Mass of water and short-cycling

Roland_18

One of these days I'm going to surprise myself and actually remember to save my posts. Maybe today is the day. Sorry, I've asked this question before. Forgive me.

Having said that, Would adding more water mass to a system with an oversized boiler have any effect on short-cycling? I'm not talking about a buffer tank in the traditional sense, but picture a storage tank in series with the boiler and radiation.

Thanks, Roland.......................

ced48

Sure Sounds Like a Buffer Tank-

and sure it will help

nicholas bonham-carter

Curing short-cycling

Is this steam, or hot water? If steam, then the first step is to find out why the boiler short-cycles. Observe it as it steams, and first check the pressure-is it cutting out on pressure? An accurate low-pressure gauge is required for this.

If pressure is not the problem, then look at the water level in the sight glass, and see if it lowers itself to the point where the LWCO will cut the burner off.

If low water is not the problem, then use a jumper wire to short out the control wires of the thermostat (setting up the temperature will not work for this test).

When you have done these tests, then post the results here to be analysed.

My guess is bad main venting, or over pressure driving the water up into the dry returns, or a badly configured thermostat.

If hot water, then is the pump moving enough water through the boiler? Is the thermostat configured for hot water, and not hot air heat--NBC

Jamie Hall

Steam or hot water?

If it's steam we're talking about, no -- would make very litte difference, as the cycling is on pressure and a soon as the burner shuts off, steam production stops and pressure starts to drop, regardless of the mass of water (the mass of hot iron, on the other hand, does make a difference, although a small one).

Ironman

Buffer tank by any other name...

Is still a buffer tank. No matter how it's piped, you're still adding mass that requires more time to heat up, thus, short cycling is reduced or eliminated.



I'm headed to a job right now where the H.O. bought a mod/con online that was twice the size he needed. We installed a 40gal. electric water heater as a buffer tank and piped it to inject into the supply through two closely spaced Tee's. This was done because the piping arrangement required that the indirect was piped directly of the primary loop (by others) and I didn't want to heat the buffer tank to 180* when the indirect called.



You want to see a min. run time of ten minutes. Take the minimum firing rate of the boiler and subtract the load of the smallest zone from that number. This is the btu difference that you have to provide enough buffer to compensate for. The formula: run time x (min. firing rate - min. load) / Delta T x 500 = gals. of buffer needed. If you use the standard Delta T of 20* and 10 mins. min. runtime, you could simply divide the btu difference by 1,000 to get the required min. buffer in gals. Thus a btu difference of 25,000 would require 25 gals. of buffer.

Roland_18

Mass of water......

It's a one zone, diverter T system. I should have said water from the get go. The additional storage would only act to park BTU's. Like adding a big, fat pipe between the boiler and the radiation.

Zman

mass and differential

Adding mass will help with short cycling. Every pound of water will absorb 1 btu per degree. Another great trick is to increase the on/off differential on the boiler control.

If you target temp is 170 you could easily have it turn on at 160 and off at 180. In some systems you could go even wider.

Carl

Roland_18

Mass of water..........

Also, this is a cast iron, atmospheric boiler, not a mod/con. I added a Tekmar 256 with outdoor reset a couple of years ago. It made a small difference in the short cycling, nothing to write home about.

Zman

Control

The 256 is the right control.

You could try manually increasing the diff.

Buffer may be your only choice.

How bad are the cycles?

Carl

Roland_18

Mass of water.....

Depending on the outside temperature and a 70*F indoor target, the boiler will cycle ~ 3 minutes on 5 or 10 minutes off, sometimes a bit more or less. 

The exhaust(stack) temperature, as measured with an accurate dial thermometer, rarely exceeds 220*F. This is measured at the outlet of the firebox.

Maybe I need to leave a few windows open to increase the run time.