Boiler run time efficiency

LLMANBRX

Is to run more efficient to run a steam boiler for a longer period of time and let it cycle on pressure limits so that the time cycle can be extended (more time between time cycles i.e. from 60 min. to 90 min?

Jamie Hall

Ask any six of us and you will get at least 8 different opinions.

What is not efficient — or as efficient — is to allow the space temperature to have wide swings. Try to keep the temperature swings smaller — that's a thermostat question.

Nor should the boiler control pressure limit be high — in one pipe or most two pipe systems. 1.5 psig cutout is plenty high enough, and in two pipe systems they will run best with around 0.5 psig cutout.

Make sure your venting is ample…

pecmsg

most units need 10 or 15 minutes run time to stabilize so anything less is wasteful.

Jamie Hall

https://forum.heatinghelp.com/discussion/comment/1876326#Comment_1876326

And this is partly where opinions may differ.

It is not a simple or straightforward topic.

I, for example, would agree, but with one important additional detail: just what is meant by "stabilize" in this contest. The fundamental question — for me — is the duration of the off — non-firing — cycle. If that off time is short enough, the boiler may never actually drop off steaming, although at a much lower rate, due to residual heat in the mass off the boiler. Thus, while the system may see a drop in pressure during the off cycle (and indeed has to, for the pressure control to reset), the system as a whole maintains full steam temperature and, for example, vents never reopen.

Under that condition I regard the system as a whole as continuing to be stable and the off time being only there to match the boiler power output to the system demand.

This means, among other things, that the controlling pressure device be set to shut off the system as soon as practical as can be after the boiler output definitely outstrips system demand — and that it recycle back on as soon as possible afterwards.

I submit my own primary system as evidence for this view. It is a Weil-McClain 580 boiler, firing at 3.25 gallons per hour. It is a "vapour" system (specifically "Hoffman Equipped"). The controlling pressure device is a vapoustat with a cuttout pressure of 7 ounces gauge and a cutin of 4 ounces gauge (verified by low pressure gauge). The system is very well power matched, but on very long runs (45 minues plus) the boiler pressure starts to rise (quite rapidly) from the running pressure of 5 ounces gauge, and the vapourstat cuts out almost immediately. The off time is controlled by the post-purge and pre-purge times on the burner, but is short enough that the pressure (again confirmed by gauge) never drops below 2 ounces when the burner refires. Steam delivery never stops. The subsequent burner on cycle would be very nearly equally long — but is interrupted by the thermostat, to avoid excess space temperature swings.

However, if the off time were longer so that steam delivery did in fact stop and the internal pressure dropped to 0 ounces gauge, then the system would not have remained stable and would, in fact, take appreciable time (depending on just how cool things had gotten or how completely steam had collapsed) to recover and become stable again. Again, drawing on observation on my main system, this may take as long as 5 minutes when the system as a whole is still quite warm — or considerably longer (10 minutes in not unreasonable) if has been off long enough to be sensibly cool.

Now @pecmsg is referring — I think correctly — to systems which, for any reason, do have long enough off times to have cooled to 0 ounces gauge pressure. This part of the discussion, though, runs into the question of on and off times relating more to the heating demand — to maintain reasonably stable space temperatures — and runs into the question of what should the overall length of a system cycle — from thermostat on demand to thermostat on demand — while both avoiding excessively short thermostat demand times (which in a system starting cool include that warmup time to stabiity ) or excessively large space temperature swings.

LLMANBRX

Thank you Jaime &

pecmsg

The boiler I am referring to provides heat and hot water to a 1. 60 unit 13 story apartment building (two pipe system) with convectors. The boiler control (Heat timer Model MPC) is running a time cycle of 1 hour. The pressuretrol is set to cutout at 2psig and cut in at .5psig. The problem is that after the heating cycle finishes the boiler boiler cools significantly and takes appox. 17 minutes each heating cycle for steam to reach the furthest convector from the boiler. Once steam is established at the furthest convector, the length (time) of the heating cycle is determined based on the outdoor temp. It seems to me that too much fuel is being used between cycles to establish steam once again at the furthest convector. It occurred to me to lengthen the run time of the boiler and let it shut down and then restart on its pressuretrol limits so it does not cool down. Then increase the length of time between heating cycles to more than 1 hour. Say to 1.5 hours as an example. This lets the boiler heat longer with steam established instead of cooling down and restarting. I think it may affect the temperature swing somewhat but if it is a fuel saver that is really what I am after.

