New Boiler Short-cycling on Pressure Cut-out

The radiators in the house have an EDR of 418 so it made sense to go with a less oversized replacement.

But still oversized. But based on your description it could be triggering the vaporstat due to surging/carryover because of oil on the water (from the new piping and boiler). Did he skim it at all yet? Skimming a new install take a couple/few sessions especially if he didn't put in some washing powder and let that boil for awhile, then skim and drain.

The near boiler piping is confusing to me, perhaps he is trying to help a counterflow situation near the boiler, not sure.

But one thing I see that is not great is the horizontal nipple at the hartford loop is supposed to be a short nipple or even a wye and he has about an 8" one there it appears.

Yes, still a bit oversized but the next size down is too small.

Not in my opinion. I'd greatly prefer 388 vs 450 for a 418 EDR (30 under vs 32 over). For those following along at home, all these ratings are boosted by 33% to make up for pipe losses, which is dramatically too much for those losses. But anyway, that ship has sailed.

For the hartford loop nipple, I believe it can cause hammer. And I was wrong about the length, it's a lot longer than that, I didn't notice the part before the elbow…it's gotta be 18"-24". This issue might not manifest as any problem, but it's good to be aware of it.

Here is an answer

It appears that there’s definitely some piping errors.

A close up pic of the sight glass would help to see if it needs skimming.

Short cycling and faulty venting may be the cause of some rads not heating.

The vaporstat setting of 2-8oz is more in the range of a two pipe vapor system. I’d make sure that the other issues are corrected first, but a one pipe system may need slightly higher pressure, but not above 2 psi.

This is proper near boiler piping. The optional drop header is preferred, but not required.

IDK why there’s a drip off of the middle of the header unless it doesn’t have proper slope towards the equalizer.

I now see it in the picture. That’s a huge mistake as now the header would have to be pitched backwards to drain.

I’d make them correct that properly and skim the boiler.

But if the boiler is oversized it may need more differential to make the cycle length reasonable.

That might be more efficient in a standing pilot boiler with no vent damper but the losses up the flue when the vent damper is open but the ignition is still working on starting a cycle and the post purge at the end of a cycle are going to eat up any miniscule gain you get by not building a small amount of pressure. Though a small amount of that energy is not transferred to the water because the water is a slightly higher temp, most of that energy is stored in the water and released as steam as the pressure is reduced as the system cools.

 There is a reason why one pipe systems tend to need higher pressures: the pressure drop through the vents on the radiators. 

If this were the true situation, NO one pipe system could ever run below………say………..10 ounces. There is more than enough data on this site to prove that is not accurate. Several folks with drop headers run with pressure so low that a 3 oz. gauge is necessary just to read it accurately. The pressure drop though the vents is negligible.

Furthermore, they can run FOREVER at this pressure and never get to a point of tripping a vaporstat. The radiation distributes ALL the energy that the boiler provides.

And be blunt. It is impossible to design and build a system with a fixed firing rate at the boiler and a fixed load area which will reach full capacity of the load under all conditions, but no more (and thus, if modulated by cycling, never cycle).

And to be blunt…………..at least five members of this forum…………other than myself…………have done exactly that.

I tried to explain that the output of a system is variable depending on water content of the steam……….and those with very little water in the steam ALWAYS result in a boiler that is not capable of overfilling the distribution system UNDER ANY CONDITIONS……………and each of these systems were designed with a boiler that was proper for the EDR available.

You are free to believe whatever………………….

First, let's get the FACTS down:

In each of the cited cases, the designer utilized a boiler with a relatively small pickup factor…………15% to 25%…………..with a very carefully calculated EDR.

In each of the cited cases the designer utilized a drop header with ample size to keep the steam velocity below 15 ft/sec.

In each of the cited cases, the designer utilized two risers that extended at least 30" above the waterline.

And, in each of the cited cases, the boiler NEVER developed anything close to 8 oz. of pressure no matter how long the boiler ran.

So, we must conclude one of two things:

1. Each of the cited cases had a designer that was erroneous in their EDR calculation
2. The design of the drop header and risers resulted in very dry steam and the radiation had an output GREATER than the calculated EDR because of the dry steam.

Clearly, there is no exact match to the radiation. The net output is LESS than what the radiators can condense even when run for an infinite time. It is not a requirement to precisely MATCH the radiation with the boiler output. What the evidence shows is that you can have a beautiful system when the boiler output is LOWER than what the radiation can deliver without compromising the capability to heat the building.

Do realize that these boilers have fully insulated piping in the basement and around the NBP thereby further reducing the piping factor and potentially provide MORE energy to the radiators than would otherwise be calculated in the pickup factor.

There is a fundamental fallacy in the generally accepted design of residential steam systems whereby one needs to fully fill every radiator in the building, and obtain the calculated output each time the boiler starts. You would be hard pressed to find a building where the heatloss is not significantly less than the the calculated EDR. Therefore, why install a boiler LARGER than what the radiation can deliver? The old standby of "you don't want to get caught short" given by all the clueless contractors really rings hollow when presented with the FACTS.

Most steam boilers should be installed with not more than a 10% pickup factor together with a a generous drop header and 30" risers. That boiler WILL NOT CYCLE ON PRESSURE.

Had a similar discussion with @The Steam Whisperer a couple of years ago, for different reasons.

Modulating burner controlled by a pressure transducer and a PLC groups those variables into a nice, controllable package.

Back to the OP’s question….I had similar short cycling behavior when my steammax was installed a couple years ago. It was localized pressure due to dirty water and surging. It took a number of skims to get it cleaned out and running properly. Hopefully your problem is as simple as that.

do yourself a favor and pay attention to how he skims it so you can do it yourself. Assuming thats all the problem is, youve likely got a few skimming sessions to go before the problem stops repeating. Itll make your life a lot easier if you don’t have to call for service every time it pops up

Problems with individual rads is probably due to inoperative or incorrect vents on them.

Since at least one of the mains not heating is one of the ones lacking vents, I'd say you have the problem in hand, just need to fix it. I would guess the main that is heating is the one with main vents, but I really don't know. Venting for balance really starts in the basement with getting the main venting sorted. On a warm start it should take less than 5 minutes to fully heat all the mains and then the radiators should start heating.

As far as the water feed, that's actually the preferred location by many. Putting it into the wet return allows it to mix with some of the warm condensate so the temperature difference from fresh water to the hot boiling water is less. That said, I don't think it matters too much either way.