Why isn't running the DHW tankfeed through a boiler coil for fast winter recovery a common practice?
When I purchased a 4 family the oil fired steam boiler had a tankless coil, and they were switching from using the 75 gal gas fired tank in summer to the coil in winter when the boiler was firing. IDK if they actually had an aquastat on it, I immediately had the oil boiler replaced with gas. What we did was run the feed to the tank through the boiler and a mixing valve. The result is when the street water is 40 deg the tank is getting preheated water so it never runs out! At the time there were several families of 5 and 6 living there and winter tank recovery seemed like it could be a real issue. I understand you get tank standby losses you would not have with using just a coil, but you'd have those same losses with just a tank.
Now I'm replacing the boiler again (trust me you don't want to know) and wondering if I should give up on this scheme. I'm told that only W-M is offering a 200k btu boiler with a tankless. But it seems like such a brilliant way to get synergy between the 2 appliances. Thoughts? Is it a waste of money?
Comments
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The question is the boiler more efficient providing BTUs or is the water heater more efficient. Nothing is free you not getting free energy from the boiler. I would say the boiler may be more efficient but the tank on the WH is probably better insulated.
If you need more capacity for the DHW then do it. Nothing wrong with preheating the tank water with a tankless coil. I assume you not maintaining the boiler temp for DHW, so it only preheats water during the heating season.
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I never said it was more efficient, but it is certainly more effective at heating more water. There's no hydrostat, so in warm seasons when the boiler is not needed for house heat it has no effect, but the street water is then quite a bit warmer and doesn't need the boost.
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My guess is you don't find oil boilers and gas fired water heater tanks in the same place often. I believe oil fired water heater tanks are not popular so if you wanted DHW it had to be done by the boiler.
Since gas fired water heater tanks are popular, gas fired boilers don't need the coil.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
There is an effect if the water is still routed through the coil when the boiler is not operating - it cools the boiler and can cause nasty condensation.
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And just keep in mind, Weil Mclain, with a tankless coil, will cost you a pretty penny. And if that coil ever gets a hole in it, you will have water spraying out of your radiators, assuming that they are one pipe.
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That's an interesting point, but I don't think summer street water temps are cold enough to cause condensation. I don't see it on my basement copper feeds even in the height of muggy summer. If I were on a well it might be different.
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what sort of pressure drop do tankless coils present? I know as they lime up, flow capacity drops off. If that is a concern?
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
that is usually a problem in spring as the outdoor temp warms but the bodies of water they pull the municipal water from and the outdoor tanks have not warmed up yet.
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I would add a bypass to it anyway - cheap and easy insurance.
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Totally fair.
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In this post:
go to comment #3, where icesail talks about using a storage tank with an indirect coil. His recommendation is just to use a regular water heater as the storage tank. But what if you took it a step further, and used that water heater as a water heater in the summer and as a storage tank in the winter? The indirect is powered by a circulator, you could put that circulator on a relay attached to the boiler on/off, so the circulator can only run when the boiler is on. That way you won't have cold water running through your boiler in the summer.
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If you used a heat pump water heater it would provide cooling and dehumidification during the summer. In the winter you'd be pulling heat for the hot water from the boiler. Which is where the heat pump would get it from, just without an extra step.
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that is a common option, people have posted the setup for it here before.
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That is what my setup currently is! In winter the boiler feeds HW to the tank, which only fires for standby losses. Firing up a steam boiler in summer for HW from the coil makes no sense. But in my own home I have a much smaller W-M cold start hydronic boiler feeding my indirect year round. I have a programmable thermostat on it so I can set 15 deg wide points, and it will not fire between morning showers and dinner cleanup.
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Around here oil boilers and gas water heaters on the same job were quite common. Tankless coils do not produce much water and in the old days indirect were not common like they are now so a lot of people had GHW so they could shut the boiler down in the summer.
Gas boilers don't usually have a tankless but did in the old days.
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At this point it looks like I can't have the coil if I don't want W-M. One contractor said he heard bad things about them recently. But I don't know much about the issues.
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