Both, today residential "steel" boilers are rare though, I believe only a few manufacturers make them such as Columbia & Burnham and Thermodynamics. They are a very small segment of boilers overall and tend to be prevalent only in certain areas. Every water boiler manufacturer lists what the minimum return water & operating temperature is. The reason is fairly obvious because the colder the water temperature, the colder the boilers block or heat exchanger will become. In boilers, when the block/heat exchanger temperature becomes at or below the dew point temperature, there will be condensation. As moisture increases, the hot surface igniters and flame sensors get wet and fail far too often creating an emergency situation that needs to be resolved. If the set point temperature in a hot water boiler is too low, say 120°F, then the exit temperature of the gas may be cooled to below the dew point. Burner turndown has nothing to do with this, only the gas temperature versus the dew point temperature. Stainless steel or aluminum condensing have no lower limit, Copper tube is around 100-110 depending on manufacturer, steel was usually 130 and all the cast iron boiler manufacturers I am aware of such as Weil McLain, Burnham, Peerless state 130 to 140. Of course you can raise cast iron temps while having lower system temps, via primary/secondary piping or injection.

Andy, I understand your frustration. Hydronic heat specialists are getting more difficult to find, I understand. Just be glad that you don't have steam. If you ever decide to replace the boiler, your engineering background might help. There a several variables and lots of calculations that need to be done to be sure you end up in a working envelope -- water temperature, heat loss from the structure, heat transfer via the radiators, circulation rate, temperature drop in the individual loops,... You could have a new hobby. The boiler heads in those two fora will be able to help you out a lot, I think. Someone there, maybe several people, will know that boiler well. Just tell them you are tired of the excess heat and give them your boiler and indirect hot water tank details. Be sure to ask them about scheduling the boiler on and off when it is only needed for water heating. Look at that this way, if you have your boiler off 16 hours a day while working and sleeping during the week, and 8 hours on Saturday and Sunday, you've solved nearly 60% of your problem. The flue can go in two ways and depends on what kind of boiler you have. There is subtlety that I am skipping. Condensing boilers have low temperature flue gas, and use plastic pipe going horizontally out of the side wall of the house. They are usually using outdoor air for combustion eliminating another heat loss point. Condensing the steam means heat recovery from that steam that used to leave boilers up the chimney. Now it runs down the drain after giving up its heat. All old fashioned residential boilers used to be non condensing. They will have a metal flue pipe that goes up to the roof line in some kind of double wall, insulated metal, or a metal joined to a masonry chimney. The problem is a mismatch between boiler and flue. Obviously a high temperature boiler will melt/burn plastic pipe. I've never heard of that happening. Exhausting wet (and acidic) flue gas in a masonry, or inadequately corrosion resistant chimney will cause a much slower failure and that happens even though it should not. Particularly bad are very old, unlined masonry chimneys no tile liner. The acid gets to the mortar and eats it up leaving gaps. Sometimes, with a change to a more efficient condensing boiler, a plastic flue can be installed within an old-fashioned chimney and go up through the roof if going out sideways is not convenient or not eye-pleasing.

I have about 40 varieties outside in my northern CA garden. We regularly get down to about 30 degrees but only at night. At that temperature my mandarin and orange trees show zero fruit damage. Of note the length of time at 30 degrees is also very important. Several hours at 30 degrees is very different than several days of consistent 30 degree temperatures. I only see fruit damage when it gets below 28 degrees and even around that temperature it is pretty mild. -Brett

