Oil fired; why no heat storage?

Using fossil fuels to heat a large amount of storage makes no sense mainly because of standby losses. However, with low mass fossil fuel boilers, a buffer tank of a predetermined size which is much smaller in scale can be used to prevent short cycling. Short cycling of an oil burner usually less than 10 minutes is inefficient. Manufacturers typically don't recommend more than 6 cycles per hour as I understand it. Think of it this way-a wood boiler can use a buffer tank to store enough heat for a day or longer. Fossil fuel boilers only need 10 min or so.

Take a look at www.boilerbuddy.com for an example.

Mike

jklingel said:

Been wondering; why don't we use large storage tanks w/ oil-fired boilers, too? Oil-fireds don't run optimally when starting/shutting down, either, so why not run them at WOT for a while?

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The main reason is this: Oil boilers can go from zero output to full output automatically in seconds, and back to zero just as quickly. Therefore, you can cycle them on and off as needed to match the heat load. They're perfectly happy running 50 minutes every hour or five minutes every hour. There's some efficiency lost, but not much. The flame is always at 100% when it's running. There's some standby loss, but not much incomplete combustion.

Wood boilers are much more difficult to modulate. They take a good deal of time and human intervention to get started, and they are much less efficient running at any less than full throttle. It's very hard to get a wood appliance to provide just the right amount of heat.

Pellet stoves and boilers would be the exception, and they typically don't have or need storage.

For some applications, we do install storage.

For example, systems using heavier oils (like waste vegetable oil, for example), where short-cycling would be a more significant problem.

Joe

I'm looking into a boiler buddy w/ low mass boiler combo, with the eventual intent of adding an outdoor wood boiler to it. My home has several small zones, and with the addition of an outdoor reset I'm finding the boiler is firing too much for my liking. I want to add thermal mass, and hydro separation via storage tank would be an added benefit.

After a bit of research I surprised to find how little thermal storage there is in a pound of cast iron/steel compared to a pound of water.

It is really simple to reduce the cycling of an oil boiler. Just increase the differential between the hi and low limit. I set mine to 145 and 180 when I use it. You can also install a programmable thermostat and increase the differential to decrease short cycling. There is very few situations where a storage tank would actually be needed for a oil or gas boiler.

Actually from a combustion efficiency standpoint, steady state is indeed the optimum. While it varies a little from fuel to fuel, any burner/combustion chamber becomes more efficient once everything is up to temp. There is a reason virtually all the new high efficiency 90%+ gas boilers have modulating burners that allow a nearly constant burn at a low rate.
Viessmann took the high volume approach in their highest efficiency non condensing boiler after their research showed the improvements in efficiency from longer burns. The Vitola (which IMHO still remains peerless in the boiler world) holds about 30-50 gallons of water depending on size which is about 10 times that of a standard gas or oil fired CI boiler.
The determining question is, when does a person reach the point of diminishing returns when you balance combustion efficiency gained vs standby losses from a large volume storage.
Think back to Siggy's statement regarding optimal burn with wood in the article posted here a week or so ago which basically said best efficiency was achieved by burning the whole load of fuel all at once.