Converting old boiler to cold start a bad idea?

[Mr. Roboto]

I have a 26 year old boiler in my house. The hydrostat was going bad (relay was bouncing) so I had my boiler guy change it out for me when he was here doing the yearly service. The new digital hydrostat he installed has the capability of being configured as a cold start boiler. This means the boiler only runs when there is a call for heat (either from a heating zone or in my case, the indirect hot water tank that I have) rather than constantly maintaining temperature.

I had him replace all the gaskets last year, and I know for a fact the boiler doesn't leak. I just had it shut down for 36 hours while I was installing the new vent pipe, and no leaks. From what I've read, this is the biggest deciding factor to determine weather or not to run an older boiler as a cold start, so after a short discussion, he set it up for cold start. But now that I've thought about it for a few days... would the repeated cool down and heat up cycles stress out the metal of my old boiler, and lead to premature cracking/failure? This makes me nervous. Or am I overthinking things?

Thanks!

 

[yeldogt]

That's how all of mine have worked ?? How did it maintain the temp previous? in the summer ?

I did a twin set of weil mclain boilers back in the late 80's that way .. all since have been Buderus .. all the german boilers work that way?

With the US boilers -- it's the return temp going into a hot boiler that's the problem. The german boilers can take lower return temps.

 

[LS6 Tommy]

Cast iron sectional? I wouldn't...

Tommy

 

[danski0224]

Only if you have piping and devices/controls to ensure a minimum 140 degree return temperature.

 

[66cj225]

36 hrs isn't much of a test. After 2 weeks, a Burnham sectional would be sitting in a puddle after being shut down with the switch.

 

[Mr. Roboto]

Thanks for the advice. I figured 36 hours was long enough test since it would likely never sit that long during any season, but I suppose it would if we went away on vacation.

Also, I really don’t know if I have a guaranteed 140F coming back to the boiler. I don’t really have a way to check. Could I just use an IR thermometer on the return pipe?

The responses in this thread and what I’ve read elsewhere alone are enough to ditch the whole cold start. I’m going to set the controller back to 160F on the low side tonight. Whatever marginal money I’d be saving in oil consumption will be outweighed by any headaches that may crop up.

Thanks again!

 

[EdT]

Due to variations in emissivity of various materials, an IR thermometer is not usually accurate unless it's been calibrated for looking at that particular color/texture/type of material. Out of the box, they are OK for determining that this area is hotter than that area if the material is uniform, but not so good at actual, accurate, temperature readings w/o calibration. So, in your particular case, an IR thermometer may be able to tell you if the return line is colder than the body of the boiler (if they have the same emissivity), but the actual temperature numbers will likely be off. I worked with this stuff for 30 years so I'm not making this up.

 

[rlitman]

Due to variations in emissivity of various materials, an IR thermometer is not usually accurate unless it's been calibrated for looking at that particular color/texture/type of material. Out of the box, they are OK for determining that this area is hotter than that area if the material is uniform, but not so good at actual, accurate, temperature readings w/o calibration. So, in your particular case, an IR thermometer may be able to tell you if the return line is colder than the body of the boiler (if they have the same emissivity), but the actual temperature numbers will likely be off. I worked with this stuff for 30 years so I'm not making this up.

Emissivity doesn't vary that much in most materials, which is why IR thermometers can be useful at all. We're not looking at stainless steel or shiny aluminum pipes here that would certainly throw the numbers off.

An oxidized brown copper pipe, or the typical black iron pipe will be close enough to 0.95 for all practical purposes. Especially since you know well that if the emissivity is lower than your device is calibrated for, the temperature will read low, which for the OP's purpose leans on the side of safety. So when that oxidized copper is actually between 0.78 and 0.87 (which BTW is very close to the emissivity you'll get off the mill scale on black pipe), a surface reading of 140 F would translate to an actual temperature a degree or two higher.

 

[danski0224]

Thanks for the advice. I figured 36 hours was long enough test since it would likely never sit that long during any season, but I suppose it would if we went away on vacation.

Also, I really don’t know if I have a guaranteed 140F coming back to the boiler. I don’t really have a way to check. Could I just use an IR thermometer on the return pipe?

The responses in this thread and what I’ve read elsewhere alone are enough to ditch the whole cold start. I’m going to set the controller back to 160F on the low side tonight. Whatever marginal money I’d be saving in oil consumption will be outweighed by any headaches that may crop up.

Thanks again!

