bolier not running correctly

sarzacsmom

I have a 10 year old oil fired forced hot water heating system. 3 zones 1-main level, 2- upstairs,3 hot water.  50 gallon storage tank for hot water. The unit is doing what I call short cycling. It comes on and only runs for 30 seconds to 3 minutes.  Shuts off for approx. 10 minutes and repeats the process. Over and over and over.  We have had 2 techs in to work on it.  the first one did a regular service cleaning new nozzle etc. before we started having this issue. (note: last heating season we had a problem where it came on every 30 seconds and ran for 30 seconds) ended up changing the thermostat on the main floor)  This time we've swapped out thermostats --no change. By pass thermostats unit runs til safety shuts it down, cools down and comes back on, so doesn't appear to be a problem with the unit itself.  Head for zone valve 1 seemed to not function as it should so put new one on.  2nd tech discovered circulator was bad. replaced it(it was bad--even I could tell that when he showed it to me).  Unit came on ran correctly, gave great heat --for one cycle.  We have since put new thermostat, new thermostat wire, new aquastat.  We have disconnected all  the zone valves(wiring) so nothing could call for heat and then hooked back up one at a time.  Each time the unit has come on when heat was called for, seemed to run correctly for one time,then not come on again til we crank up a thermostat.  Then it comes on and starts short cycling.  The temp was 60-70 degrees out, thermostats set on 70, room temp 70 and it still kept doing it.  Last night out of depseration I even turned the thermostats down to 60 and no change.  I have heard it's gotta be the transformer/relay thing, it's an air bubble, it's the eye, it's the zone valve (but pipes get hot on both sides of all of them), the inside of the unit wasn't cleaned well enough, etc,etc.  I should also tell you that even with the constant cycling on and off, the heating baseboards only get luke warm.  Sometimes the boiler water reads 200, 190, 160, 130, or almost 0. but mostly 160 -190 in 3 minutes or less.  Does anyone know what REALLY would becausing this? I live in northern new england, USA and it's gonna get real cold real fast.  I can't afford to keep having techs out just to end up with the same problem.  I know it's not supposed to be runninng this way---please don't tell me it's all in my head and that it's runnning correctly. I have lived in this house for 10 years and it's normally come on and run for about 20 minutes then shut off--occassionally had to go through 2 or 3 cycles to bring room up to temp in the coldest months.  I'm getting desperate.  Ideas or answer anyone???

CMadatMe

I have an Idea

Let's start basic. What is the heat loss of the structure? What size is the boiler? Then from there let's take a look at the heat emitters? Which zone has the longest run from and too the boiler? What is the lenght of that total run? Let's calculate total system GPM requitement. How many total foot of baseboard is on the system?  Could be as easy as the pump is the wrong size thus no or very little flow so the aqustat gets fooled. My point is that this sounds like a flow problem. What type of delta-t are you getting? Meaning what is the supply temp and what is the return temp? That difference is the delta-t. Typical system should be a 20 degree difference between supply and return.

World Plumber

1 control at a time

Does your call for heat go through an aquastat? I had one that did similar things it ended up being the Aquastat. You need to test 1 control at a time. Look in side the aquastate if it is a triple aquastat which it sounds like if you have hot water connected to it. Disconnect the blue wire that pushes into the white porcelain. (Turn the power off first.) Use an ice pick or small screwdriver to push the spring lock in. For it to be continually coming on without a call for heat the low temperature side of the aquastate is more than likely malfunctioning. By removing the blue wire it will only run the burner when a zone calls for heat. If that stops your problem you know what to do. Have you tested for resistance across the TT wires to be sure there isn't a partial short causing the control to think it's calling then dropping out. But if your boiler is supposed to maintain a set temperature and it is bouncing around and cycling then there is a problem with the aquastat.