Adding an inexpensive Dwyer A6 switch, instead of an expensive vaporstat?

andrewa · March 2017

I have a pretty standard residential boiler setup with a pressuretrol. Unfortunately, my pressuretrol doesn't seem to be very precise, and at the absolute lowest settings, on a longer run, my boiler still builds up to 4-5 psi of pressure.

A vaporstat is fairly expensive - a friend who does energy consulting thought I wouldn't get a reasonable payback on the expense from an energy savings perspective. Plus, I've read mixed things about the new design (non mercury) ones.

I was thinking of adding a Dwyer A6 in the same circuit as the pressuretrol. That way, if the Dwyer fails, the pressuretrol would still regulate pressure.

I might add a "delay on break" timer to reduce cycling.

I'm new to all this, though I've done a lot of reading. Is there anything I'm missing? Can anyone come up with why this is a bad idea? I haven't seen anyone else on the forum discuss Dwyer or alternatives to Honeywell pressure controls.

Specificiations:
http://www.dwyer-inst.com/Product/Pressure/SinglePressure/Switches/SeriesA6#specs
I'd buy the A6-15322, since I want to keep pressure below 1 psi.

Jamie Hall · March 2017

I see nothing in there about a cutin differential, which leads me to believe that either there isn't an adjustment, or there is no differential. If that is the case, you have two choices: one is to us two such switches and a relay, wired to simulate the differential action of the vapourstat. The other is to use a delay relay to simulate the action by introducing a timed delay on reclose. The former option is a little complex, but will respond to system pressure conditions properly. The latter is simpler, but won't respond to how fast the system pressure drops.

Otherwise, particularly if the differential in the Dwyer is very small (which I suspect it is) you will wind up with extremely short cycles, with the off time of the cycle controlled by the post-purge/ pre-purge of your burning controls (if any), since the switch will reclose almost immediately after the burner is switched off (a matter of a few seconds).

Fred · March 2017

The other option, which costs you nothing is to re-calibrate the Pressuretrol (if you have the model 404, gray box). Many of us have done it and can get it very precise. Here is the procedure:
Inside the Pressuretrol, right below the micro switch, there is a pivot arm. At the end of that arm you will see a screw pin that is activated by the diaphragm at the bottom of the Pressuretrol. If you look very carefully at that screw pin, you will see it actually has a tiny (I mean tiny) hex head on it. It takes a .050 hex wrench and you can turn it clockwise (Towards the bottom of the Pressuretrol to decrease the Cut-out pressure or counter clockwise to increase the cut-out pressure (which none of us want to do but who knows, your Pressuretrol may be really screwed up!). Turn the power to the unit off first. You may find the first attempt to turn that screw a little bit stubborn (relatively speaking) because it has some Locktite on it but it does turn. Don't turn too much, a fraction of a turn goes a long way towards getting it adjusted where you want it (maybe 1/32 inch turn to start with). You may need to play with it to get it exactly where you want cut out to be.

jumper · March 2017

Still asking why nobody uses temperature instead of pressure?

Jamie Hall · March 2017

jumper said:
Still asking why nobody uses temperature instead of pressure?

Probably because the margins are too small, and are affected by local atmospheric pressure. Gauge pressure is easy to measure -- quite accurately and reliably.

EBEBRATT-Ed · March 2017

Never mind the vaporstat, that's not going to solve your problem.

If your building pressure you could have 1 (or more) of 2 problems.

You don't want to build any pressure.
1. the boiler may be oversized for the load. You need to find out weather it is or not
2. You need to make sure your venting is adequate

neilc · March 2017

does the gage go to zero when the boiler has been off, between thermostat calls?
is the pigtail clear?

HVACNUT · March 2017

> @neilc said
> is the pigtail clear?

>>That would be the first thing to check. Its probably clogged. Remove the pressurtrol and anti siphon (pig tail) and try to blow through it. Can be cleaned with hot water, wire, pipe cleaner etc.

andrewa · March 2017

Update: I checked the pig tail. Wasn't clogged, but I took the opportunity to clean it. I then calibrated the pressurtrol using the 30 PSI gauge that came with the boiler. It's much better now! Will fine tune once I get a better gauge.

The system is does cycle on pressure (as expected), and now I will work on venting.

I'd be curious to see if anyone goes down the road of creating a DIY pressure controller using two switches & a relay.

Thanks for everyone's help - especially Jamie Hall & Fred.

Fred · March 2017

@andrewa , it's been done. Check out this link:
http://forum.heatinghelp.com/discussion/156690/a-diy-vaporstat-alternative

Adding an inexpensive Dwyer A6 switch, instead of an expensive vaporstat?

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