Why do mod-con boilers even need an offset setting?

[ah2bwise]

As I am making adjustments to my Lockinvar Knight boiler, I have spent a lot of time staring at its display and see that the unit decreases firing rate as it approaches the setpoint, and lightly increases firing rate as it falls away from it, just as I would have expected it to. But as I am watching this, I got to wondering why the unit even needs to have an offset setting above the setpoint if the unit is merely attempting to hover at the setpoint until the spaceheat needs are met.

It seems like the differential setting is what would have an impact on short cycling but not the offset setting?

thanks,
DW

 

[NY_Rob]

I have never heard of "offset" setting... what does it do?

My reset curve settings are: max outdoor temp/min outdoor temp and max CH temp/min CH temp.
Four settings, that's it.

 

[Sponsor]

 

[kalex1114]

DW what model is your lochinvar? I have KHN-110. Offset is there in order for the boiler not to cycle. My boiler seems to behave different from yours. It overshoots often and can't adjust its firing rate in drastic moves. I have an offset of 10f and it does help in my case. As an example there is a call for heat it fires up and gets to high 90% or so. I watch it and it starts to back down very slowly with about 2-3 degrees to go. My 10f offset helps pretty much every time. It gets to about 5 degrees of it. Sometimes it gets to its max offset and then shuts down due to anti cycling.

I have it hooked up to conxus for monitoring.

Here is an example of the firing:

 

[justcallmeglen]

Kalex, enable your ramp delays. You can set them to hold firing rate as little as 20% for as much time as 20 min. You have 6 blocks you can fine tune.
I have mine set for 25% for the first 20 min, keeps me from shooting past the set point temp, especially in shoulder season temps. But be careful you don't cripple yourself in bitter cold because at 20-25% you many have a hard time reaching a higher set point if your firing rate %'s are held too low.

This could all be poop if the KHN isn't like the WHN

 

[kalex1114]

@justcallmeglen - KHN is similar to WHN . I thought about doing that but I might have to adjust it for cold snaps. I did load calcs and adjusted my water temps based on the recommendations. 145f at 7f and 105f at 68f. But i did run into an issue with master suite. When we had a cold snap here and it was windy, cold outside my master suite couldn't get to temp. It hovered between 66f and 67f after sun stopped shinning. So I'm afraid that if i enable ramp settings I will have an issue.
I was thinking of replacing master suite baseboards with low temp ones but still not sure. Rest of the house has no issues keeping 69f except for master suite. I could probably take the curve down even more and house will be fine but master will not be. My master bedroom has 2 outside facing walls. Front also has 5 windows, side is just a wall. Only front has baseboards, side wall doesn't. Master bath has skylight, high ceilings and window. baseboard in there is 5.5 feet and attached to internal wall that goes into second bedroom. Bad layout and build.

 

[Leon82]

my bedrooms barely reach the setpoint but there is a constant flow of heat so it is very comfortable

I have my ramp setting at 40 going up to 70 at slot #6

 

[kalex1114]

Hi @Leon82 we meet again I may adjust the ramp settings. I noticed it too that even though its 66-67 its still comfortable since heat is running constantly

 

[DewPoint]

@dwassner Check the user manual or service manual for a description of each setting. I assume the offset helps avoid nuisance cycles in some installations. Also, That setting might be more applicable in a cascade/multi-boiler situation. Perhaps the manual will give more insight.