2 stage gas valve

upstateben

I've got a customer that has a 2 year old boiler double the sq ft of steam than what he needs. I've purchased a honeywell  2 stage gas valve and a vaporstat. I was thinking of setting the low fire at about 70 percent of the high fire. I'm wondering about 2 things, one is where I should be setting the vaporstat to drop to low fire and wondering about combustion numbers obviously I want to watch the CO, but I'm just wondering if I need to be concerned about o2 numbers on low fire, also how worried should I be about low stack temps and condensing in the chimney. The chimney is lined. Thanks

Tim McElwain

Going two-stage with steam may not be a good idea. I would if I were you get some of the experts here on steam help you out. The way you posted just Two Stage valve will not get there attention. Change the post to Two Stage with Steam.

EBEBRATT-Ed

@upstateben

You always have to be concerned with the combustion numbers.
Co should be under 50 and must be under 100. Stack temp should be about 320 minimum. I don't think stack temp will be low but check it. O2/Co2 #s may suffer a little on low fire.

If you run 70% on low fire and the boiler is that much over sized I doubt you will ever need high fire at all. As far as the pressure control goes cut in should be as close to 0 as you can get it. If you do the two stage and you start up on high fire I would dump it to low between 1-1.5psi and sut the boiler off at 2 psi max

The Steam Whisperer

Yeah, no point in 2-stage if you are that oversized. We find this on virtually every Weil Mclain LGB install we come across and nearly always run on low fire only. Often quiets the system down and evens out the heating.

EBEBRATT-Ed

@The Steam Whisperer

Any combustion issues down firing? How much do you down fire? What does your stack temp run at?

JUGHNE

Do you leave the existing single stage gas valve and just adjust the pressure down and check combustion?
Any problems on light off at lower pressure?

EdTheHeaterMan

To be sure you get proper ignition on all burners, you will want to start on high fire always. I’m not sure how you wire that except to say that you can make some type of holding circuit to prevent ignition until both pressure switches are closed. You can probably set high fire at about 90% of the rating with good combustion numbers. Getting the 70% pressure to get good numbers will be a challenge. Once you get good numbers on low fire, recheck high fire again.

Your idea has merit... good luck on the implementation

Yours truly,
Mr.Ed

P.S. gas combustion issue is @Tim McElwain wheelhouse. And @The Steam Whisperer already answered the steam part of your query

AMservices

@upstateben
What boiler and what gas valve are you using? 

Quick story. 
Once I down fired a oil WM gold boiler over 60%. 
It was a 7 section boiler and the customer only needed a 4 section. I think the boiler was rated for 240k and I was firing under 100k.
I was also planning around with vacuum pumps and was try to see how the system worked with 180°F steam. 
With all the right burner adjustments, my combustion test was 86% efficient. Gross stack temp around 425°. 

Point being, if your boiling water, your not condensing flu gas. 
Correct me if im wrong.

EBEBRATT-Ed

@AMservices

I pretty much agree. with 212 degree water the stack will be 100 deg over the steam temp but still needs to be checked

The Steam Whisperer

Like most larger boilers, the Weil LGB model is low fire start and then goes to high fire. The gas train is designed to run at 2 firing rates, so combustion isn't an issue with the newer models. Low fire ( 1.2 inches WC) is about 60% of high fire (3.5 inches WC) on the newer larger models ( gas valve with blue plastic top). On the older LGB's (probably over 20 years old), low fire (0.8inches WC) was about 50% of high, and CO tended to be on the high side. I've found that the LGB boiler does not seem to suffer much loss in combustion efficiency at low fire, which makes it unusual compared to most atmospherics. You probably will gain some fuel usage reductions running the boiler on low, whereas its probably a wash for most atmospherics. Power Burners are another story. Low fire usually provides considerable efficiency benefits. We have some Old 1920's Pacific steamers which we have tuned and we can get stack temps well under 300F at about 9.0% CO2 ( gas) if we bring the firing rate down to the system needs. We end up pumping up the firing rates to keep the stack temps up.