Fireplace

[jessethompson]

Dave,

According to several sources I have talked to, an EPA approved modern wood stove generally needs to be supplied between 5 - 25 cfm to burn properly.

I would talk to the technical department at one of the Scandinavian manufacturers (Morso, WIttus, Scan, Rais, etc). There are Morso units with outside air intake knock-outs in the rear.

There are varying viewpoints as to the effectiveness of outside air intakes: http://woodheat.org/outdoorair/outdooraircode.htm

[Kat]

Hi everybody,

I like to take the opportunity to share what we recommend in terms of fireplaces and wood or wood products as a heating option for Passive Houses. Many of the points of concerns have already been touched on in this discussion.

One note upfront: There has been the suggestion that it is ok to just try and to learn from it. We would like to advise against it. In our projects we always advise the client, if they desire a fireplace, that it can be done in a Passive House, but that it can be potentially risky, that it requires special detailing, higher investment, and knowledge on their end on how to safely run the fireplace/pellet stove. Mistakes and negligence in the planning, execution and use of a wood or pellet burning device can have in the worst case scenario consequences of bodily injury.

The firebox has to be entirely independent and sealed from the interior air space of the building in regards to the combustion process. It is absolutely necessary that the firebox is truly airtight and has a dedicated and sufficient combustion air intake. The doors here are especially of concern. It is important to get good data on that from the manufacturer. This is essential to avoid back drafting of potentially dangerous byproducts from the combustion process such as carbon monoxide as well as particles. Both products are low if combustion works properly, but are more likely if the combustion process is inhibited (for example lack of oxygen). Pellets are producing less particles which are also less toxic by nature than particles that are produced during the burning of firewood. In the case of an inhibited combustion process, firewood particles contain larger amounts of aromatic hydrocarbons which can be cancerous - in comparison, they are 10x more dangerous that the byproduct from burning diesel. So, this is important.

In a Passive House we have typically a balanced ventilation system which introduces another variable that needs to be considered. Firstly, even the best rated airtight firebox will still have minimal leakage and secondly, the ventilation system could potentially significantly depressurize the house, if for example one of the two fans breaks. If the failure goes unnoticed until the house has been depressurized smoke can be drawn into the living space from the firebox or stove, no matter how airtight it is. Therefore, in Passive Houses where balanced ventilation systems and stoves/fireboxes are installed there should always be a pressure warning device present.

It has already been discussed in this forum that any exhausting device like a range hood directly vented to the outside, directly vented dryer and central vacuum cleaners can significantly depressurize the house and cause back drafting. All those appliances should be avoided in Passive Houses, period, for the obvious reasons! It is correct, that efforts are needed by a Passive House planner to overcome existing consumer expectations, preferences and even codes. There are ways around the code requirement for directly vented exhaust hoods if argued correctly as there is a balanced ventilation system installed. Directly vented dryers can be placed in the garage or somewhere else outside of the airtight envelope if really desired. Most of our clients are happily using the Bosch condensing dryer even though most of them were very skeptical at first. The individual appliances, not the washer/dryer in one ones, seem to work just fine. The same is true for recirculating vent hoods. So far only happy homeowners.

We like to advise against make-up air solutions and accommodating the client's desire of large kitchen vent hoods directly vented to the outside where ever possible, as they have significant energy losses, thermal bridging as well as higher cost and systems complexity attached to them. All of those we want to avoid in Passive Houses. In regards to fireplaces, make-up air systems pose another increased risk. Just like the ventilation system they can malfunction and cause serious back-draft conditions.

In addition to all those planning aspects, the homeowner needs to be educated about the potential dangers and on how to use their wood/pellet burning devices safely.

To avoid overheating thermal mass is very helpful as well as open floor plans where the rooms are connected with eachother.

Flue penetrations need to be detailed in an airtight manner as well as they need to be as thermal bridge free as possible.
The connection between the fireplace and the outside should be via a class A insulated metal chimney, or if possible one should be using a zero clearance chimney. There are ceramic insulating materials that are rated for very high temperatures. A double wall flue could also be used to provide intake of combustion air at the same time while needing only one envelope penetration.

We feel, from a consulting point of view, that the fireplace is enough of an issue and potentially risky if not executed correctly that there should be a disclaimer on contract documents. Please find the wording of a disclaimer that could be used below.

I hope this is helpful,

Katrin


Disclaimer Regarding Fireplace

The use of a Fireplace in a home can be dangerous due to the risk of carbon monoxide poisoning. The fireplace specified for this home must be completely airtight to the room air. Combustion air must be provided by a dedicated pipe connecting to the outside.

Because it depressurizes the house, the kitchen range exhaust hood could potentially cause a back-draft, especially if the makeup air system malfunctions. (If a directly outside vented kitchen range hood is installed, a makeup air system should also be used) If smoke is seen to be leaking from the stove into the living area, immediately open all the windows, and discontinue using the stove. DO NOT LEAVE A FIRE BURNING UNATTENDED especially when going to sleep at night. Carbon monoxide detectors should be installed in locations so that they can be heard throughout the house.

