Hydronic short cycle question

Eli

Any help would be greatly appreciated:
My gas fired hot water system is short cycling when heat is called for. It's about 3 minutes on/ 3 off. Seems to be going off on temperature. I hadn't noticed it before, but I can report two changes. 1) one or two winters ago I replaced the old, failed circ pump with the small ($50/$60) Taco (the green one without a coupling) and 2) a couple months ago I insulated all the pipes in the basement (which brought the basement temp down at least 15 degrees!). My best guess is that the insulation is slowing the heat loss so that the circ pump can't keep up with moving out the heat input of the heater. That's my guess... What do you guys think?
1) Is the little taco pump good or junk? Should I replace it with the big red B&G?
2) In general, what are the likely causes of short cycling?
3) How should I address this problem or is it not a problem at all?
Finally, the pressure is perfect, locked on 20 psi all the time.
Thanks again,
Eli
(When I get some photos, I will show a really cool old two pipe steam system with VECO rad traps... and my solution for the handful of missing ones!)

hr

Do the \"tighten up\"

Archie Drells and the Bells?? wasn't it?

Have you also insulated or tightened up the home? If so you may have reduced the load on the boiler. In which case the boiler may be oversized for the current load.

Not uncommon to find oversized, sometimes grossly, equipment in homes.

Do a heat loss calculation, a free calculator is provided to the left of us at this site, then determine how closely your boiler is matched to the actual building load.

Reducing your heat load, or you buildings load , is a good thing, you may be able to downsize the firing rate of the boiler if this turns out to be the case. Always check with the manufacture for their blessing before downfiring any equipment.

I like to see a good 10 minute minimun on cycle. Generally this is needed to warm the boiler, and the venting system, above condensation temperatures. Often times short cycles end up destroying vent materials and even heat exchangers. An expensive, and often dangerous failure.

Keep searching for the cause and solve the problem. Get a CO detector installed, for additional protection. Plugged flue and or HX's can cause unseen or un smellable problems!

hot rod

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Nickk

what's your anticipator on your thermostat set at? that might need to be adjusted

Eli

It's a programable, newer type. It was set on +/- 1 degree. I just bumped it up to +/- 3 degrees. The problem is definitely a new phenom, though and the thermostat has been the same for several years. Is it at all possible that it's a circulator pump problem?
Thanks,
Eli

Bill Nye

Eli?

How cold is it outside? Are you sure the boiler is not cycling on high limit? What is the boiler temp when it shuts off?

I'm thinking three minute cyle time is not all that short or uncommon if the boiler is heating the house in cold weather and the boiler is reaching high limit before the thermostat is satisfied. Most high limits have a 10°F fixed differential [i.e. , reach 180° , fall to 170°, repeat] Hard to say w/o being there

Eli

Yes, I think it is shutting off on high limit... the temp at shutoff is around 170 and it comes back up (3 minutes later) around 160. And it is esp. cold out these past couple weeks (here in philly). But shouldn't the pump be able to move the water fast enough to keep the boiler from shutting off on high limit (given that the tstat is not satisfied). (To be clear, this is a hot-water system, not steam)
Thanks again,
e

Bill Nye

It Depends......

On your heat emmitters. Lint, dust, carpet, furniture placement, etc, all play a part in this. It is not the pump. Piping arrangements, fittings, etc. , will affect the flow too.

But I think when the weather climbs back up into the 30's things will go back to normal for you. If your house is warm and your thermostat gets satisfied you're good. Hey it wasn't all that long ago our ancestors were huddled in a cave wrapped in bear skins trying to stay warm around the fire. Rent a movie or DVD and forget about the burner.

Eli

Forget the ancestors,

that describes most of my co-workers!

hr

If you are close

to design outdoor temperature, and the equipment is sized correctly, that boiler should be running much more than 3 minute cycles.

Ideally a perfectly sized boiler would run non stop on a design day. The structure would be losing heat at the exact rate the boiler was putting heat into the building. Usually a design is somewhere around 70 inside with 0 outside.

Without some hard numbers we are all guessing as to your building loss and the equipment match up.

hot rod

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Joe_13

I'd rather see a 20 degree differential

Might want to set high limit to 180 and see if the diff can be set to 20 degrees, let the boiler chew on the load a little longer and then have more time between cycles.

Joe_30

differential possible with 8148E??

Joe, Thanks for idea of adjusting the differential on the aquastst to lengthen cycles.. . However, do you know if it is possible with the Honeywell 8148E aquastat which seems to be a high limit device only??

This cycling stuff is my thread -of - interest these days. We are finally getting 4 1/2 minute ON's and 10 minute OFF's, at minus 14* (near my minus 20 * design temp.}. This with Buderus 110 net hot water boiler driving a 4 zone valve, 95K load , in a not so big 2400 sq feet 1940 house . Got the anticipators right and that brought it up from 2 1/2 minute runs. It's about a 40 gallon system of oddly mixed iron and fin tube. ( a future project}. House is nicely warm at all temps.

Good enough on the cyclling?, or go for a simple in- line 30 gallon buffer tank to work toward Hot Rod's 10 minute ideal of of ON time's.? No electrics.. just a fat addition of water after the circulator and before the supply manifold..?

hr

To adjust differintal

Not all aquastats are adjustable. I install a second control, in series, run the temperature up on the exisiting control for a high limit control. Which is what it was intended for!

All kinds of adjustable setpoint and cap tube type aqustats out there. I like the Goldline with thermistiors. If possible use a well into the boiler or supply header for accurate senseing.

I have a handful of cast boilers installed, running 40° delta tees, they work fine if your emitters can work in that range. Generally mine are in radiant applications.

You still need to determine the match between actual load and boiler size. A downsize may solve your problems. A 3 minute burn cycle is not a very desirable operation mode, in my opinion. All the components will se a shortened life expectancy cycling that much. In addition to a potential flue condensation issue!

hot rod



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