I asked this question years ago when we first bought a home with a boiler. 2 or 3 different boiler professionals said it was not a good idea to get your boiler serviced. It just mucks things up. If your boiler is working fine there is nothing for the pro to find. Wait until something goes out and then call someone to fix it if you can't figure out what happened yourself. You had it checked for a gas leak and there is none. Ours ignites each time so doesn't have a pilot light. Yours might be the same? You might have mistaken the electronic ignition being off because there was no call for heat for a pilot light that wasn't lit? Did you try turning the heat on before you looked for the pilot light? It looks modern enough that it should have electronic ignition. To test it turn your heat off and wait for the boiler to go off. Then look for the "pilot light". Then turn your heat back on and see if the boiler ignites. If there was no pilot light but your boiler still heated then you have electronic ignition. Your pressure looks good at 20psi and the boiler is getting to temp at 180 degrees. The bell shaped device is your autofill. It is in the correct position "off". Never allow a technician to leave it in the on position. It can flood your house in the event of a leak. It is used to fill the system with water after it was installed and if there is any need after fixing something. i.e. it is almost never used. I copied this from the web site below: https://inspectapedia.com/heat/Water_Feeder_Valves.php "B&G wants you to keep the boiler water supply valve shut except during service - as a precaution that allows easy detection of a boiler leak by noticing the reduced water pressure in that system. The company also points out that too-frequent feeding of water into a heating system can increase the effects of corrosion and system damage. In my [DF] opinion, a typical building owner or occupant almost never thinks to check the water level in a hydronic heating system (hot water boiler) until there is some indication of a problem (like no heat or a damaged leaky boiler). In our experience most hydronic heating boiler installers and service techs leave that shutoff valve "open" or "on" so that the boiler won't be at risk of being ruined or unsafe by operating at low or no-water level - their experience may be similar to mine. The manufacturer is telling you what's safe in some regards, but they may not have the same view of what people actually do in the field. Add a low water cutoff safety valve A safety improvement on hydronic boilers that gets around this argument is to add a low-water cutoff on residential hydronic heating boilers (hot water heating boilers)." ------------------------ Answers in-line below: In particular should I test any safety switches? Can I do this myself? Nothing to test. Just check the pressure hasn't dropped. A pressure drop would indicate a leak somewhere or a faulty expansion tank. That is the grey Amtrol tank. If you rap on the top with a knuckle of your finger where water comes in it should sound full. Near the black nipple at the bottom should sound hollow (in your case the black nipple is on top due to how the tank was installed). There is a bladder in between that allows the water in the system to expand and contract while keeping the system under a constant pressure. I understand boiler is a closed system filled with water. Do I ever need to drain and refill this water? I don't think it has been done ever to my knowledge. No, never drain and refill your boiler unnecessarily. The water in there gets deprived of oxygen which stops it from corroding the inside of your boiler and the other components. It only needs to be refilled if it has been drained to replace something. Aside from safety switches do I need to test anything else? Whenever you are in the basement give a quick look at the pressure gauge to make sure it is holding pressure. If it drops to zero you may have a faulty pressure gauge, a leak or a faulty expansion tank. Given my situation, do I need to have anyone come look at my boiler in professional capacity to do something I cannot do? I don't think so. They have nothing to look at while everything is working correctly. In one picture you show your backflow preventer. That insures that the dirty brown boiler water does not contaminate your fresh potable water supply. There is nothing to service on it. In your last picture it looks like you have pointed to the drain valve below your boiler circulation pump (I can't really see what it is due to the picture being cut off - if it looks like a outdoor hose faucet it is a service drain). That is there to drain the pressure off the system before something is removed and replaced. You don't need to do anything with it. The yellow tagged switch is an automatic pressure relief valve. It goes off if for some reason the pressure in your boiler gets too high. They are normally set at 30psi. Nothing for you to do with it either. Just leave it be. If you follow it to the end of the pipe (which should lead to a floor drain) and ever see water coming out of it then call someone. The boiler internal pressure has gotten too high and something is going wrong. The blue and red hand turn valves look like shut off valves. But it is kind of hard to tell for sure without seeing all the piping. Usually they are not the hand crank style anymore but a linear handle. They are there to allow you to isolate parts of the system without draining the whole system when replacing parts. Don't mess with them. The picture of what looks to be a cup at the top of the piping is probably an automatic air vent. They allow any oxygen in the water that gets trapped to eventually work it's way out of the system. It does its thing all on its own. If you ever see water coming out of it then something is wrong with the little float device inside and it either needs cleaned or replaced. If you really want to you could hire a pro to walk you through the entire system and explain everything to you. But I'd be suspicious of anyone telling you that it needs yearly servicing or that they are going to test something. More than likely they would either inadvertantly or on purpose mess something up so they could charge you for a repair.