Can you modify a cast iron boiler so it modulates?

john123

It appears that there are some good reasons (overall costs of purchase, installation, maintenance, repair and fuel?--and dependability?) to pick the cast iron over the newer style wall hung boilers; at least some people think so.--but the sticking point seems to be they don't modulate. Is there some way of overcoming this.

I would think it would be helpful if we could just turn down the power by 50%? in the Spring and Fall. Could you just insert a valve on the gas line before the boiler and manually turn it off by 50% (approximately of course)? Why doesn't someone make and sell a modulating gas valve?

JohnNY

Yes. You can install a 2-stage gas valve and an aquastat or pressuretrol to trigger the second stage, or not.
About 10 years ago this board was full of chatter about how to make it work. I've done it three times for clients and it works well.

edited to add:
The gas valve for boilers up to 300 MBH is the Robertshaw 700-053.

john123

Will any gas valve or generic? gas valve fit any boiler?---or be made to fit? I thought you had to get the gas valve as spec'd by the mfg. Would the Robershaw 700-053 fit a slant fin galaxy, do you think? It looks the same (or a little bigger perhaps?).

But is there any design problem in running a cast iron boiler very much below its (advertised) BTU/hr spec? If you were to jury-rig some sort gas control.

I spoke to someone familiar with Laars Teledyne and he thought that they can't get the efficiency high enough if the boiler modulates---so they don't do it..

Jamie Hall

Efficiency is a major handicap. But if the flame pattern can be controlled so as to provide good heating to the combustion chamber and other passages... it isn't just a matter of running a smaller flame.

fentonc

I'd love to see a comparison of the efficiency loss from reducing the output (what exactly is the cause, btw? increased O2 in flue gases?) vs the efficiency loss from short cycling. My nominally 140K BTU/h boiler only clocks 120K BTU/h at the meter, but it also usually only fires for 70 seconds at a time before the aquastat cuts it off it off when only 1 zone is calling for heat.

SuperTech

I modulate the high limit temperature of my cast iron Peerless WBV-03 oil fired boiler with an outdoor reset control on my aquastat.  It needs to be setup so the return temperature stays above point where the boiler condenses.
The high limit minimum is 140⁰  when the boiler is running during milder weather 190⁰ for heating during the peak of winter.  I don't know if this will work as well with every cast iron boiler installation, my boiler is piped primary/secondary with variable speed injection pumping to decouple the load from the boiler and keep the boiler above condensing temperature during operation. 

Jamie Hall

There are two problems with efficiency (at least), @fentonc . One is, as you surmise, control of the fuel to air ratio. There are ways to do this -- otherwise modulating boilers wouldn't be feasible -- but the burner and controls have to be set up for it. The other is firebox and heat exchanger design, which has to be arranged in connection with the burner that all, or most of it at least, is heated by the flame. The newer modulating boilers are designed this way, and work well enough. There is a third factor, which is that the entire boiler must be veery well insulated -- the heat loss from the boiler is pretty well fixed by the boiler water temperature, so as you reduce the heat input by modulating the fraction lost to that loss becomes greater.

I'm not suggesting it can't be done -- obviously it can. But it's not simply a matter of cutting back the flame and hoping.

Big Ed_4

I would also modulate a cast iron boiler with temperature output ... outdoor reset control

Mad Dog_2

Yeah...not something to Tinker with. Just manually shutting a gas cock off halfway, can throw off your combustion numbers dangerously.  This is for a very experienced burner man who can work his Combustion analyzer in his sleep 💤 😴   mad dog 🐕 

random12345

Wouldn't the stack temperature get so low that there's a risk of flue gas condensation in the chimney?

EBEBRATT-Ed

The average cast iron atmospheric boiler is not designed to be run with less than full fire. Can it be done yes, but only by using combustion test instruments to check combustion and flue gas condensation must be avoided.

And in the end you may find you paid good money for for a gas valve and an extra pressure control or aquastat and cannot use them.

@john123 If this is a hot water system there may be other things you can do that will be more cost effective.

1add an outdoor reset control may save fuel

2 Add a strap on reverse acting aqua stat to run the pump after burner shut down to extract the heat from the boiler. You need to shut the pump down when the water has cooled to 130 or so.

