It's true regardless. A 50% duty cycle at design conditions is Short Cycling. While the 79 gallon buffer tank may keep burn times long enough to avoid most condensation issues, but you are still wasting energy. Every time you start the boiler you are wasting energy, because the initial energy is needed to bring the system up to operating temps before it can put out heat, and also due to incomplete combustion as it warms up. Energy is also wasted at the end of combustion due to incomplete combustion. This is where Mod-Con boilers are able to save energy, they are able to better supply heat at the rate of heat loss.

In a low heat loss home, a lot of times Domestic hot water demand can outstrip heat demand, the buffer tank can help a lower output boiler keep up with demand. If there is a huge mismatch it may be better to separate out the uses. This allows the system to be more flexible, and possibly increasing system efficiency.

Also Solar Thermal? Unless you are hard up on solar access it's just not worth it. It may be cheap enough if you build you own flat plate collectors, but factor in your time, and the decreased system reliability and I doubt the cost/benefit is there.

Dend, your:
“It's true regardless. A 50% duty cycle at design conditions is Short Cycling.”

U R confusing a boiler that is oversized for the home heating load at “design” temperature
(2*F) here, with Short Cycling.

It’s not oversized for my house DHW load when 2 showers are on or filling a large bathtub.


Hot Rod Rohr (HR) who is Caleffi’s USA technical communications lead defines “Short Cycle” as:

“More than 3 boiler cycles per hour or boiler cycles less than 10 minutes in length.”

“If you look at it from simply a cycle life of the components, regardless of efficiency, that 10 minute on/ off is the guideline I have used.”


My boiler firing time will be: 30 minutes minimum in Summer, and much longer in Winter.

BTW, my boiler don’t waste heat at the end of a cycle, it burns clean until the moment that the gas valve is closed and then the residual boiler heat from the cast iron is pulled out & used by an adjustable post purge cycle of the boiler pump.

ModCons never again!

I plan to do in floor(concrete) loop design as BBP described, closer spaced exterior to wider interior runs in sq spirals, as it will also help redistribute direct passive solar gain from direct sunlight areas to non. My space is not large(one long zone) so I can do it with two loops to max of suggested length. Would it be more efficient to go with three shorter loops? Easier to balance? I would think this would allow for quicker response and ability to heat more if needed(we can have unexpected cold demands here, in MN average weather is just a statistic)

For 1/2” Pex-A

Max recommended loop length is usualy 300’

I recommend making loops shorter to reduce pumping energy.

1 GPM through 300’ needs 5.1 PSI

Cut loop length in half to 2 x 150’

1/2 GPM through each of the 150’ needs .765 PSI

That’s a reduction in pumping energy by a factor of 6.67X

There is not a real need to balance loops if they are within 10% of each other.

A 10% difference in flow makes less than a 1% difference in heat delivered.


ECM pumps like the Grundfos Alpha are a great way to reduce pumping electricity,

But you will need to have a cleanable magnet somewhere in the system to collect the magnetite that is always created by Iron in contact with the system water.

Magnetite can gum-up ECM pumps.

I used a Caleffi Dirt-Mag on my Sons boiler return pipe, a good but pricy solution.