Jamie Hall

I like your approach, @LLMANBRX . The only real downside I can see to that is that you may have more temperature variation in some of the units than might be really desirable.

You are correct, in my view, that that time spent establishing steam throughout the system is rather inefficient — when the boiler is finally running at its more ore less steady state, that is your best efficiency.

The only thing I would wonder is if anything can be done to improve the time to establish steam. I presume all your mains are well insulated? There is a way to check main venting — and it's not hard, although you will need a good low pressure gauge (0 to 3 psig). If you have one, then observe the pressure from a cold start. What you should see is 0 until the boiler starts to steam, then a fairly moderate speed rise to some intermediate low pressure (which will vary with the system piping — on the system I reference it is about3 ounces gauge). And then — and this is the important part — it should hold very close to that pressure, perhaps rising very slowly, until all the radiation is really filled with steam. Then it will rise about as fast as it did in that very first initial rise, and that is where the pressure control should shut the boiler off. If, in the other hand, the pressure continues to rise significantly, although perhaps more slowly, after that first rise then you need more main venting. This is all a bit harder to describe than to do! Judgement calls…

LLMANBRX

Jaime,

This two pipe system uses combination vacuum/condensate pumps pull the steam through the system and return the condensate to a tank which then pumps the condensate back to the boiler. There are no vents on the steam mains. You had mentioned that a two pipe system should operate on .5psig. I will have to observe the boiler pressure gauge to see what the reading is when the apartment with the furthest convector gets steam. If I can cut down the pressure that the system is currently operating at so that the boiler goes on and off by pressure limits thereby allowing me to keep steam in the system longer without the boiler running the entire time cycle. It obviously does no good to run the burner when the system is filled with steam to capacity. That is where I thought some savings could be had by lengthening the time between heating cycles. The downside maybe a bit wider temp. swing for some units.

Jamie Hall

There is a very common — and simple — assumption that the steam will simply push the air out of cool main line — or in your case, follow the air out as the pumps evacuate it.

It would be nice if that were so, but it isn't. What takes time is for the steam front to heat the pipe up enough so that it no longer is condensing at that location. This is why enough venting is enough — and more won't make any difference. And why concentrating on that low pressure operating plateau is so important. And why insulating steam mains is so important. And why that initial warm up is inefficient — the steam is happily condensing in the mains rather than usefully heating the space.

mattmia2

It will have some sort of crossover trap that acts as the main vents, at least usually they do.

LLMANBRX

You state it very succinctly. All the mains by the way are well insulated. All return lines are also well insulated. I did not know that a two pipe system should operate at and cutout at 0.5 psig. I imagine that the cut in must be 0.0psig. Thank you for that information. I will now observe the boiler pressure gauge reading when steam is established at the radiator furthest from the boiler. Thank you for taking your time to help me.

Jamie Hall

A cutin pressure of 0.0 psig is a bit risky, as not all systems will accurately drop to 0.0 psig. and reset the control. Better to leave the cutin a bit higher — say 0.02 to 0.1 psig.

mattmia2

It depends on the type of 2 pipe as well. 2 pipe systems with some form of thermostatic traps can work at much higher pressure differentials than vapor systems.

LLMANBRX

mattmia2

We have thermostatic steam traps on all building convectors. We perform maintenance on them (replace the trap or trap element every 3 years) so that no steam gets into the system on the condensate side. I believe that the steam trap itself acts as a vent also.

mattmia2

the steam trap does not act as a vent but it allows the air out of the radiator in to the return so that the steam can get in to the radiator then the return is vented or in your case has a vacuum pump on it.

if the mains don't have crossover traps and all the air in the mains vents through the emitters that could explain the last emitter being slow to heat.