#2 is the most cost effective.

john123

@EBEBRATT-Ed : so can you help me here: I have noticed that there appears to be no difference in the specs of the Slant Fin 75/100/125---same gas valve, same HX, all the parts seem to be the same, (except the 75 has a 5" exhaust)--Except for the number of burner tubes 3/4/5. Does the gas valve have settings? Could you just remove a couple of burner pipes in the Spring and Fall to "modulate" the boiler? How does the does the gas valve work anyway?

bburd

A standard on/off (non-modulating) gas valve has a maximum capacity, but below that the gas flow rate is controlled by the pressure regulator setting and the size of the burner orifices. The same gas valve can be used on appliances with different firing rates.

clammy

Many years ago hydrotherm had a 2 stage hi low fire cast iron atmospheric vented hot water boiler it can w a 2 speed inducer fan . They worked fairly well and there were a fair share of them out there at one point . I would think a 50 % turn down would leave to some serious issues combustion issues , a 30 % reduction of input on cast boiler using a 2 stage gas valve and the proper controls and secondary limits should be fairly doable and still maintain proper combustion with in specs . Again this is not for the tinker or YouTuber thinking any one can do , it s for a professional and after All is said and done he assumes the responsibility being most manufactures warranties would be void from altering the equipments design so the pro would assume all responsibility does sound like the wisest business decisions .
On a hot water boiler I doubt I would get involved but on a cast iron steam boiler it’s not as big deal to bounce a 2 stage valve off a pressure troll and in the case of a over sized boiler it’s a no brainer. There where a few non condensing 3 to 1 turndown wall hung boiler Utica cub comes to mind not to expensive and chimney vented and designed w some modulation I ve installed 1 cause it fit the bill for the job and it’s been running without any issue knock on wood .
Peace and good luck clammy

fentonc

I can understand 'physical' issues with flame stability, igniting all the burner tubes properly, etc., and condensation issues if the return temp never gets high enough, but what would cause issues with combustion byproducts? Naively, it seems like turning down the input would still always result in complete combustion (and from what I've read, having more O2 in the output actually lowers the condensation point of the flue gases, helping somewhat with that issue).

STEVEusaPA

Put a buffer tank in. Run the zones and their water temps off of an outdoor reset, off the tank. Let the boiler run to recharge the tank, using a high differential (40°?) No condensing, no short cycling, etc.

john123

thanks for all your comments, and @clammy, I am right with you in terms of getting a specialist (or knowledgeable) HVAC tech to do any modifications. I am interested in getting a better system and it helps if you know why something can't be done! (or what might be considered).

EBEBRATT-Ed

@john123

What @STEVEusaPA suggested may be the most bang for the buck. The boiler can probably be downfired somewhat but I don't think you will get what your looking for. JMHO

pecmsg

Two boilers. 1st stage 2nd stage. 

Of course the install cost is doubled but you have a back up. 

john123

fentonic questions why turning down the flame would create combustion problems. Is there an obvious answer?

john123

pecmsg said:

Two boilers. 1st stage 2nd stage. 

Of course the install cost is doubled but you have a back up. 

Do you think it could be done by running the 2 boilers in series without hydraulic separation since the cast iron hx's are so large?

EBEBRATT-Ed

@john123

I wouldn't recommend the boilers in series. I would use primary secondary piping.

Turning down the flame creates the problem of flue gas condensation which must be avoided.

Also, reducing the flame on a cast iron boiler usually lowers the efficiency because there is no way to reduce the combustion air when the gas is reduced. They just are not designed for it.

You can probably down fire the boiler a little bit. The gas pressure on the burner is normally 3.5" of water column. You can usually reduce this to around 3" which will reduce the gas flow but this must be proven with a combustion test.

When the gas flame is reduced by whatever method the airflow drops some because as the flue gas runs cooler (from less gas input) the draft which pulls the combustion air through the boiler is reduced. That is why combustion testing is a must

WMno57

Is there a gas power burner with a variable speed fan?

Jamie Hall

john123 said:

fentonic questions why turning down the flame would create combustion problems. Is there an obvious answer?

There are at least two. The most obvious one is controlling the air/fuel mixture ratio. It is possible, with some types of venturi mixing (think carburetor) to get reasonably tight control, provided there is no fixed air inlet in addition. It is instructive to go a bit outside the box, though, and take a look at how a modern fuel injected internal combustion gasoline engine controls this (and they do it very well indeed). Basically, variations in power demand are met by controlling the air flow, which is then sensed by a mass air flow sensor or other similar device, and a computer is used to control the amount of fuel injected to match -- which is then further modulated by sensing the oxygen level in the exhaust which feeds back to trim the fuel flow Modern gas turbines do it you might say in reverse -- changes in power demand are met by varying fuel flow, but various sensors and computers are then used to alter the air flow to match. Either way works. Neither approach is even remotely inexpensive, if it is to maintain any kind of tight control (and efficiency).

