Keystoker - advice on size

Asking this question for a friend (who should be joining this forum soon).

He’s considering buying one of the following:
Lightly-used KAA-2 in excellent condition at a fair price.
Used K-6 in good condition - priced a bit above the smaller unit.

Currently has a 110k btu hand fed bit boiler in a detached garage with a 98k modine dump zone. Taco 0010 at the boiler.
1” insulated underground PEX runs approx 50’ to his house - feeding 100k coil in an electric furnace/AC unit in a new construction home. Has a 10 plate on the input to his electric hw tank. Also a pex-hydronic slab in the basement and a 50k modine dump zone in the house garage. Home is well insulated and well planned-out regarding the insulation under and around the cement floor. The hand-fed has been in place for 3 or 4 winters and is easily keeping up with the load. His dump zones run from time-to time when the heat load is low- medium.
I don’t have any details such as square footage and a heat load calculation has not been done. He hasn’t had any problems satisfying the house load with the Existing pump and 1” pex. He keeps his house WARM - 74-76
I had a very similar set-up until I replaced my hand-fed with an EFM520.
He is considering doing the same, but prefers the hopper feed of the keystoker boilers.
He is considering keeping the hand fed in place and adding the stoker boiler in series or in parallel. He thinks he may still run the hand fed during the days and weekends (bit coal and free wood is cheaper) and let the stoker idle, taking the load when he chooses to let the hand-fed go out. He acknowledges that in a few years, he will likely remove the hand-fed boiler from the system. He will not be supplying DHW with the system.
The hand fed holds approx 50 gallons of water - similar to the larger of the keystokers.
The KAA-2 has half the water capacity and a lower btu output.
He wants to run as efficiently as possible, but doesn’t want the stoker to be undersized and struggle.
I hope he joins in to provide more info.
Opinions are welcome.

Hello. I would choose the KA-6. Although I think the KAA-2 is a large enough unit to do the job, I prefer the design of the KA-6 stoker. I believe the KA-6 also has a larger hopper, and can take a larger ash tub.

I would also remove the hand-fed. Once he sees how easy it is to run the stoker the handfed will just gather dust - not worth keeping it around just to burn a few sticks.

Thanks Rob.
I'm assuming the Keystokers "idle" well when there is no load.
I'm familiar with my EFM520 in that regard - it's pretty simple to maintain the fire using the air/feed and timer settings.
Thanks for your input!

Well, let me ask you this;

How well do you think your 50-93 would heat your friends house?

I heated my 1700 square foot place with a 50-93 from the basement. The 65' un-insulated basement is also 1700 square feet. The 50-93 did a great job, All from one end of basement. It was a little chillier at the other end of house, as well as cellar. The 50-93 would be cranking right along...

I replaced the 50-93 with a Kaa-2 boiler. The little Kaa-2 heats every inch of the house, and basement from one end. And does it with ease, using less coal... My concern would be that 50' of underground... Never had any problems with the Kaa-2 stoker.... Nice little boiler... Very happy with it... The K-6 may be too much boiler for your needs, but would give extra capacity if adding on. And then there's that 50' of underground again... Without being too technical, the Kaa-4 I think would be a nice fit.

Robo wrote: ↑

Fri. Mar. 06, 2020 11:43 am

Thanks Rob.
I'm assuming the Keystokers "idle" well when there is no load.
I'm familiar with my EFM520 in that regard - it's pretty simple to maintain the fire using the air/feed and timer settings.
Thanks for your input!

The idle well once you get them dialed in, but are more picky about the timer settings than an EFM.

The kaa-2 has the same size stoker with three fire grates as the ka-4 and kaa-4-1-which I have to heat my home.

The key to keeping a flat bed stoker running well is maintaining the boilers water temperature at all times AND
making sure it is feeding the rice coal at a slow rate of feed to permit the combustion air fan runs well and provides enough combustion air to burn the rice coal to a fine powder before the klinker falls off the end of the fire bed and falls apart as it hits the ash bucket.

Letting the hold fire timer run for 14 minutes every 30 minutes will provide plenty of hot water for heating like mine does using 4 minutes at zero time, 3 minutes at 7 minutes, 4 minutes at 15 minutes and 3 minutes at 22 minutes. This keeps the water inn the boiler hotter for longer periods and you will not have to deal with out fires.

This is accomplished by using the hold fire timer to its greatest advantage where the hold fire timer maintains a hotter water temperature no matter how often there is a heat call by running more often.

A kaa-2 will provide enough heat for the system as long as the high limit temperature is kept at the summer operating temperature range being 140 low -160 high or 150 low - 170 high which is what I use for my temperature settings.

You also need to keep in mind that the pex must not be damaged by higher temperature water.

If the forced draft combustion fan system on this kaa-2 coal stoker boiler has the smaller secondary combustion fan you will want to unplug it as it will not be needed.

I would avoid plumbing boilers in series, when one is off it is giving up heat from the one running. Both up the chimney and to the room. And if you have to service one it will complicate that process.

He drew a diagram of his system.
I was mistaken when I mentioned running the boilers in series.
It’s piped as a primary / secondary - he’ll have the ability to isolate the hand-fed.