Modulating Honeywell pressuretrol without a differential?

KnowNothing

I was recently at a building with this pressuretrol set up. It appears that the one that I have enlarged is the modulating pressuretrol due to the bellows at the base. However, I can't understand how there is a modulating range, when it looks like there is just one setting (2.5 psi). I can't find any literature that says what the default differential is, or if it is additive or subtractive. Does anyone have any suggestions on how this is working? I'm recommending that the owner replace the control to allow for proper setting, but I'd like to have a sense of how the system is operating currently. Thanks in advance!

Tim McElwain

What are the control

numbers?

GBriggs

L91A modulating pressure control

You are looking at a Honeywell L91A series modulating pressure controller. This controller may drive a firing rate motor on the burner.  This control will not have a differential as this term is used for a digital switch. We call it a proportional band or throttling range. Some of the models have the feature of adjustment and some do not. Please refer to [u][color=#0000ff]www.customer.honeywell.com[/color][/u] and type in the search located to the left of the page L91.  I am enclosing a link to the “Product Data for you use.

[u][size=12][color=#800080]http://customer.honeywell.com/techlit/pdf/PackedLit/60-2152.pdf[/color][/size][/u]

KnowNothing1

L91 range

Thanks GBriggs. If there is no differential, then what are the high and low limits for the firing rate motor? It seems like the non-adjustable throttling range at mid-scale on the L91 is 0.5 psi. If the main scale is set at 3 for example, would the high and low be 3.5 and 2.5 respectively? It seems limiting, but I can at least see how it would work if that were the case.