Approaches to reducing boiler size

about 2,000. Cedric is fired for 1,700.

The logic behind keeping the burner off part of the cycle ishort is to avoid the efficiency loss from reheating the boiler itself — and to avoid allowing any air back into the system.

The overall HEATING off cycle is controlled by the thermostat, not the boiler, and is a different matter altogether. In my case, Cedric is controlled by a mercury T87, centrally located, and there is a temperature swing in the structure of about 1 degree between the call for heat and the temperature reached after any overshoot from residual heat in the radiators (the T87 switches back off at about half a degree above where it switches on, the rest is residual heat). The time between calls varies with structure heat loss, and can be several hours — or less than half an hour (actually even less, if it both windy and cold — which, thankfully, rarely happens; I've never seen the system actually overpowered).

I estimate that the overall heating power available from the system is within 10% of the maximum historic demand — hardly what I would term overdesigned, either from the standpoint of the boiler or of the installed radiation.

I quite agree. I try not to, but…

@PMJ said: "My point is that there is no actual advantage to totally filling these systems with steam ever. They were not designed to do that, and because as we all know lots of extra radiation is installed and totally filling it is never required to heat the structure."

I might Change one word here:

My point is that there is no actual advantage to totally filling My systems with steam ever. My system was not designed to do that, and because as we all know lots of extra radiation is installed on many systems just like mine, and totally filling it is never required to heat the structure.

And that, in truth, makes sense within this case.
For Paul’s steam may never reach one pound,
Nor Jamie’s “Cedric” build a single PSI —
(And who bestows a name upon a boiler?)
Thus radiators filled with steam throughout
May happen only on design-day cold,
And rightly so when winter shows its teeth.

Yet other plants, oversized in scale —
Both boiler mass and radiator spread —
May find full steam in every section cast
Invites an overshoot of roomly heat.
For every system lives its private law;
No twin behaves in quite the selfsame way.

In such a house as PMJ describes,
One call per hour may prove too long sustained.
Two measured calls within that hour’s span
May better suit the building’s tempered loss
And guard against excess beyond the mark.

The thermostat now governs much of this:
Whether the old round T87F,
Whose anticipator warms by wire’s glow,
Or modern stats with cycles set per hour —
Their logic shapes the overshoot we see.

Yet if, mid-call, the pressure climbs too high
And forces short returns of flame — two on,
Then three minutes off in restless turn —
Some tuning should be made before we dream
Of casting out the boiler for a new.

Perhaps reduce the firing rate with care,
Or set the pressure control somewhat low,
Or mend the venting so the air escapes
Before the gauge begins its upward march.

For steady flame within a call for heat
Is far preferred to cycling born of strain.

This poetic verse was brought to you by ChatGPT

I asked it to change this into a poem:

And that makes sense in this situation.

Paul’s system may never reach 1 PSI of steam pressure — nor does Jamie’s Cedric (and honestly, who names their steam boiler?). In those systems, the radiators may only completely fill with steam on true design days, and that may be entirely appropriate under those conditions.

On other systems — particularly those that are oversized in both boiler capacity and radiator EDR — completely filling the radiators with steam can lead to excessive overshoot. Every system behaves differently.

For someone with a system like @PMJ is describing , it may not be desirable to drive the system to full steady-state operation and limit it to only one call for heat per hour. In that case, two calls for heat per hour might better match the building’s heat loss characteristics and reduce overshoot.

Thermostat behavior plays a significant role here. The accuracy and anticipator logic — whether it’s the classic anticipator in a Honeywell T87F or the cycles-per-hour (CPH) setting in a modern electronic thermostat — largely determine how much overshoot occurs.

However, during an active call for heat, if the boiler pressure reaches the pressure control setting and begins short-cycling — for example, two minutes on and three minutes off — that indicates a mismatch somewhere in the system.

Something needs tweaking, short of replacing the boiler with one more closely matched to the connected radiator EDR, other adjustments should be considered, such as:

• Reducing the firing rate (if combustion parameters allow),
• Adjusting the pressure control setting,
• Or improving system venting to reduce pressure buildup.

The goal is stable operation during a call for heat — not excessive pressure buildup followed by rapid cycling.

@EdTheHeaterMan , Thanks for the comments. I see folks running for the exits under the "Every System is Unique" banner but ok. My Mouat system is 100% stock piping as installed 100 years ago and obviously they did many systems to this standard. Granted, many systems have had unfortunate modifications done. My purpose is to discuss fundamentals which I don't think change all that much. Some systems are so baduly unbalanced that there is simply no way to get steam at all to some areas without pressure. I get that. But it is possible to discuss fundamentals still.

I am in this space on this thread because yet once again I see a case crying out for PWM which would help this OP's situation a lot more than any suggestion here. I get that there is no product available to help with that. It is very sad how all attempts to address this - even really good ones like Ecosteam have been rejected here as not worth it. Given the control tools developed especially in the last 30 years I find this astonishing. Extra boiler is easily tamed with PWM and is a lot less expensive that anything else.

It's the same drill, get a smaller boiler, adjust your pressure device better, buy some new vents, find someone to safely reduce the fire. All of which I think just avoid the elephant in the room. I like my big boiler, I don't use a pressure device anymore, I removed all the vents, and I didn't touch my 69 year old boiler fire. This is on completely original piped 100 year old Mouat system. My system is not some one- off.

The orignal vaporstat contol is still in place. The system just will never run long enough to develop enough pressure to trip it. If my plc control failed on somehow then the vaporstat will stop the boiler.

In addition, I have a clearly marked switch on the front of my control box that reverts all control right back to original.

How are you going to condense steam well into the burner off time?

If the burner is off where's this supply of heat coming from?

Do you have a way to create steam without heat?

The tiny amount of energy you can pull out of a modern residential sized boiler is so tiny it's irrelevant. And then you get to put it back the next time the burner fires anyway.

But we're talking a whopping 5000 btu/h if that and that's a pretty deep vacuum.

I'll stick with the Ecosteam.

I think I finally get it. There are no fundamentals that apply here that can and should be discussed because every situation is different. Um well, ok.

Regardless of the resistance here, I will stick to the fundamentals anyway, and discussion of how they apply, which I think is what most readers are here for. Such as my statement that by definition how long the calls are is directly related to how even the heat is. For if the heat is perfectly even, the tstat would never see any change at all and there would be no cut ins or cut outs and calls for heat will be endless. Can we at least agree on that as a fundamental that applies everywhere to all systems? From there I simply observe that the standard control design will produce the shortest call for heat the boiler can possibly create, running it on high stopping only for max fill and pressure in the system until the tstat is satisfied. I then conclude ,correctly I think, that the standard control design is really not aligned very well with a desire to produce even heat. I'm not saying that everyone needs to desire even heat. I'm simply saying that if one does, the standard control is not well suited to produce that result, or certainly not near as good as it could be.

My Engineering School training brought me to this fundamental realization pretty early in my experience with my first steam system 34 years ago. It was the dramatic temperature changes the radiators went through every heat cycle that I didn't like. That is why I ended up working to improve the situation, which I have immensely, by identifying and applying the fundamentals like I was taught. I have a stock 100 year old piping system exactly as originally designed. My guess is that it is very similar to a lot that is out there and the things I am saying do apply quite generally.

good plan. carry on!