70 deg indoor (the T/S setpoint). It's a wifi thermostat so I could check how often it was calling for heat over the course of the day — that was about 60% of the time. When the temp dropped a few degrees, it couldn't maintain the 70 deg setpoint and was running flat out.