cast iron hot water boiler and radiator system efficiency

megaretro

Hello, long time reader first time poster, greatest heating resource on the net here IMHO.

I bought a house last year with cast iron radiator heat. the 40 y/o weil-mcclain conventional cast-iron boiler was still humming along but was just too old to be relied upon any more and was a huge gas hog (150,000btu). Yes this boiler was way oversized as the house is about 1300sq ft livable (2200sq ft if you include unheated attic and basement space). Needless to say the boiler could probably heat this house with the windows wide open all winter.

The replacement boiler put in was a 125,000 Dunkirk conventional cast-iron with a similar output to the removed boiler. Still oversized yes but this was chosen based on the BTU ratings of the cast iron radiators in the house, as opposed to heat-loss calcs. And it is an old and drafty colonial with no insulation in the walls. The boiler went in just before the end of winter last year, so I at least know the system works well.

The new boiler was put in with a primary/secondary loop setup with a 6-inch or so crossover pipe between the boiler flow/return to allow some hot water to cycle directly back to boiler to reduce possibility of condensation- forgive my lack of proper terminology here. There is a continuous duty circ pump on the primary loop (house radiators) and a regular high-efficiency taco circulator on the boiler driving the 'secondary' loop when the boiler calls it. This said, the primary loop continuous duty circ. pump was wired by the contractor to the heat call relay and as such is only running when everything is running when there is a heat call, and does not run otherwise- which leads in to one of my questions below.

I want to maximize my efficiency of this system. first off I future proofed things by re-wiring my thermostat from the 2-wire cable that was previously installed and left in by the contractor that put in the new boiler. So I re-wired with a proper cable while re-locating my thermostat and now have power at the thermostat and extra leads for any changes I want to make in the future.

1. I changed the CPH (cycles per hour) on the system (thermostat) from 3 to 2. This seems logical to prevent short cycling as it should force longer cycles from the boiler at the expense of higher temp swings in the house (but even at 2 CPH the temp swing is hardly noticeable when you have HUGE cast iron heaters in the house). Is this thought process correct? My hope here is to lengthen cycle time to raise efficiency and reduce wear-and tear and possibility of condensing.

2. Max temp. What should I do here. I believe the boiler is currently set at a 180deg (f). max temp. This kind of seems overkill for an oversized system with big cast iron rads. and I was thinking of dialing that back to 160f, any reason not to do this?

3. boiler built in hot-water temp reset/economy function. The boiler has an economy feature which limits water temp to a varying temp. based on previous number/length of heat calls. Essentially if a heat lasts long enough, the boiler will kill the gas burner while continuing to cycle water through the boiler. This economy feature would most often come on only during long heat calls, like when returning from an overnight temp setback. I can turn 'up' this economy function to force the boiler to kill the gas burner sooner/more often during a heat call- should I do this in an attempt to minimize gas usage? My gut tells me to raise the economy feature since the system is already likely to run shorter cycles than a properly sized one- or would this just increase wear and tear?

4. setback. we don't need to beat this question to death as the answer is elsewhere- but I am still undecided on using my programmable thermostat to dial back the temp at night because the system must run so long in the morning to bring all that mass back up to temp. It seems it may be better just to leave the thermostat at one temp. I compromised by the end of last winter and had set the system to go back only a few degrees and recover only 1-2 deg at a time- 2 times and hour apart in the AM. But I've also read the wisdom that there's nothing wrong with the boiler running a long cycle for recovery as it's efficient. So I'm torn here. Any input or thoughts or a preferred answer regarding temp-setback on a higher mass system?

5. I mentioned earlier how the primary loop continuous duty circulator is just simply wired to the heat call relay and simply runs with the rest of the system. I've seen a couple ways this might be improved. For example maybe using a 2-stage setup where the 1st stage is calling just the continuous-duty primary loop pump, and the 2nd stage is used to call the boiler for heat. this setup could be done pretty easily here I'd just need to buy a 2nd relay. The other thought here is to just wire the continuous duty motor to run all the time regardless of thermostat status, the only issue here is that it seems somewhat wasteful and the pump will run even when there is no heat needed at all unless you kill the boiler's electric. thoughts on this front?

sorry for the long winded post. Just wanted to have some input on this while I still have some time to deal with it and I didn't want to have to start a dozen threads. Thanks in advance!

