Where to set Cut In/Out and Thermostat Swing on Steam System

5horizonsrr

Hi everyone,

I guess its been since last winter when I posted haha. Its that time of year again, and I just fired up the heat, but with a few adjustments.

Since last season I've learned from one of our forum experts (and the future replacing plumber) that my boiler is way oversized (Weil-McLain 185,000BTU, Recommended was 138,000BTU), but I need to make it through another winter with the current unit before replacing for financial reasons.

The good news is the boiler fired right up. The bad news is the Nest thermostat didn't. The nest has been a disaster since day one, so I pulled it out and reinstalled a Pro1 IAQ T705 thermostat, which was there when I bought the house and worked immediately with some new batteries.

Due to the oversizing, the boiler fires up and then pings on and off the PSI limit for 20-30 minutes to keep temperature at the setting- which is not good, and seemingly worse than last year with the nest. It does one full cycle (meaning firing until thermostat shuts) per hour, using almost a therm of gas. Outside temp today is 55 degrees, inside thermostat is set to 71.

So the three questions:

1- Can someone recommend a boiler cut in and cut out knowing all this? It is currently in at ~.5PSI and out around 1.75PSI. Recommendations?

2- The thermostat manual shows an option to set a "Heating Swing", which is currently at .3 degrees F. Should I alter this at all?

3- Any other ideas?

Any tuning help is greatly appreciated, and thanks! If relevant i can post an image of near-boiler piping.

Steamhead

The thermostat manual is here, for those who want to see it:

https://www.pro1iaq.com/images/700Manuals/PM-705-IM-Installation-Manual-Alitho.pdf

The swing setting should not be set at minimum, since steam systems take longer to heat up and cool down. I'd start at .5 .

The Pressuretrol settings are about as low as they'll go, which is where you want them.

The Nest thermostat can be very troublesome. What problems were you having with it?

5horizonsrr

Thanks for the reply! I've put the swing at .5, it goes to 1 so I'll monitor tonight and see. I'm glad I asked since I thought shorter would be better. So I understand the tuning here, I'm seeing if the bigger swing setting means less active boiler time when heating to temp? Or something else?

Re: the nest, you name it. The C wire scenario when I first bought it (easy fix, but a long diagnosis before I turned to google!) Then erradic cut outs and messages and other "quirks", then just now it decided the wiring was incorrect and would never leave the electrical connection screen, I couldn't even reset it. I'm over the thing, its a multi family home so the learning portion was unusable anyways...

Jamie Hall

Nests are a nuisance, aren't they?

As @Steamhead says, your pressure settings are about right, no need to mess with them. The greater swing setting will allow for more temperature change in the space, but also for a longer run when the boiler does fire up -- which is good.

And being oversized like that, the boiler will indeed cycle on the pressuretrol, and honestly there's nothing you can do about it as is. However, some Weil McClain boilers can be downfired -- that is, the burner output reduced. Some can't. What model boiler is it, exactly? If it could be downfired, the "on" part of the cycle would be longer.

All that said, however, while cycling on pressure isn't the best thing in the world, it's not the worst, either. There is, true, some efficiency lost because of cycling during a run, but not that much -- a percent point or two -- as the boiler and system never has a chance to cool much. The two places where steam boilers do lose efficiency are during the start from cold or barely warm, and in raising the pressure above what is needed to supply steam to the system. Reducing the total number of big cycles, by increasing the swing, will help greatly with the first, and setting the pressure low enough -- as you have -- helps greatly with the second.

Deltat210

I had a honey well programmable, and I absolutely hated it. Now I have a LUX. Honeywell programming brought us to the moon, and that is the degreee of engineer you have to be to program it..

PMJ

@5horizonsrr ,

Question #3: You asked so here is another idea.

None of these thermostats will do what you really need which is to stop your boiler before any wasteful pressure develops at all. That pressure is not required to heat the place... it just wastes fuel and overshoots the temperature.

Install one of these: https://www.galco.com/buy/Macromatic/TR-6512U in series with your thermostat(any thermostat).

With this timer you can pace your big boiler and never have any pressure at all. You can limit the longest time the boiler can ever run straight with T1. With T2 you can set a wait time before the next burn can start. 60 minutes divided by (T1+T2) is the cycles per hour( I like around 3) - but you can set the cycles per hour to exactly what you want. As long as you make the total T1 run time per hour enough to heat on design day(for me that is 30 minutes per hour) you can always be sure to get the job done. By spreading the burn out this way the heat will be more even and you will never have any pressure at the header beyond a few inches of water. Dead cold starts are a little slower, but during times you actually need heat there aren't any of those.

This is the simplest pacing control. It doesn't make any difference how oversized the boiler is. The result is way better than life with a pressure control - especially with an oversized boiler.

5horizonsrr

Thanks everyone! All very good feedback. I'm seeing how things develop as the temperature drops (outside), and monitoring gas usage as well. From there I may try your suggestion PMJ, because its just pinging off the PSI cutoff and fighting itself. (when trying to get the heat up inside)

PMJ

5horizonsrr said:

Thanks everyone! All very good feedback. I'm seeing how things develop as the temperature drops (outside), and monitoring gas usage as well. From there I may try your suggestion PMJ, because its just pinging off the PSI cutoff and fighting itself. (when trying to get the heat up inside)

RIght. It is maintaining a pressure you never needed to get to in the first place. There is no reason to ever be at 1.75psi or close. You don't need a smaller boiler. Just don't let the one you have run so much on that first burn of a call. Even out what it is doing, give the steam a chance to do its thing. Much more pleasant at so little cost.

I have built a more sophisticated control now but ran as described for several years. At $65 it is more of a question why not do it.

ethicalpaul

> Due to the oversizing, the boiler fires up and then pings on and off the PSI limit for 20-30 minutes to keep temperature at the setting- which is not good, and seemingly worse than last year with the nest.

Curious-- is this from cold start, or during normal operation?

Also, is your main venting OK? It seems like it must be if you've been around here before but I have to ask.