[sjohnston]

I recently looked at a Vermont Castings stove called the "Aspen". Smallest I could find with an air intake fitting on the back which seemed tighter than the Jotul's sliding-door air intake adjustment. It's rated at 18,000 Btu's/hr.
As Mike said you can light a smaller fire or use a lower Btu wood fuel- willow, poplar etc.

I'm sure others know of gasification boilers, but I will just throw this out for those who don't.

For those of us in colder climates this can heat your house and hot water and these types of stoves burn cleaner and more completely than any other type of biomass heaters.
You don't get the lovely pleasure of watching the flames or sitting next to these units but they are clean and safe. They can be installed indoors or outdoors. For an ultra-safe Passive House installation they would be located outside the house's air barrier and the heat would be piped into a water storage tank inside the house. This could be attached to a side wall of the house or even niched into the house exterior walls to absorb some of the ambient heat from the unit's room.

For a Passive House this unit would only need to be fired every fews days or so. The smallest units I've seen tend around 85,000 Btu with an efficiency of 91% -according to the manufacturer. http://www.cozyheat.net
They are not cheap- $4,000 to $5000 for just the smallest boiler plus piping etc and installation. Of course a nice wood stove can be few thousand dollars just for the stove.
The other downside is that they require electricity to run the fans which modulate the burn to its optimum state and to pump the water through the system.

This setup seems optimal for multiple buildings, ie- farms/house and outbuildings/greenhouses/workshop, and multi unit buildings would do well with this system I think. The fanciest/larger systems even have automatic feeding equipment.
Biomass is, I believe, the best option as an energy carrier in my area- Northern Michigan. Especially when done with clean burning units. I love wood stoves, but on cold mornings the populated valleys around here fill with the unburned smoke and particulates of damped down wood stoves and old style wood boilers. I swear it looks worse than LA haze sometimes. Gasification stoves do not produce this kind of pollution.

Steve

[jmorosko]

Mike

What can you tell me about wood gasifier heating systems... Garn or Barrett? You should be able to get 10-20% more energy out of the same log? by using a gasification method. Cleaner burn. I have plans for a 3 unit PH and want to use a wood gasifier boiler (water heater) system for space heat... probably either wall radiators, or in floor radiant. Advantages/Disadvantages to standard wood systems?

[mlebeau]

Jason,

I haven't looked closely at Garn or Barret. I have looked at Tarm and a couple of the Eastern European import boilers. The imports are getting smaller which is what I am looking for. They all work better with a tank so that they can be run hot and the energy stored. We get back into the whole question though of complication and overkill in a very low energy building.

All of the gasifying units I've seen take combustion air right from the building and they are induced draft. It really doesn't matter if there is nothing else to backdraft though. The energy penalty is a minute fraction of the production of the device and the combustion air quantities are very small. Leakage when it is not running is another matter.

As far as wood stoves goes, meaning closed combustion devices not fireplaces, several North American products have outside air connections. None of them , however, are direct sealed connections and this needs to be understood. I have discussed this with stove manufacturer representatives and combustion safety experts and experimented with various devices over the years. The reasons are significant. There have been some problems with embers being drawn back into combustion air ducts. This is a design issue, of course, but it is a very significant one with the current equipment. If the system were sealed, meaning from the combustion air intake through the stove and up the flue, a gust of wind can affect the draft and cause a brief reverse flow in the entire system. A few hot particles of burning embers sucked back into say an insulated flexible duct can create a risk of fire. What the industry did to reduce the risk was to create indirect connections to a chamber under or behind the firebox. So, there might be a combustion air connection port but it will be connected to the room air and only indirectly connected to the firebox. It is safer but ends up being a cold leak in an otherwise tight house. There may be other systems in Europe but that is what is going on with the equipment available here right now. This is the same reason that atmospheric drafting water heater, furnaces and boilers have a draft hood. A gust can blow the flame out the intake door. I'm happy for them to go away.

The direct combustion air connection does nothing to reduce the possibility of back drafting caused by negative pressure. The whole system is not tight enough to withstand more that 5 or 10 pascals of pressure. To use this stuff requires neutral pressure in the building. Giving it outside air does nothing to that at all. Sealed combustion boilers and furnaces, for instance, are tested and rated, usually to 50 pascals. Other rating categories, like direct vent or power vent, are rated differently. often 25 pascals. No atmospheric device I've seen is tested and rated for any standard of pressure resistance.

[jmorosko]

Mike,

So, from that standpoint, is it safe to conclude that if instead I locate the gasifier boiler unit outside and tranport the heat into the building via hot water (heating and circulating water back into the building, into a very nice insulated tank) that most of the safety hazards are eliminated... I just have to go outside (get cold and wet most likely) to put wood in the darn thing!... though that should not be a large complaint!

[mlebeau]

Jason,

I would look closely at the device. There are a whole bunch of outside wood boilers that are very dirty and inefficient. Some may claim to be gasifiers. All of the real gasifiers I've seen are indoor devices. Any of them, out in the cold, will lose efficiency of course.

