When to test pressure

Zipper13

I've been told again and again here that my 2 pipe Hoffman vapor heating system requires really low pressure (~7oz) and that the best ways to achieve that pressure is with a vaporstat and well-insulated mains.

While I can install the insulation and pressure control on my own to save a few bucks, the parts themselves are real money.

I figured the first thing to do before making these investments would be to verify the operating pressure to assess the utility of the upgrades.

I installed the gauge as shown below, opened the valve and let it run for about 35 minutes. The mains started cold, but the boiler was already a bit warm so in about 10 minutes, I the entire main was hot. By 30 minutes, all rads were hot across the top (some were fully hot, but my really big rad which has never been 100% hot all over was still cool toward the floor).

The highest pressure I read was just over 2oz plus the reported 3% error on the gauge.

Sounds great, right? No need for the vapor stat since I'd never reach the cut out anyway! Well, I don't know actually.

How frequently and in what conditions (basement temp, setback,run time, etc) would I expect to see my pressure really start to build I should really only take a reading in a "worst case" scenario, right?

Hap_Hazzard

Highest pressure would be reached on cold days after a setback. The greater the temperature increase, the longer the boiler needs to run. If the desired temperature is reached before the pressure builds up to the cutout point, it will be shut down by the thermostat rather than the vaporstat.

Jamie Hall

Is your system really fully Hoffman Equipped? Like... the Differential Loop is still there and hooked up? If so, there is an easy way to see if you need to worry about a vapourstat (and yes, they are pricey -- sorry about that). That Differential Loop is designed so that if the pressure difference from the boiler to the dry return goes over 7 to 8 ounces, the Loop will "trip". Which, in brief, simply allows live steam from the boiler to get into the dry return at the loop. So... in normal operation, that dry return should never be hot. Warm, perhaps, but not hot or even close. If it's hot, the Loop tripped -- which won't hurt anything except your fuel efficiency.

As @Hap_Hazzard said, the time you're likely to see it is when you are recovering from a significant setback -- or when you are down close to design conditions and the boiler is running a long time.

acwagner

How closely does your boiler output match your radiator EDR? I'd start there to determine what conditions would cause the pressure to build. Right-sized boiler means you may never build that kind of pressure. Oversized boiler means you could hit it fairly often.

Zipper13

I guess I was of the belief that pressure was a more omnipresent problem. In reality, for me at least, it sounds like it may only be something that we encounter on particularly cold days when when the system needs to work extra hard.

I will definitely be checking the pressure reading periodically - especially as we get into the cooler months as it runs longer and as the basement and the uninsulated(!) main gets colder.

Not sure of the EDR. I have 5 different styles of radiator and have not tracked them down yet to calculate.

I do have one larger radiator that has never gotten completely hot. I interpret that to mean that steam never fills it enough to
reach its trap to close it, which mean that it continues to be able to vent throughout the entire burn, so there would be little opportunity for pressure to build since the system remains open at to the main vent ( an original #11, I believe, mounted on the differential loop) through that open radiator trap.

I've never noticed any of the piping connected to the differential loop to be warm at all. So assuming that the thing isn't all gummed up and is in working order that suggests that it never trips I suppose!

ethicalpaul

> when the system needs to work extra hard.

I always find this type of description interesting. Not picking on you, but the system has no idea of how cold it is outside and isn't really working hard or differently than any other time. It just runs longer before the set point is met.

> I do have one larger radiator that has never gotten completely hot. I interpret that to mean that steam never fills it enough to
reach its trap to close it, which mean that it continues to be able to vent throughout the entire burn, so there would be little opportunity for pressure to build since the system remains open at to the main vent ( an original #11, I believe, mounted on the differential loop) through that open radiator trap.

Sorry for the additional nitpick but I really like the finer points of these topics.

I wouldn't think of it as "continues to be able to vent throughout the entire burn". That is true, but implies that there is continuous venting going on. A better description I think is that you have a nice large radiator that is giving up a lot of heat during your burn cycle. It is probably reaching an equilibrium where the steam gives up its heat and condenses to water resulting in little to no actual venting occurring once the radiator gets to the point where you notice it stops heating new sections.

And yes I agree completely that pressure isn't going to ever build as long as there are one or more cases of this radiator equilibrium happening in your system. There is a continuous cycle of water -> steam -> your big radiator -> water and no pressure will result.

It may be that you actually have a correctly-sized boiler, congratulations, it's all too rare. And if I'm wrong about any of this, I count on a veteran slapping me into correctness which is a great way to learn!

acwagner

I'd run the EDR calculations before doing any changes or buying anything. It only takes a few minutes and will give you a better understanding of your system.

Zipper13

@ethicalpaul thanks for the nitpicking - I appreciate the clarity. Somehow the concept of the steam entering and condensing in a radiator in equilibrium (or pretty close at least) hadn't occurred to me before. So OK, even though the trap remains open and able to vent there is no net steam volume increasing to displace air out - neat!

ethicalpaul

The idea took me awhile to get a handle on as well. If you had enough of an oversized boiler (and I'm not saying you don't--your measurements will tell you), then the boiler's ability to create steam would be greater than all your radiators' ability to condense it, and they would then heat all the way across and close traps and cause cycling on pressure.