Another good question, this one from our volunteer guide, was about the specifics of our radiant hot water and domestic hot water piping. Could I provide a diagram about how it all worked together? I had never put both systems on the same diagram. I started with the diagrams I had of the fireplace/gas boiler interaction. This is the logic of how the fireplace boiler operates to send warmed water to the heat exchanger to transfer heat to the gas boiler water.
The fireplace boiler water forms a closed loop separate from the gas boiler water in the radiant system. The fireplace water has some steel pipes while the rest is all pex, copper, or stainless steel. The steel pipes degrade the water so the stainless steel heat exchanger keeps the water systems apart and both boilers are piped to transfer the heat from the fireplace water to the gas heated water.
The gas boiler water is always routed through the heat exchanger even when there is not a fire. The return water should be at least 20 degrees cooler than the water that is sent to the flooring so the extra trip through the exchanger helps cool that water and improve the boiler’s condensing efficiency.
When there is a fire, the return water picks up heat from the fireplace boiler water and is warm enough to circulate through the boiler and on to the radiant pipes. I noticed that the previous fireplace boiler diagrams all had the storage tank which I removed some time ago. This is the diagram with the piping going directly to the heat exchanger.
There is an unfortunate error in this diagram. The expansion tanks are shown on the cold side of the pipes when of course they are on the hot side as they are meant to expand as the water in the pipes expands with heat.
I noticed that and fixed it when I was adding the domestic hot water loop to the old diagram. I made a few other changes to show the household hot water path and explained that when the boiler functions as an instant hot water heater the radiant heat is off while the water is being heated for household use. If needed, it automatically turns back on when water is no longer flowing through the separate pipes inside the boiler that heat our hot water.
This diagram also shows that the return radiant pipe water is connected to the primary loop and not to the secondary loop as in the old diagram. In the primary loop on the gas water boiler the water just circulates into and out of the boiler. It is the required primary loop size for the dual loop system to function. The secondary loop pump pulls the heated water from the primary loop and circulates it to the valves that are controlled by the thermostats and the valve controller electronics. In this diagram I include the hot water entering the distribution valves and returning to the boiler to pick up more heat. When the room is warm enough the boiler and the pumps will shut down. So one thermostat must always call for heat when the fireplace boiler is on. That is not difficult as the radiator fed bedrooms over the crawlspace of the house are usually colder than the slab area. So we send the heat there by turning up that thermostat over 70°.
Cold water that is heated for use in the house is piped to a separate loop in the gas boiler. This is an open system not a closed loop like the radiant piping. Water flows freely from the well pressure tank into the boiler. There is a hot water recirculation pump that pulls water from the pressure tank through the loop in the floor that is the “trunk” of our hot water system before we open a faucet. The water then flows through the gas boiler’s instant hot water piping and back to the hot water pipes.
The recirculation pump runs for 15 minutes after being signaled on by a remote control. But it also has a built in thermostat (thermo sensor) that shuts it off at 120°. If within the 15 minutes the temperature drops below 120° then the pump turns on again. So from the first signal to turn on the entire trunk is heated within in a couple of minutes and it won’t begin to lose heat for 15 minutes when the circulating pump turns off because of the 15 minute timer. The “twigs” in the diagram below are the shorter lengths of pipe that feed the faucets from the nearby trunk flow. Only the cold water in these short pipes needs to be replaced by hot when the faucets turn on. This wastes as little water as possible before the warm water leaves the faucets. The boiler keeps heating household water until the faucet or appliance is turned off. The dishwasher has its own hot water heat function that boosts the heat from the boiler and we usually wash clothes in cold water.
The Triangle Tube Challenger Combi boiler that we installed in 2012 is a very efficient use of gas to run our household heat and hot water. Right now the push is to eliminate gas use at the home level and electrify homes including electric induction stoves and cold weather heat pumps. It is something we will think about when our boiler stops being repairable. But we use much less gas for heat, cooking and hot water than an average Colorado home uses just for heat (Electric heat converted to therms). The figures were from a Colorado Extension report.
I spoke with a vendor at the Metro Denver Green Homes Tour Expo who used to sell the Challenger boilers but said his customers had reliability issues. I was hopeful that would only be true if the boilers were not maintained or fed the specific water ph and mineral content that was specified as we do. We have not had the problems but we do flush the boiler and the pipes every couple of years and we make sure the pipes have no air in them before the boiler is turned on for the year. The water is within the recommended ph and is treated for mineral content and carbon filtered. The calcium is suspended in the water with an electronic magnetic filter that keeps the molecules from clinging tightly together so less likely to be deposited in the boiler or pipes. So far that has kept the boiler reliable for us. I’ve read that the lifetime expectation for these boilers is 15-25 years so maybe in 5-10 more years it will have to be replaced. By then I hope there will be a much more energy efficient alternative.