steam boiler timer for short cycling

Gary Segal

Hi all, Happy holidays to all!

I have been pondering ways to further reduce energy consumption in my 2 pipe steam system (single home, oil fired0, Thanks to firsthand help from Steamhead and advice from others, the system is running pretty well. It makes steam in about 21 minutes from a cold start on a typical winter day. We do run a setback at night. However, even when running at steady temperature, the boiler often will not fire for over an hour at a time, because of either temperature overshoot or lack of a need for heat.

Here are the basics:
Boiler: Slant Fin L40 PZ
Oil Burner: Beckett AFG (I believe it is 1.8 gph)
House: 1938 frame house, 2 floors, insulated walls and new windows (Maryland)
Pressure controller: Vaporstat

I have the cutout pressure set slightly below 2 PSI (supported by calculations and observation). So here is my question:

In normal operation, from a cold start, the boiler will run for 21 minutes to come up to pressure (1.5 psi give or take. Then it will cycle off for 1 minute 45 seconds, then back on for 1 minute 30 seconds until the temperature set point on the thermostat is reached. The cycling appears to be due to the relatively low cutout pressure and the fact that it takes very little time to dissipate this pressure in the pipes. The traps on the convectors and the radiators appear to be working properly, but I suppose that if some were not closing entirely, pressure would dissipate rapidly. There are no obvious external leaks.

I've observed that the convectors stay very hot for a good 30 minutes or more after the first cycle, even if I turn the boiler off. Making fresh steam as soon as the room temperatures begins to drop only takes 10-11 minutes. The I'm good for another 30 minutes of slightly increasing or stable room temperatures.

Thus, I feel that much of the cycling that the boiler does until the thermostat set point is reached is just wasted energy, primarily just repressurizing the pipes. So, I was thinking of implementing some sort of restart delay timer on the system to see if I could reduce the duty cycle a little.

Any thoughts to the practicality of this suggestion and (if practical) the best way to implement it? I see that there are a lot of start delay thermostats on the market for heat pump systems, in fact the thermostat I'm using has a 4 minute start delay for an AC compressor (have not figured out how to trick it into working for the boiler)

Thanks,

Gary

Fred Harwood

Boiler control

I find that a Honeywell T87 thermostat with the anticipator set to provide two to three cycles an hour works wonders. Simple, no overshoot, no gadgetry.

Henry_6

Steam Thermostat

Hi Gary:

I've had the pleasure of steam heating in my home for several years with one pipe steam by oil. I have installed a Honeywell adaptive intelligent recovery thermostat which is set up with a seven day schedule and it works great. As you may know, we get 40-50 degree delta T conditions some days in the Boston area and this thermostat ensures that the house is up to temperature before my feet hit the floor. The setpoints allow for a gravity system and that is what I set it for. I have tried other thermostats for steam setback, but this one seems to be the best, IMHO. Happy New Year to the Holohan brigade.

Gary Segal

thermostat options

Fred and Henry, Thanks for your suggestions.

I did a little digging into both thermostats. Itonically, I think the house had a T 87 thermostat installed when I bought it 12 years ago. Nevertheless, I do not really understand the anticipator function. Can you explain how it works or how it would prevent short cycling?

I also looked at the adaptive intelligent recovery thermostat. It is a really neat device. After reading the installation manual, I'm still not sure that it will keep my boiler from short cycling, since I did not see any settings where the thermostat would control the boiler firing operation, other than to assure that it fired appropriately to get the house to the right temperature at the chosen time.

Additional thoughts are appreciated

Guy_6

basic

I would try something a bit more basic: That 1.8gpm firing rate seems high. How about dropping the rate a bit, say 1.5 or 1.35, and shooting for more of a steady state run. The only downfall may be that the initial start from setback may be extended a bit.

Gary Segal

following basic as we speak

Guy, Henry, thanks for the additional suggetions.

Steamhead came by this morning to look at the system ( a real luxury to hae him local). He found that the oil pump pressure was off his gauge (well over 130 psi). He has set it down to the recommended pressure. The boiler no longer starts with a big bang and it is running longer on the first cycle to make steam.

After he dropped the pump pressure, he checked the draft and found it was way too high and decided it needed another damper to get the draft down. I guess the 7" flue is too big for this firing rate.

As expected, though, the short cycling is still present, since at the moment, it is simply a function of the pressure in the system. The boiler is running longer (a little over 2 minutes) to make steam on the second and subsequent cycles, and is off for a bit over a minute in between cycles.

Thanks again for all the tips,

Gary

[Deleted User]

Kind of convoluted.

But here goes. Install a 2nd pressuretrol. Wire it 120 volts & set higher than the existing pressuretrol. This will be your high limit (safety) control. Rewire your existing control low voltage & in series w/ the thermostat. Between the load terminal of the pressuretrol & the thermostat terminal on the cad cell relay, install an adjustable "delay on make" timer.

Then, when the pressuretrol breaks, & if the stat is calling for heat, the burner will be delayed when the pressuretrol makes again. Set the timer for up to 10 minutes delay. The downside is - the timer will also delay when the stat first calls for heat.