Zman

It sounds like you have done your homework and have an good understanding of your system. Figuring out how to make your system more efficient can be confusing and with all the possible settings, it would be pretty easy to chase your tail indefinately.

You will save some energy by turning down your boiler temps. You can do this right on the boiler's aquastat controls. You do need to pay careful attention to your return water temps. Be sure the return temps stay above 130 degrees or you will damage your boiler. If you find you need to run the boiler hotter on the coldest days, there are outdoor reset controls which will adjust the temps automatically.

Your other questions all seem related to boiler cycle lengths. How long are your typical cycles?

Changing the cycles per hour at the t-stat, as you mentioned, is trading length of heat call for thermal comfort. Unfortunately a longer heat call does not necessarily mean longer boiler cycles.

Indoor temp setbacks are similar to cycles per hour. I would be surprised if you saved any money as you are just cooling the house off just to heat it back up again.

Boiler cycle length is dictated by the size of your radiators, output of your boiler, mass of your system, boiler water temps, and boiler on/off differential. The whole thing can be a bit confusing as there are several control strategies to tweak it. Most are just changing the on/off differential on one way or another.

I think your next step is to observe your boiler when it goes through a typical heat cycle. How often does the burner turn off and on? At what water temp does it turn on and off?

If you can get your system to the point where the boiler is running 10-15 minute cycles at the lowest allowable water temp, there is little else you can do to improve efficiency with your existing setup.

megaretro

"Inconceivable!" :-)

Thanks for the response, let's see if I can answer back some of your questions.

"You will save some energy by turning down your boiler temps. You can do this right on the boiler's aquastat controls. You do need to pay careful attention to your return water temps. Be sure the return temps stay above 130 degrees or you will damage your boiler. If you find you need to run the boiler hotter on the coldest days, there are outdoor reset controls which will adjust the temps automatically."

Ok so this boiler has a "fuel smart hydrostat" in it. Per the manual for the control: "the hydrostat utilizes Thermal Targeting Technology to operate the boiler at reduced target temperatures during off peak demand or at the high limit setting during peak demand"
Basically, the boiler will likely never hit 'peak demand' and be pushed to it's max temp setting- BUT *if* it were to hit that max 180 degree setting for some reason ever, I'm concerned the rather small expansion tank on the new system may not be enough and there could be a possibility of high pressure forcing the pressure relief valve to open. This is the main reason I am thinking of lowering that max temp to 160-170f. As far as return temps go, the auqastat controls the boiler's circulation pump to limit cold water coming back to the casting- when the system is cold it doesn't even circulate the water through the boiler until it's heated up a little. I'm hoping that it's designed properly. :-) Return temps do not hit 130 until the system has been running for about 5 minutes into the heat cycle.

"Your other questions all seem related to boiler cycle lengths. How long are your typical cycles?"

During normal operation, the system cycles would last no more than 15-20mins. Only when using temp setback would the system have longer cycles when it was recovering from the setback.

"I think your next step is to observe your boiler when it goes through a typical heat cycle. How often does the burner turn off and on? At what water temp does it turn on and off?"

During a normal heat cycle the burner would just normally stay on, with the boiler never reaching a temp where it decided to kill the burner and keep circulating water. The exception to this would be during recovery- in this case the heat call would indeed last long enough for the boiler to hit it's pre-determined max temp and temporarily shut off the burner for a few minutes while still circulating water. This is something I have some control over- by adjusting the auqastat's economy setting I can make it turn off the burner at lower temps than it currently does, which could indeed get it to kill the burner more often during a typical heat call/cycle- *but* I don't know if this would have any benefit or maybe even the opposite it could cause more wear-and-tear with more variation in the casting temp during a cycle?

"If you can get your system to the point where the boiler is running 10-15 minute cycles at the lowest allowable water temp, there is little else you can do to improve efficiency with your existing setup."

Following this logic, and with the boiler being oversized as it is, we might already be close to this 10-15 min cycles and lowest allowable return temps.

So I'm still unsure about lowering my high-max temp limit, as the system probably will never even hit that max..

