Oversized 25-Yr-Old Boiler Cycling Very Often - new aquastat

CindyfromRH

I have a 25-yr-old Slant/Fin v-90 (90,000 btu) natural gas fired boiler for water baseboard heat for my 1620 sf single-storey CT home. It's been cycling on/off in quick succession, particularly when I move setpoint up 2 degrees. I don't know if it's always done this but just notice it this winter which is my 3rd winter in this house. During a thermostat call cycle, the circulator goes on, triggering the boiler which heats the water to 180 degrees and the 180 max point triggers the boiler to shut off, circulator keeps running, and then (sometimes within 30 seconds and other times usually 1 to 2 minutes) the 10 degree setback is hit (170) and the boiler goes back on for 1 to 2 minutes, back off, etc. This on/off continues until the set point is reached in the house and the thermostat shuts it down and ends the cycling. To raise it 5 degrees (in a test), it cycled on 19 times and off 19 times, for 70 minutes, mostly 1 to 2 minutes at a time for each on/off, before it got to set point. Outside temp was 56 that day. Once it gets to set point, it usually has a few more quick cycling episodes and then regulates to being off 7 to 15 min with 3-4 minute "on" cycle. Occasionally it is off 30 minutes; occasionally it is off 4-7 minutes. Rather inconsistent, but I know many conditions can effect this. To prevent so much cycling, I've set the thermostat low (61) and not touched it for days (and I use an electric heater in whatever room I'm in to supplement, until I get this resolved.) My boiler guy replaced the aquastat (Resideo S8610U) that he called a "universal" though my hot water heater is a completely separate system and I think I read that the universal is for combined systems. I'm also thinking the aquastat needs more than a 10 degree deadband because it rises and falls so quickly, but I don't know if it's adjustable and don't know the model #. My neighbor has the same setup and it seems to be doing the same thing when I raised setpoint 2 degrees (I didn't test it in a stable setpoint environment yet), and I'm starting to think this is normal (though ridiculous) behavior. I can't get my boiler guy to return my calls because he's so busy and I'm low priority since I have heat, but after spending so much on him I'm afraid to call another outfit who will start all over again. I don't think his 3-hr job improved anything. (Additional info: my test's starting boiler water temp was < 70 degrees but zoomed up to 180 and never below 170 entire test; water pressure was steady at 17-18 psi the whole test time which seems in range, though my neighbor's was 25 psi; both feed and supply lines felt hot the whole time once the test got underway; one room in house doesn't heat well at all but it could be because it's at the end of the line, but then why is return water line about as hot as supply? otherwise house heats up OK; and lastly from upstairs I hear an exhale noise at end of cycle that I don't hear when I'm right in front of the boiler). Thank you for listening. I may take a ride out to a local heating company and have a talk. I have learned a lot in my research & testing, but I still don't know much so will need plain English in any responses please. Thank you.

CindyfromRH

I've attached 2 photos of my setup. In one photo, the pipe marked x is what I believe is the supply; and the pipe marked o is what I believe is the return. The other photo shows the side of the boiler without the panel.

SuperTech

Turn the low limit all the way down.  You should only have a low limit if the boiler has a tankless coil for domestic hot water 

CindyfromRH

Wow I hope it's that easy; thank you very much. I'm not sure I have a low limit set, but the behavior makes me think it's 170.

I corrected what I called the low limit in my orig post -- the 170 was probably just a preset set-back. Low limit is probably already low around 140. My mistake.

CindyfromRH

I just read this below response on a different post and it sounds like what is happening now and makes me think that changing the low temp may not change what's currently happening at the high limit; maybe you can help me better understand; or maybe mine is working as designed which makes a lot of on/off cycles. I corrected what I called the low limit in my orig post -- the 170 was probably just a preset set-back. Low limit is probably low around 140. My mistake.

Pulled from a response to a different post: If you have the high set for 180, the low set for 140 and the diff set for 15 and there is a call for heat,the boiler will go up to 180 and the burner will cut off. If there is still a call for heat, when the boiler drops to 170 (the Hi has a fixed 10 degree diff) the burner will come on again. So basically, the boiler will cycle between 170 and 180 degrees durring a call for heat.

If there is no call for heat, when the boiler cools down to 130 degrees (always 10 degrees less than the Lo setting) the burner will come on again. If there is still no call for heat, the burner will stay on until the boiler gets up to 145 degrees (Lo setting minus 10 degrees plus the differential setting).


When there is a call for heat, the boiler stays "near" the Hi setting.