I was involved recently on a project that is now getting built. They are putting a Tarm gasifying unit in an attached garage (speaking of IAQ issues). The waste heat off of the boiler then at least gets to warm the garage a bit. And the mess will not be dragged into the house.

It will be connected via a glycol loop to a tank in the basement that is also the storage for a solar thermal system. That tank, heated by either solar or wood, is the heating source with a gas boiler in parallel as backup. This is not a Passive House although it is a very low energy building. I think more and more that it is valid to question the complexity of some of these elaborate mechanical systems. Simplicity is a good goal.

As far as other impacts though I think we need to question the efficiency and sustainability of central gas, coal and nuclear power plants if we are looking for something to judge wood heat by so there are much bigger issues to consider.

[sjohnston]

I agree with Mike that it can be an overkill system in terms of investment cost, especially if you need to add another backup when you want to leave home for a few weeks in the middle of winter. But it does use local energy (for Space Heat and DHW) and does so in a clean manner.
If it is coupled with wise land/forest management practices it can achieve a carbon emission level that is better than neutral.

A neighbor just installed an outdoor gasifier from "Central Boiler". It is a downdraft gasifier not a traditional outdoor boiler which, I agree, are quite bad in terms of air quality. Putting it outdoors saves on the cost of building it shelter and on house insurance rates, but I would be inclined to put it indoors, though as I said earlier- outside the house's air barrier.
I assume from Mike's comments that a backdraft from the gasifier that is inside the home's air barrier would only be an issue when the unit is not running, but that could still be an air quality problem- smelling like tar.

In terms of complication-
A solar DHW collector system with collector panels, storage tanks, heat exchange systems, antifreeze and pumps. A pex tube system run underground with pumps and through a heat exchanger to preheat HRV air. In northern zones, an additional Space Heat system to get through the Peak Heat Load periods. OR

A wood gasifier with fans, pumps, antifreeze and an exchange/storage tank to satisfy Space Heat and DHW. Then a small electric heat system to keep the house above freezing if you go away for a few weeks. You could still add solar panels to this system for the 50% we can collect here. Or stoke the gasifier less frequently during the milder parts of the year for your DHW.

I can't put a ballpark dollar figure on these two different systems. Anyone?
The solar energy inputs are free, but inconsistent.
The wood system requires your time and energy, that shouldn't be forgotten. But it also provides energy security. That is unless the electric goes out. That's where the little tiny, wood stove excels. But I have the sense that having a wood stove or fireplace inside a Passive House would require redundant occupant warning systems and careful and observant operation. My inclination is to keep safety as passive and foolproof as possible.

Steve

[mlebeau]

Steve,

Good point about the gasifying boiler back drafting when not running. It would be no different then than anything else connected to a chimney.

Another thing to point out here is that there is nothing magic about how this stuff works in a Passive House building. They are just well insulated buildings built to a certain set of envelope and air tightness standards. Very few new buildings built in this area now are leaky enough to not take exactly the same precautions we are discussing here. We have simple utility Energy Star programs that achieve 1.5 or even 1 ach/50 in many program homes. There has been intensive builder education efforts in airtight construction methods here for many years. There are a lot of wood stoves and fireplaces in fairly airtight homes.

These pressure problems and safety needs are true in any relatively airtight home.

[Roger Woodbury]

As we get deeper into the planning for our new home which we intend to take as close to Passive House certified as possible, we become more and more convinced that wood burning is simply out of the question. We have largely given up the idea of any form of wood burning appliance at all, although we do cling to the idea that our home will be entirely off grid.

Then we do something stupid like go out to Valentine's Day dinner at our favorite restaurant that has a nice warm fire in the dining rooom, and we get the table right next to it. My wife, with her undiciplined back discs gets to sit right next to it, and the entire discussion about fireplaces gets reopened.

We have no illusions about trying to find a way to make a fireplace, wood stove or other combustion heater compatible with Passive House. But last Tuesday we were on the house site and with the temperature around freezing under a bright sun and clear skies with no wind at all, the temperature felt like fifty or more. We discussed that had the house been built then would could easily have been sitting on a deck in a light sweater having lunch.

Right now it is a theory. What we are thinking about is building a three wall sunroom onto the southwest wall of the house. This sunroom will not be intended to be an integral part of the main house, but will contain a masonry chimney and fireplace. During periods of bright sunny and windless weather in all seasons, this room could be used as a general utility room, taking advantage of the heat output from the fireplace when appropriate, and the passively collected solar heat through the roof and other large windows.

When the sun was gone later in the day, if the overall temperature was extremely cold or in periods of high wind, the sunroom would be closed off from the main house, and the normal augmented HRV/ERV air circulation method for heat would be used. Our plan at present is to install hydronic tubing in the bathroom and kitchen areas at least, and perhaps an additional zone in the master bedroom. Hot water to be supplied by solar collectors with storage.

So my question is if there is an additional room that is NOT intended to be, nor IS Passive House certifiable, does the entire house get rated for the entire living footprint? This might sound like a foolish question, but since our intent is to be off grid entirely, the house will be constructed once and will not be added on to later, so we are planning the living envelope as one exercise.

Roger