The other problem is one of incomplete combustion. One would think that less gas would burn more completely -- but that it is not necessarily so. If there isn't enough fuel/air mixture in a space, the flame temperature cannot get high enough for complete combustion to take place. It simply cools off too fast. There is no way to fix that short of changing the combustion chamber geometry.

hot_rod

If this were a viable option I suspect the boiler manufacturers would offer it?

That would prevent the mod cons taking all the cast boiler replacement market

For me the modulation is more important on system that may never condense anyways.

john123

I think the problem from the Mfg's point of view is that modulation drops the efficiency below the government mandated level, perhaps even because the electric current required to run the modulation reduces the overall efficiency. (What do you think their problem is?) The Mfg can meet the limit if there is no modulation and therefore is allowed to produce and sell the boiler. But would that mean that a tech who changes things to allow modulation and reduces the efficiency is in any way compromised (legally)?

I would agree that the modulation is important to smooth out the heat cycle and reduce damage to the electrical components and (perhaps the HX too--what do you think?). A couple of points decrease in efficiency sometimes is unimportant, particularly if you have a modest gas bill (consumption) overall.

Jamie Hall

You ask if the tech who modifies the boiler to either a lower firing rate or to modulate is in any way legally compromised? Um... yeah. He has just voided the warranty on the boiler and opted out of manufacturer support, and may very well have voided any UL or CSA approvals or other code approvals on the boiler, which may mean that no insurance is legally available for the boiler or the structure in which it is installed. You may also be in violation of local codes or ordinances regarding efficiency.

Now if he did it at your direction, then there is a neat legal question as to whether you are responsible for all costs or damages or even correct operation or he is. Lawyers love that sort of situation.

Dave Carpentier

fentonc said:

My nominally 140K BTU/h boiler only clocks 120K BTU/h at the meter, but it also usually only fires for 70 seconds at a time before the aquastat cuts it off it off when only 1 zone is calling for heat.

Fwiw, I had no issues at all after disconnecting the end-switches on my two smallest load zones.
The zone valves dutifully open and closed according to their respective tstats, but they couldnt instigate a heat call on their own.. they have to wait for any other larger zone to call (which then starts the circulator and eventually the boiler). The bathroom and laundry room didnt seem to change comfort levels, the flywheel effect of the slab probably helped.

JohnNY

To clarify: The boilers I've made this field modification to were all oversized steam boilers. Condensation was always a concern because there was the potential that the heat exchanger would transfer the heat faster than it was taking it in but that never happened. The stages were controlled by vaporstats and when pressure dropped to a set value, the 2nd stage would bring the temp back up for a short time. I know this sounds like short cycling but it's not, in the classic sense, and it's interesting to listen to the boiler simmer at low boil while heating the last few sections of radiation in the building.

Jamie Hall

Not that different, thermodynamically, from what I've been muttering for years. If you have a boiler which is either on or off -- a typical residential boiler, gas or oil, single stage burner -- if you keep the off cycle very short, as short as you can, the thing never really goes off the boil during the off cycle.

fentonc

@Dave Carpentier - good to hear that works, that probably gets you like 90% of the way towards an 'optimal' zone synchronization system for a setup with tons of small zones.

@EBEBRATT-Ed - Is the gas output just proportional to pressure? I don't have a manometer, but my nominally 140K BTU/h boiler clocks at 120K BTU/h at the meter, which would be exactly 140x(3" WC / 3.5" WC).

I found a chart showing condensation temperature vs percent oxygen in the flue gases, with the dewpoint being 133F at 3% O2, all the way down to 116F at 10% O2. With the output down to 120K BTU/h, my boiler showed 10.2% O2, so around 116F dewpoint. The aquastat lets you set it all the way down to a 140F high limit (so 120F before it kicks on again), which I had assumed would condense, but might actually be fine on mine. I've never noticed a hint of condensation after running it with a 150F high limit all last winter.

@Jamie Hall Interesting re: incomplete combustion when the flame can't get hot enough. I'd love to see a chart of that, although it's probably boiler-specific.