The near boiler piping has to be installed with the components that ensure the return temperature. All boiler install manuals that I have seen include piping diagrams for the near boiler portion of the piping and for indirect water heaters.

There are a lot of cast iron boilers in the Chicago suburbs, and I have yet to see one that is configured to be hot all the time. I have not yet seen one leaking in the off season, however I also do not see many that are piped correctly (well, maybe one or two). I do see lots of efflorescence on the bricks near the chimney and piles of rust flakes under the heat exchanger, though...

Thermometer wells are hideously expensive, so very few get installed on residential jobs...

There is no point to keeping the system hot all the time. If it is piped properly, including flow-checks, the primary boiler loop will get up to temperature very quickly for your indirect water heater.

 

[Mr. Roboto]

I understand conceptually what you guys are saying, but I am certainly no HVAC expert. Can some of the questions you asked about how it la hooked up be answered with a photo? My setup is in the attached photos. Definitely some mixed opinions from different areas of the country as to weather or not cold start vs constant temp is best.

 

[danski0224]

That piping is not done right.

I suggest downloading or reading the I&O manual for the boiler.

 

[yeldogt]

I understand conceptually what you guys are saying, but I am certainly no HVAC expert. Can some of the questions you asked about how it la hooked up be answered with a photo? My setup is in the attached photos. Definitely some mixed opinions from different areas of the country as to weather or not cold start vs constant temp is best.

Why don't you try and call the manufacturer ?

Every boiler I have ever had used an indirect tank and typically would only fire once or twice a day in the summer. I guess since i wanted an indirect I alway got one that would cold start. The Buderus are specifically designed for colder water temps on the return -- to eliminate the need for a double pump primary secondary -- they use the radiant as the primary.

 

[Mr. Roboto]

I guess I left out a key element. The boiler originally had a tankless coil to produce hot water, which is why it was most likely always configured to remain hot. I had to have the indirect tank installed right after I bought the house because the coil was scaled up and could not adequately produce hot water. The old hydrostat did not have the ability to be programmed as a hot start boiler. The new one that was just installed does.

I never thought to look online for info on a boiler this old. I didn't think I'd find much, but I will see if I can pull a model number off the unit and do some digging.

I'm sorry this question was much more complex than I thought. I though it was a generic question, but is seems that a lot more info goes into weather or not to configure it as a cold start. And a lot of what is mentioned is unfortunately over my head.

 

[zak77]

I was thinking of doing this with my 28 year old boiler since i only need it for heat in the fall/spring since i burn wood. I disconnected the hot water coil when i got a hybrid electric water heater so why have the boiler running all summer? I was able to shut it down a couple times with no leaks but this year i cant shut it down without it leaking. But, when i was able to shut it down it was for months on end and it never had an issue coming back online. I was able to set the temps relatively low on mine so it's not wasting a lot of oil but i'd prefer a total shut-down, especially in the summer. When i asked the boiler tech about the newer cast iron boiler, he said they're meant to be shut down completely but for $8k, that's expensive for something i really dont need.

In your situation with the super store, i dont think it's going to work but in my case, it'd probably work if it didnt leak.

 

[yeldogt]

I guess I left out a key element. The boiler originally had a tankless coil to produce hot water, which is why it was most likely always configured to remain hot. I had to have the indirect tank installed right after I bought the house because the coil was scaled up and could not adequately produce hot water. The old hydrostat did not have the ability to be programmed as a hot start boiler. The new one that was just installed does.

I never thought to look online for info on a boiler this old. I didn't think I'd find much, but I will see if I can pull a model number off the unit and do some digging.

I'm sorry this question was much more complex than I thought. I though it was a generic question, but is seems that a lot more info goes into weather or not to configure it as a cold start. And a lot of what is mentioned is unfortunately over my head.

I guess this is all about the type of boiler ....maybe age? It could be that boilers with internal DHW coils don't have to be built to worry about ever being cold? I'm not sure because I know it's common to disconnect the the DHW from the coil and go with a standard hot water heater -- be it for cost reasons or the scaling of the internal coils.

I would call -- the twin WM CI boilers I did in the 80's although connected to radiators were set up with outdoor reset and would run cooler -- did shut off in the summer.

 

[danski0224]

The Buderus are specifically designed for colder water temps on the return -- to eliminate the need for a double pump primary secondary -- they use the radiant as the primary.

The control on a newer Peerless will turn the circulator off to bring up the boiler temperature on a cold start.