And I'm still wondering about raising the economy setting to make the burner cut off more often, (at lower temps than it currently would shut off at)

So the only other question I really have is how to take advantage of my 2 pump setup (primary and secondary loops).
Right now the system is wired with both the boiler and the primary loop circulator connected to the same heat call relay- where the primary loop (radiators) circulator only runs while there is a heat call. I'm curious if there's any advantage to setting this up differently (it is after all a continuous duty motor).
The secondary loop is the boiler's loop and that runs only during a heat call and is controlled by the boiler's aquastat- obviously this would not and should not be changed.
I have stumbled on a cool solution where the primary loop circulator could be wired to the 1st stage on a thermostat in 2-stage setup (with 2 relays). So the primary loop runs whenever the thermostat hits it's first stage heat call and then the 2nd stage heat call is used to call the boiler for heat.
The idea here is this keeps the 1st stage (primary loop) circulator off until the house temp drops to the thermostat set point, at which point it starts circulating the radiator water for the season (or until not needed), and when the house temp drops a little lower the 2nd stage then calls for boiler heat.
For some reason I'm drawn to this solution as it seems a clean way to have the continuous duty primary loop circulator run for the season (instead of kicking on and off with each heat call) while still making it easy to shut off the primary loop just by lowering or turning off the heat at the thermostat.
Any thoughts on this? I guess it comes down to "is there an advantage to having the radiator water circulating constantly (or almost constantly) for the season?" (Maybe it keeps temps more balanced from radiator to radiator or keeps heat from being 'wasted' by sitting still in a pipe somewhere?)

leonz

Someone more knowledgeable than I can answer you this for sure;

I do not know if Installing trv's on each radiator to control the temperature with a two pipe system would work or simply throttling back the wet steam feed with the shut off valve would be a better way for you to heat the home. I know that some of the plumbers on the board have been converting 2 pipe systems to one pipe systems to simplify the steam boiler operation.

hot_rod

Sure, the two stage concept has been around for many years, I remember an IBR class that cover it in the 1970's! Run the pump first, then fire the boiler on second stage.

Without a drawing I not sure how your system is piped and which loop is which?

There are two stage T-stats for doing this and the offset is adjustable.

Constant circulation is common in European systems, they just pulse heat into the loop. Usually controlled with an accurate ODR control. It can even out temperature swings and provide ideal comfort. I'm not sure of the energy savings?

Canucker

The system I originally had when I moved in to my house was set up like that @hot rod The thermostat controlled the boiler firing, pump ran constantly. It was very comfortable

Paul Pollets

Cast iron boilers don't come with integral computer controls. To achieve highest efficiency, install a Tekmar outdoor reset control that can be selected for the various methods of piping, zoning or use of zone valves or pumps. The control can also sense return temps with a sensor and prevent thermal shock. A manual bypass is not the best way to assure correct return temps. See:
http://www.tekmarcontrols.com/

hot_rod

sketch up the piping and post it. Seems odd that you have primary secondary and a boiler bypass, or that it is needed if the boiler is way oversized.

Any idea of the actual radiator output, were they measured and output calculated. Or a accurate, current heat load calculation? Those are key numbers to know when trying to fine tune as you are.

Of course any upgrades to the building envelop works against an oversized boiler, as far as efficient operating conditions due to short cycling.

Because the heat emitters, radiators in your case, in all cases, are in charge of the boiler operating condition, not the opposite.

Left to it's own devices, all controls removed from the boiler the system would seek and find a point of thermal equilibrium. With an oversized boiler that point would probably be way above a safe operating temperature, well above 200F.

With a single output boiler, there is only so much you can do to try and match it's output to the radiator sizing, and load of the home. Controls mentioned can help limit cycling and possibly lower operating temperatures. You have a minimum temperature that must be maintained to protect the boiler from condensing conditions.

If you get 10 minute or more run time with a system that has an oversized boiler, you have done well Mild, low load days will probably shorten that boiler run cycle as the boiler has only one speed, full speed ahead whenever it fires.

Mod con boilers are a good option when you have the ability to run low temperatures and want an output modulating feature. If the system could supply adequate heat with 140SWT on a design day, and lower on milder days, the mod con may run in 90% plus efficiencies most of the year. A conventional boiler, maybe mid to high 80%. You cannot "fool" or design that efficiency number higher in a non condensing boiler.