firing rate of he boiler?

steamfitter

Looking over old notes from a seminar and trying to answer the questions regarding boiler firing rates, I'm struggling to get things clear in my head.

When dealing with hw boilers and steam boilers, what device or devices control the firing rate? Is it the thermostat, a timer or an outdoor reset control? Is it a combination of different controls? Are there choices according to the contractor or engineer?

Trying to answer the questions of over-firing and under-firing. And its connection to the common problem of short-cycling. As I look through my old dead men's school workbook the troubleshooting answer to certain issues says: the burner isn't firing to the connected load. I think I know what that means, but how does one go about fixing that problem. Is it just a control problem if the boiler is sized correctly?

nicholas bonham-carter

Under/over-firing

If the radiators have been properly measured, and then the correct sized boiler installed; then at first firing, the gas meter is clocked, to ensure that the correct amount of BTU's are being supplied to the burner.

Some larger boilers have a 2, or 3-stage burner, but this is usually controlled by pressure from a Vaporstat. On a call for heat, the burner is on full fire, and when the pressure rises to maybe 4 ounces, the Vaporstat will either make or break a circuit, causing the burner to go to low fire to maintain that pressure, and not exceed it.

Short-cycling can be caused by poor main venting, as the boiler (and gas company), try to force the air out of the radiator vents. It can also be the result of an over-sized boiler, or a badly setup thermostat.--NBC

steamfitter

vaporstat

Thanks NBC!,

When you mention vaporstat are you refering to a vapor system only? Will other steam boilers use a pressuretrol to control the firing rate? How about HW boilers, are they controlled by an aquastat and thermostat?

Jamie Hall

And a follow up to NBC...

Vapourstats can be used on any steam system, but they are much more common on vapour systems which simply have to have low pressures to work properly.  Pressuretrols can be used on most systems other than vapour, but they have to be set low -- pound and a half cutout -- which is right at the lower end of their range.



Overfiring can cause short cycling on steam, although any steam system with a single firing rate burner will begin to cycle on pressure at the end of a long (say half an hour) run, when all the radiation is full of steam.  If you are so fortunate as to have a two stage burner, it will, as NBC says, be controlled by a vapourstat, to switch from high to low fire when pressure starts to build.



Short cycling much earlier in a cycle either suggests poor main venting -- the most common cause -- or a seriously over sized (or over fired) boiler.



The wrong thermostat setting (cycles per hour or anticipator) can also cause short cycle problems with steam.



Hot water systems are controlled by both a thermostat and an aquastat -- the thermostat tells the system that heat is needed (it does that for steam, too, of course!).  The aquastat -- in more efficient systems, in conjunction with an outdoor reset -- tells the boiler that the circulating water isn't hot enough, and turns on the boiler.  There is a fairly large "dead band" -- the difference between boiler on temperature and boiler off temperature -- in most hot water systems, and it is the width of that dead band combined with the heat demand of the system which will determine cycle length.  If you have the outdoor reset set properly, if available, and the aquastat set properly, the cycles should be reasonably long, but the system will always cycle except when it is delivering the maximum design heat -- which is going to be pretty rarely (you hope).



In answer to what actually controls firing rate, rather than cycle time -- in most residential situations it is fixed at installation.  Average heat delivery is controlled by the cycling, and therefore by whatever devices are controlling the cycling.



Clear as mud?