However, the boiler, indirect water heater and the apparent 2 zones in the OP's system (probably) all have different BTU and therefore GPM requirements.

At the very least, it is highly unlikely that either zone has a GPM requirement that is equal to the boiler GPM requirement. Given that a 20 degree temperature drop across the supply and return is common design practice, I'd hazard a guess and say that the system is flowing too much water and is inefficient.

The indirect water heater probably preserved this boiler. If it was just connected to cast iron radiators- especially if it was configured as a cold start system, the boiler heat exchanger probably would have rotted away due to flue gas condensation and low return water temperature. If the flue is connected to a masonry chimney, I'd certainly look at the chimney really close.

 

[Jackfre]

Run it cold start with the proper piping to prevent cold return water. The guy who piped your indirect did you no favors building it in like that, and with no unions. I just don’t get that.

 

[yeldogt]

The control on a newer Peerless will turn the circulator off to bring up the boiler temperature on a cold start.

However, the boiler, indirect water heater and the apparent 2 zones in the OP's system (probably) all have different BTU and therefore GPM requirements.

At the very least, it is highly unlikely that either zone has a GPM requirement that is equal to the boiler GPM requirement. Given that a 20 degree temperature drop across the supply and return is common design practice, I'd hazard a guess and say that the system is flowing too much water and is inefficient.

The indirect water heater probably preserved this boiler. If it was just connected to cast iron radiators- especially if it was configured as a cold start system, the boiler heat exchanger probably would have rotted away due to flue gas condensation and low return water temperature. If the flue is connected to a masonry chimney, I'd certainly look at the chimney really close.

In the summer on a call for DHW the boiler is cold -- obviously when the circulator starts ... it's actually going to pull some heat from the indirect tank .. but will quickly come up to temp. I guess it's a question of design the total mass of the boiler -- but since the boiler is going to go to 180 for the DHW .. it's going to quickly burn off the moisture.

 

[yeldogt]

Run it cold start with the proper piping to prevent cold return water. The guy who piped your indirect did you no favors building it in like that, and with no unions. I just don’t get that.

I stopped using unions due to the problems .. leaking. I found using good threaded valves to be the way to go. Isolate everything a bit back from the appliance and leave a space to cut the pipe. I have never been lucky with the next tank being the same .. but if it is .. you just rethread the old onto the new and sweat in a connector.

Buderus changed the tank design ... but the valves made the switch out easy

 

[LS6 Tommy]

Due to variations in emissivity of various materials, an IR thermometer is not usually accurate unless it's been calibrated for looking at that particular color/texture/type of material. Out of the box, they are OK for determining that this area is hotter than that area if the material is uniform, but not so good at actual, accurate, temperature readings w/o calibration. So, in your particular case, an IR thermometer may be able to tell you if the return line is colder than the body of the boiler (if they have the same emissivity), but the actual temperature numbers will likely be off. I worked with this stuff for 30 years so I'm not making this up.

Emissivity doesn't vary that much in most materials, which is why IR thermometers can be useful at all. We're not looking at stainless steel or shiny aluminum pipes here that would certainly throw the numbers off.

An oxidized brown copper pipe, or the typical black iron pipe will be close enough to 0.95 for all practical purposes. Especially since you know well that if the emissivity is lower than your device is calibrated for, the temperature will read low, which for the OP's purpose leans on the side of safety. So when that oxidized copper is actually between 0.78 and 0.87 (which BTW is very close to the emissivity you'll get off the mill scale on black pipe), a surface reading of 140 F would translate to an actual temperature a degree or two higher.

That's why you get an IR thermometer with adjustable emissivity. Most IR thermometers are factory set at 0.95, which is what the majority of materials being measured calls for.

Tommy

 

[rlitman]

That's why you get an IR thermometer with adjustable emissivity. Most IR thermometers are factory set at 0.95, which is what the majority of materials being measured calls for.

Tommy

Yes. Adjustable emissivity can get you more accuracy (IF you can calibrate it), but when a degree or two isn't critical (as it isn't here), the 0.95 gets you close enough for anything by shiny metal surfaces.

The bigger risk of getting bad readings is not understanding the spot to distance ratio.

 

[LS6 Tommy]

Yes. Adjustable emissivity can get you more accuracy (IF you can calibrate it), but when a degree or two isn't critical (as it isn't here), the 0.95 gets you close enough for anything by shiny metal surfaces.

The bigger risk of getting bad readings is not understanding the spot to distance ratio.

I agree.

Tommy