Vaporstat setup

Rightrudder

Just traded out an old pressurtrol for a new Honeywell L408J (0-4 PSI). Trying to get the boiler to cut in at .5 psi, cut out at 1.5.  Right now I have the left scale (DIFF) topped out at 16 oz, the right (MAIN) set at 2 PSI. This is yielding a cut out at 1.5 which is what I want, but a cut in at just under 1 PSI (too high). If I change the DIFF to 8 oz, she cuts out at 2, cuts in at 1.5. 

I’ve tried different settings and nothing seems to be getting me where I want to be (1.5 out, .5 in)

I have 2 3# gauges on separate clean pigtails, so I’m confident my readings are correct.
Any insights out there?

Jamie Hall

With the main scale -- which is the cutout -- set at 2, it will cut out at... 2.

The vaporstat is subtractive -- that is, the main scale sets the cutout pressure, and the cutin pressure will be the main scale minus the differential. So you want the main scale at 1.5, and -- for a cutout at 0.5, the differential at 16 ounces

Rightrudder

Thanks for your input! Actually, I’ve tried that set up, but it looks like I may have a unit that’s completely defective right out of the box. If I set the main to 1.5,  it cuts out at 1 pound. And even with the differential set at 16 ounces, it kicks back on at half a pound. 

I’m not sure if this is a stupid question, but is it possible that I have it wired incorrectly? 

The original pressurtrol was a two-wire set up, but there are three connectors on this. I am on the R and B. 

ratio

On the switch, there's a common, a make-on-rise, & a break-on-rise. I can't remember the colors, so I always check with a mutimeter. In any case, you'd know if you were on the wrong set, as it wouldn't shut off once it got going.

Jamie Hall

Rightrudder said:

Thanks for your input! Actually, I’ve tried that set up, but it looks like I may have a unit that’s completely defective right out of the box. If I set the main to 1.5,  it cuts out at 1 pound. And even with the differential set at 16 ounces, it kicks back on at half a pound. 

I’m not sure if this is a stupid question, but is it possible that I have it wired incorrectly? 

The original pressurtrol was a two-wire set up, but there are three connectors on this. I am on the R and B. 

Let me be sure I have this straight. If you set the main at 1.5, it cuts out at 1 pound? Then it may be out of calibration. That's possible. However, if the main is at 1.5 and the differential at 16 ounces, it should cut back in at half a pound -- which you say it does -- so it would seem that it's not out of calibration.

So... I'm a little confused here.

Rightrudder

Got it. Thanks for your input as well. 
So I guess the darn thing may be defective from the get go. 

mattmia2

What altitude are you at?

Rightrudder

About 270 MSL. Intriguing question!

Rightrudder

And I just saw Jamie’s  post- 

This is what is odd to me – the main is 1/2 pound out of whack, I get that and I can treat it as just a number and work around it. But I’m surprised that the diff would also be out of calibration. It keeps cutting back in at an 8 ounce drop, even though it is set for 16 ounces. So 2 out gets 1.5 in, 1.5 gets 1.0 in, and so on.  And if I move the diff down to 8 ounces, it drives the main cut out UP an additional half pound (on the gauge). Isn’t it pretty slim odds that both would be out of calibration?

mattmia2

what if you set the setpoint to 2psig and the differential to 1 psi? does it get 1 psi differential that way?

Jamie Hall

Very slim odds there. Could you possibly post a photo of the face of the vapourstat and the settings?

Fred

You should be using the top and bottom terminals on the Vaporstat. The middle terminal has no wire.

Rightrudder

Only top and bottom terminals connected.So here is the set up that is yielding a cut in at just under a pound, and a cut out at 1.5:

Jamie Hall

Just for curiousity... the differential is, I see, maxed out. Without taking one of the things apart (the ones I have are all the mercury sort, which are bulletproof) I'm not sure just how the linkages to the diaphragm work, but it occurs to me that having the differential maxed out may affect the overall operation.

Therefore, just to satisfy my rather selfish curiousity, what happens if you set the differential at mid-scale -- say around 8 ounces -- and leave the cutout alone?

Rightrudder

Had to wait till the wife was out of the house- she was complaining that the house was so hot and it’s 70° out- and why was I running the boiler?
Anyway, interesting idea. Tried it, but all I did was reduce the differential cut in from 8 ounces to 4 ounces.
so it looks like I’m stuck with a lemon of a vaporstat – a shame Honeywell couldn’t keep their QC in check.
Thanks to all for your suggestions and insights!

gfrbrookline

Not necessarily a lemon just a calibration issue on the main. Now you know what setting gets you the co/ci you want. What are you using for a gauge? That could be another culprit.

Rightrudder

I’ve got two 3# gauges on separate pigtails, and they’re within <1/16 # agreement. 

It appears I can finagle cut out without a prob, (,just find a number that works on the guages) but I can’t create a sufficient spread for cut in. 16 oz (max on the setting) only yields 8oz  differential. That’s too quick, and will result in excess fuel useage. 

JohnNY

Did you put some water in your pigtail?

Jamie Hall

8 ounce differential is too quick? Excess fuel usage? Actually, no. In fact, the quicker you can recycle on a pressure cycle, the better your efficiency will be, since the boiler doesn't have time to cool off any (this is not true for seriously oversized boilers, by the way).

Cedric, the 580 in the the main house I care for, has been running just fine for a number of years now with a7 ounce cutout and a 4 ounce differential; the off cycle time is actually controlled by the post purge/pre purge requirements of the burner.

Rightrudder

Both pigtails clean and full of water.

So I guess the Holy Grail of 1.5 out/0.5 in may not be all it’s cracked up to be.
Thanks again to everybody for your input!