I’m a new member to this site and I like to gather some input on a very specific topic. I posted this topic a week ago in an other forum on this site without much succes. It looks like this forum on radiant heat is the right spot.

I’m involved in the design and manufacturing of indoor wood boilers with visible fire. In recent months we were contacted by several professionals involved in the construction/design of passive houses, with the question if we could build/design an indoor wood boiler with visible fire (water stove) with an overall heat output of around 15,000 BTU/hour. Part of this heat would be radiated into the room where the unit is installed, the balance would go to the water and used for heating other spaces or domestic hot water preparation.

We did some research into this topic and came up with the following concept: the "ideal" indoor wood boiler, construction wise and towards EPA particul emissions, would have an overall heating output of around 30,000 BTU/hour of which around 20,000 BTU/hour will go to the water and 10,000 BTU/hour will be radiated into the room.

The unit would have a single pane viewing glass. The 10,000 BTU/hour of radiant heat can furthermore be limited by the use of a double or triple pane viewing glass, and consequently more heat would go to the water.

This 30,000 BTU/hour wood boiler would also require the implementation of a small buffer tank (50 to 100 gallons), to store the heat for later use. This buffer tank can also be used in conjunction with a solar hot water collector system mainly during the spring and summer months when the wood boiler is not used.

Question: Does this size (30,000 BTU/hour) indoor wood boiler with visible fire seams right for the passive houses currently under construction/designed?

For more information on the concept of these indoor wood boilers you can visit the website below.

Thanks for your input.

Marc

Hydro-to-Heat-Convertor

DHW would be stored in an indirect. Output would be sized to the heat load. Storage-not really a buffer-would also be sized to the heating load.

Glass will not significantly effect output, DHW or otherwise.

This doesn't seem very "passive"... costs involved would have to compete with some of the very extreme other elements in a passive house design. Also, 10k to the room is more than the total heat load for some passive houses under design conditions.

I think you'd have a hard time not cooking people out of their passive houses with these kinds of outputs. For reference, a true passivehouse usually has no more than a 1 or 2 kw electric duct heater.... 6kBTU/hr load is high for passivehouse. And that's "coldest day of the year".

I think your strategy would need to dramatically dampen the heat radiation to the space and dump nearly all of it to a tank. Or, find a way to burn a small stick of wood for a long time for ambiance! Mirrors?

EPA regs were drafted to by OWB Manufacturers to give their smokies some credibility. Hardly a high bar to aim for.

With a 10k peak, even that might be 5k norm so you would not really be aiming for passivhaus. But that begs the question exactly how large that market is.

One thing not mentioned is make up air, you would need to include a direct outside air supply.

There a few gassifiers over 90% efficient, 80% would be more a norm and I do not know how far you can down scale the technology. I do not think it really matters.

Cord is 128 cu ft, say 25m btu per cord. so 200,000 btus pes cu ft at 100% efficiency, divide by 2 so a cu ft produces 100,000 btus which may be the daily demand for this sort of construct. And a pretty small fire.

So one evening fire should be enough to heat the house for a day. The bigger issue in these circumstances is domestic hot water. Sounds to me that you would need a bigger tank with a much larger percentage going to storage.

Or you could fire all day and store enough heat for a week if you had enough storage..

It would be nice if you could vary the heat split, might be a tricky design issue.

Thanks Como!
Exactly the input I was looking for.
More heat to the water we can accomplish by a double or tripple pane window. Each pane takes away about 8 to 10% of radiant heat.
But, you are wright, STORAGE is key.

Just a few more thoughts.

You need hot water year around, where I am there is not much difference in the ground water in winter or summer, cold all the time.

If the use could add or remove the glass that would be once way of providing heat and hot water in the winter and providing a little heat but mainly hot water in the summer.

Or you could buy and outdoor or separate room boiler with DHW and space heating storage, the GARN unit comes to mind.

I burn wood in a small Morso and love to see the flame. It is 75% efficient and used when I am in the mood.

To address the DHW during summer you can do a combi system:
Indoor water stove for winter and transition seasons to a storage tank.
Same storage tank fed by a Solar Hot water collectorsystem integrated in the roof and designed for the family that lives in the house during spring/summer
By depending on Solar hot water only in summer you can go with the cheaper flat plate Solar Hot water collectors (versus the vacum tubes)
The sun is free all year round but especially in summer.

I am familiar with Garns, that is actually the route we are taking. But gross overkill for passivhaus. I have lost the link, there is a condo development in Switzerland, totally solar for heat and hot water. The tank is mounted vertically and 3 or 4 stories high...

To keep from overheating a PassiveHouse most of the season you'd need to be able to reduce the min-heat emitted into the room back to under 3KBTU/hr (maybe even under 1KBTU/hr) if you need to run the thing at 30KBTU/hr long enough to satisfy domestic hot water heating loads.

At 3K of output it's adding the output-equivalent of 5-6 active adult humans to the background cooling load, which can be managed during the winter by adjusting the ventilation rates upward, but in summer it might require a small compressor to dump the additional load in some designs if there's much hot water demand. At 10KBTU/hr min-output it's like having a party in there- they might be kicking the windows open to stay cool enough even in cool/not-cold weather if it runs for more than an hour or so at a time.

