Modulate On Diff / Modulate off Diff

Danscrew

   What is the best way to set up my Mod /Con Boiler for the Turn on and off above and below Target temp.To help it not short cycle as much?   Now I have it set 10 degrees above to Turn off and 5 degrees below on  to give it a swing to slow the short cycle What is the best way to accomplish this? I have my ODR set to provide the lowest temps to cover the Load.  It runs good and I get long run times its over-sized like the rest of them out there just trying to get it to run a little better.  thanks Dan 

SWEI

What kind of zoning

if any, and thermostat(s)?

Danscrew

2 zones

Thanks Swei

           Two zones Two stats Basement is fin tube, first floor is CI recessed radiators What I'am doing now is setting the boiler off at 10 degrees above target and 5 degrees below what are your thoughts? to give it more of a swing between cycling. I would like to change it from a WM 110 to a 70 to give me better low end I would be going from 22,000 to 14,000 on the low .  I am having trouble finding a 027 orifice for the Honeywell venturi valve. thanks for all your input Dan

SWEI

The best way to reduce short-cycling

in my book is to minimize or even eliminate zoning.  How close can you get with the curve if you leave both thermostats at 85ºF?  Hydraulic balancing probably required.

ced48

Can You Adjust-

ramp up, and what is the modulation rate, 5-1? How much oversized is the boiler?

Danscrew

The Mod rate is 5-1

The boiler is a WM 110 almost 35 % or more over sized.  If i could get it to run below minimum by changing it to a WM 70.  I would be in better shape that would give me a 14,000 minimum  instead of 22,000 I also have the Max fire rate down to 2250 giving me 40% of max. Then I get longer run times at that until one of the zones gets satisfied.   thanks Dan

ced48

What Is a Short Cycle?

By this, I mean what happens? Is the boiler reaching set point, shutting down and then re firing minutes latter before the heating cycle is complete, or is the heating cycle itself short?



I am messing with my own modcon which is also oversized, by double, I need about 25K net, got 44K net- by cutting maximum fire and water temp, I run between 20% and 50%, with no short cycling. I don't see why you can't do the same.

Danscrew

My Short Cycle

It reaches the set point . Then I have the Boiler off set to 10 degrees over set point and the Boiler on at 5 degrees below to slow the short cycling it only short cycles when one zone is calling when both zones are calling it's enough load to keep it running at 20% Thanks Dan

ced48

So, the Problem

seems to be caused by the existence of 2 zones, making the boiler much more than 35% over sized, when only 1 zone is calling for heat. As SWEI suggests, how about having only one zone? Will one or the other be over, under heated? Worth a try, otherwise, a smaller boiler, or buffer tank, seems to be the options. You are right in trying to get lower modulation, but you can't. Just a thought, can the radiators be regulated? If with 1 zone, that space was over heating, you could turn them down.