When there is not a call for heat, the boiler stays "near" the Lo setting.

neilc

exhale?
can you determine from where?
are there auto vents upstairs on the rad(s) where you're hearing this?

when the boiler and circ is off, what pressure do you see on that gage?
use the service switch and shut it off for a minute to check,

CindyfromRH

I sat there about 5 mins with the service switch off, and went to about 15-16 psi. I cannot determine where the exhale is coming from; when I'm in the room over the boiler, it sounds like it's from the boiler at it's last breath as it shuts down, but when I'm at the boiler I hear nothing. It doesn't seem to come from the baseboard radiators. It's not every time, and maybe I have bigger problems than that right now.

neilc

ok, pressure seems ok,
I was thinking low pressure and poor circulation,
still thinking circulation,

CindyfromRH

Thank you; that's what my brother-in-law's hunch has been too, but no other signs of a bad pump.
The frequent cycling may be just the way it works and is designed to work .. so different from my expectations and history of furnaces (this is my first boiler).
Funny that the bedroom is cold and the return water is still hot; might be a blockage or something shut off.

neilc

show us the bedroom radiator

hot_rod

How many feet of actual fin tube?
compare that to the boiler output.
one or two zones?

CindyfromRH

One zone. Fin tube throughout the house is roughly an 80' stretch; in the bedroom is 12' of fin. I attached a photo of a section of the bedroom baseboard heater as requested. (The white cable running along the bottom of the radiator is an old tv cable.) I noticed that the carpet is sort of pushed up under the radiator; maybe that is interfering, but I wouldn't think so.

hot_rod

If you have 80 feet of actual fined tube, not just the cover part, but the finned part inside.

80 X 550 btu/ ft = 44,000 btu/ hr is what you can put into the home regardless how large the boiler is. So a 55,000 btu boiler would match that best, running 85% efficiency.

So on a really cold day, called a design day, maybe 0 in your area the boiler is almost twice the size you need. On warmer days, above 0 for example the over sizing gets even worse as far as the short cycling. Spreading the actual operating differential may help some.

The exhale noise, maybe the burner shutting off?

bburd

It's hard to be certain from the photo, but it looks like the air inlet at the bottom of that baseboard is mostly blocked by the carpeting. That will interfere with heating for sure. The air inlet opening at the bottom should be about the same width as the air outlet at the top. Generally 1 inch clearance at least.

CindyfromRH

Thank you hot_rod and bburd. Both responses were very helpful. I'll pull and cut that carpet.

hot_rod: I didn't measure actual fin but approximate radiator length, but I think fin length would be even less, making oversize even greater. I've been suspecting that, but there seems to be much controversy over size; a local boiler guy friend I talked to today said it was NOT oversized; I kept quiet, but I did think he was wrong, and you just confirmed it.

I have a different boiler guy coming out to inspect things on Friday; he will try to increase the aquastat's high limit differential (currently 10 degrees so temp fluctuates between 170-180 when it calls, and he'll try to make it 20 degrees so temp fluctuates between 160-180 to reduce cycling, though that won't reduce it by much), but there seems to be little else for options. I'm being told by several that this is how these systems are designed today; they go on and off frequently to keep the house at an even temperature. Thermostats with 2-degree setbacks aren't recommended because they don't want the house cooling and reheating; no more thermostat adjustments either -- they want us to just set the thermostat set point once all winter. It goes against my lifelong beliefs, but it means less temperature climbs and therefore less cycling so I may just have to play along.

bburd

@op keep in mind that by increasing the differential and not raising the high limit, you'll reduce the system average water temperature from 175 to 170° F. That may help short cycling, but it will worsen your underheating problem.

Since the system is pressurized, the boiling point is significantly higher than 212°F. You might try raising the high limit to 190 or 200°.

minneapolisboilers

2 thoughts, your pump make be making vibrations like it is working, but you lost an impeller and it actually isn't doing anything. Or, your pump may be working, but your system could be air bound and just require some purging if you aren't getting heat to the furthest parts of your home? The return water could feel hot near the boiler because of the heat transferring by gravity near the boiler. Is there an isolation valve you can turn off on either side of that boiler drain in the return line you could purge against? I would start there.

CindyfromRH

Thank you minneapolisboilers; I'll have the new boiler guy (who is coming Friday) check the pump and I'll ask about an isolation valve. Thank you.

Thank you bburd; I will pass this on to the new boiler guy who is coming Friday. I hadn't thought of raising the high limit but that seems a good idea.