You may be able to get there using aerogel to insulate viewing window, and adjustable (or even thermostatic) insulated panels or convection vent louvers to fine tune down to near-zero the heat emitted from the rest of the unit. Even the exhaust venting insulation & water heating loop plumbing insulation would need some attention. The "view" through aerogel is pretty neutral-density- more similar to the view through finely ground glass rather than something capable of seeing the fine detail of the flame-front. An aerogel viewing port might be considered a nice aesthetic to some (but probably a PITA for boiler engineers trying to get a visual on what the fuel/air mixture is doing. ;-) )

Overkill, in what respect?

The design-condition heat load of most PassiveHouse designs is well-under 10KBTU/hr, and the seasonal average heat load is typically under 2K ( which is why they don't bother building heating systems for them and just add a modest amount of resistance electric heat.)

Most of even the smallest off-the-shelf wood-fired units today is 8-10x or more oversized for the heat load, forcing the occupants to dump heat in the dead of winter to stay comfortable.

The Garn gassification is a batch burner with storage, suitable for solar, dhw, etc. Radiant floor with sensor inside, outside and reset can control all.

Even the smallest Garn WHS 1500, has a burn rate of 350,00 0btu hr.

So a couple of hours burn would be good for a couple of weeks heat.

For that reason I think it is overkill, with batch systems and storage you do not have to match load with boiler size, but I think there are limits. Plus it is big. I would have thought 500 gallons storage, wood for winter, solar for summer should do it.

A small biomass gasifier would be nice, but a bit overkill unless you use a lot of hot water. The ambiance of a visible fire, coupled with hot water generation would appeal to me. And be able to give you an instant ish heat boost.

So, if I follow the conversation, we are down to 3 to 5 kBTU/hour for Passive Houses. Does this included DHW?
If so, I will need to incorporate tripple pane glazing in the stove door, what wil limit the radiant heat into the living areas and more heat towards the water.
500 to 1000 gallons of storage, and a storage tank with minimum a dual heat exchanger: one for the indoor wood boiler and one for the solar hot water collectors.
The ambiance coupled with the generation of hot water is indeed the selling point of this woodboiler, as also the contemporary looks/design.

Como, what is like the average square footage of these passive houses?
I'm asking just to see if there is "space" available to integrate this indoor wood boiler concept as a freestanding or insert configuration.
Or does space comes at a premium?

The Garn is exceptionally large. A bit brutal in its approach but simple and effective.

I can only comment on the ones that I have seen, which were probably a bit smaller than you might otherwise expect for a new build of that size family etc etc. A McMansion Passivehaus seems contra logical, but I am sure there are some out there.

If you want more ambience out of it without cooking the occupants, again, aerogel (not triple-pane), will give you a much lower U-value, allowing for a much larger viewing window.

Hot water loads will be much higher than the heating loads, but they're batched. Think of this wood boiler more as a highly-insulated hot water heater, not so much a space heater, but give it the capability of an operator-controlled heat release to the house of 10KBTU/hr (peak), but on the sub-3KBTU/hr side when space heating isn't desired or necessary.

The amount of tank insulation required to make a Garn practical at such low loads is a bit silly, as is the notion of heating a PassiveHouse with radiant floor, where at design conditions the floor temp would still be barely above room temp.

It's easier to hit PassiveHouse annual BTU/ft^2 with a McMansion than a cottage (the ratio of exterior surface area to floor area isn't linear), but the PassiveHouse spec recommends an upper bound of floor area of 540 square feet per occupant. This is somewhat on the small side compared to most US homes, and a fraction of a McMansion. Energy use per occupant has been proposed as another metric to include, but SFAIK that's not part of the PassiveHouse cert (yet). The recommended floor space upper bound is not as severe a limitation as it might at first seem- measurements are from the interior-only and doesn't include plumbing chases, staircases, the thickness of interior partition walls, etc. and basement floor area is discounted by a 0.6 multiplier. This is in contrast to the US standard of counting every square inch that's inside of the exterior paint (to deliver the largest arguable number possible, and independent of how much usable space exists.) A 2160' house by US measurement standards might feel slightly cramped for a family of 4, but has a fair amount of elbow-room (but not at all McMansion-ish) as measured by the PassiveHouse protocol.

To get an idea of what you can see through an aerogel window, check this sample:

http://img2.allvoices.com/thumbs/event/609/480/48057290-aerogel.jpg

A window that thick would have a U-value between 0.05 and 0.08 (yes, the decimal point is in the right place.) With the fire behind it it would glow & flicker, but the finer details of the flame would be lost. Going half that thick would give a crisper view, but would emit more heat an would still outperform any triple-pane.

It's not cheap when you're talking about daylighting a building with the stuff, but for a 30KBTU specialty/luxury boiler it won't be a product-killer on cost. With aerogel you could make a decent sized window out of it with plenty of light ambience, yet extremely limited heat emitted to the room.

Where to find technical information on this aerogel product and do you know some manufacturers.
Wondering if it holds up to 1,700F temperatures inside the firebox.
Also the impact resistance is important, it is not allowed to crack on impact (UL-testing).