Tankless coil vs. Indirect

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Old 10-14-18, 09:15 AM
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Tankless coil vs. Indirect

I keep hearing / seeing online people say how inefficient a tankless coil is compared to an indirect tank. But it seems to me that they wouldn't cost much difference to operate. Ignore the upfront / installation costs.

If you have a tankless coil, and take a 15m shower, the boiler runs for probably 15m. If you have an indirect tank, the boiler won't come on until the tank temp drops enough, but once it does, won't it be running for close to 15m to get the tank water back up to temp? I think that in the winter, a tankless coil and indirect would not cost much different to operate. Can anyone speak to the savings on this?
 
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Old 10-14-18, 12:02 PM
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I believe it's the whole coil-in the-boiler that is inefficient. Not the storing of the water in an indirect tank.
 
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Old 10-14-18, 01:48 PM
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But the only thing that matters is boiler on-time when we're talking about money. I'm just trying to understand how the boiler would run less with a tank.
 
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Old 10-14-18, 02:21 PM
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I don't think the boiler would run any less with a tank.
Actually it would be slightly more because of the small tank losses.
The benefit of the tank is more hot water without reduced flow or waiting.
 
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Old 10-14-18, 02:37 PM
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With a coil the boiler must maintain a certain temp 24/7 365 days of the year if you want heat or not. Even if you don't use hot water when the boiler water cools it will refire back to temp.

With an indirect you eliminate the constant maintaining of the temp and the boiler only runs on a demand from the indirect which is much more insulated than your boiler and will keep the hot water a lot longer than a boiler will.

With an Amtrol Boilermate their claim is a 2 deg. heat loss per hour which means that unless you are drawing hot water your boiler hardly ever runs in the off season. They have 2" of insulation in there tanks.

Another plus with the tank is that because of that insulation it takes very little time to heat the water in the tank.

With a coil you have the possibility of the coil liming up and severely hampering the transfer of heat from the boiler to the coil with the results of running out of hot water.

With the coil the boiler water heats the coil. With the tank the boiler water is inside the coil which heats the tank of water which means no liming.

Hope this helps a little.
 
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Old 10-15-18, 11:06 AM
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Yeah makes sense. Thanks.
 
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Old 10-15-18, 10:13 PM
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slade8200: I keep hearing / seeing online people say how inefficient a tankless coil is compared to an indirect tank. But it seems to me that they wouldn't cost much difference to operate.
Round tank type oil and gas fired water heaters are very inefficient. In winter main boiler is more efficient as source of heat for hot water. Just look at the difference in stack temperatures

A simple solution to use boiler tankless coil for more economical hot water in winter. In summer direct fire separate water heater.

In winter use small circulator activated by aquastat/sensor on water tank circulated tankless coil hot water tank. For summer flip switch and aquatsat activates water heater burner.

This hybrid setup provides both most economical heat source in winter and the tank storage more capacity. The circulator, aquastat/control and plumbing cost less than $200. At today's oil prices that can be one year payback.

Key is using tank aquastat with small,adjustable delta-T. In winter deltaT is 3F and summer 5F. A eBay PID control (120 Vac and F/C) is good control: https://www.ebay.com/itm/New-REX-C10...a~r3:rk:4:pf:0
 
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