Geo water to air + high effciency wood burning fireplaces

[gabby]

An interesting question was poised to me recently that I couldn’t answer with certainty. Maybe someone more experienced with residential aux heat could chime in with their opinion (s).

There are many manufacturers of wood burning inserts and wood burning fireplaces, with some offering a means to connect to existing ductwork to distribute heat to other rooms, and some offer stand alone ductwork for this same purpose. This option usually includes one or two small blower accessories. Which is better?

The second question has to do with disruption of the geo heating cycle. If distributed aux heat from a fireplace raised the ambient temperature of rooms or room with a thermostat, wouldn’t that cause the geo to shut down the same as if the set point was reached? If so, then the distribution of this aux heat would no longer be as efficient since the blower is no longer running (warmer air from the cold air return). Does this mean that a manual setting of “Blower On, Fan On” would be the normal course of action once the heat source took over the primary heat function? When it is cold and stays cold then the two could work more in conjunction with the fireplace adding that “emergency heat” option, but when temperatures fluctuate by 20 or 30 (+,-) degrees from morning to night I can see this as being problematic, especially if the blower is set to run continually.

Once the major room gets warmer than desired, one could change the damper to direct more heat up the chimney (summer operation), or reduce the airflow to reduce the burn rate to moderate room temperatures to a more comfortable level, a common practice for night time burning to extend burn time from 9-14 hours (depending on size and make of fireplace).

[Choice Mechanical]

I'm glad you brought this up because it seems like a logical combination of technologies. Geothermal and wood burners tend to appeal to the same type of customer, and I'd like to know the various options for using them together in an efficient manner. I had heard that it would be possible to run a hot water coil from the wood burner into the return ductwork, but would like to hear from others the most efficient ways to use both.

[Pine Mountain Energy]

I can contribute as both an HVAC contractor and homeowner with one installed, I have a Quadrafire 7100FP in my great room, and although I dont use it for heating my entire home, it will do the job.

When I built my home I was concerned about uncomfortable temperature in the great room when the Fireplace was firing, so I installed extra supplies, and
2 Large (10X18) returns high in the great room (cathedral clg), they are controlled by Dampers, and
space temp. sensors, that will opperate the dampers, start the blowers in the GEO units, and over-ride the ECM motors to full speed, when the temp in there goes more than 3 degrees above set-point, I used it last week for the first time and it worked great, except that the room temp went about 5 degrees above setpoint with a roaring blaze, but it was able to keep the rest of the house at 70 degrees very easily, since we are a sheet metal contractor, I took my manual D designs, and went almost 25% greater capacity
to account for this, and quiet operation at high air flows. The extra cost of the ducting was well worth it, as far as heating with wood, I think its romantic,
once in a while, but mostly a pain in the a$$.

Mark

[gabby]

I don't understand the overcapacity increase of 25% for the ductwork. The design should be for low to high speed for the ECM blowers. Are you saying the cold air return was made bigger than the Manual D design or the whole system, including supply lines? If yes to the return, then I do understand because Manual D and the industry standard nearly always under-sizes the return duct, more for convenience of fabrication of the transition pieces, or limited space around the air supply return. The only time I can remember where this was not true was for coal, gravity furnaces that had no fans. The later versions that had natural gas conversions lowered the duct size on the supply and return because of the fan. Even then, they were huge compared to today's standards.

[Pine Mountain Energy]

Air movement noise was my main motive for oversizing
both supply,(additional supplies) and larger returns were designed for balanced SP in addition to being oversized, The ECM motors are pushing the required air flows thru the unit, & so far there have been no problems.

Most of my 6" rounds are only putting out 70-80 CFM
as compared with 120Cfm that most guys out here are trying to push, a side benefit that I noted was lower
blower current, but Mech noise, is a big issue for me
as this is a Heat Dominated climate, and most new construction here desires wet heat, it was more of an experiment, and so far Im quite pleased with the results.

Mark

[gabby]

LOL....10-15 years ago the smallest duct for supply was 6" and now they are down to 4", so I understand your "oversize" choice. I'm also in a heat dominated area and my home has a 42" x 10" cold air return for the main trunk, with 6" round drops off a rectangular trunk on the supply side. Today's standards would call them over-sized, but this was the standard 30 years ago. Besides derating motors as a modern change for lower power requirements, they seemed to have shrunk the sizing of ducts, reduce the steel contents, and cheapened them to paper...noise not being a major concern.
I have a two stage compressor that develops 200 PSI driven by an open frame 1 1/2 HP motor.
I would have to replace it with a 5 HP motor to meet the torque and speed of the old style. I believe they call that progress????? It is 47 years old and hums right along whenever I need it. My Sears upright freezer is 52 years old and still gets -10 below. I'm totally in favor of improved efficiency, but not if it shortens the product life, so anything that lessens the load allowing something to work easier is a plus. Today's products have a very short life in comparison.
Thanks for clarifying your statement regarding over-sized ducts and the explanation.

[Mark Custis]

I like moving BTUH with water. I also prefer to use the most efficient fuel as long as possible therefore I dis-like having to shut down a heat pump(s) to protect it from high temperatures, like trying to duct a fireplace to the return-air side of a system.

If I were forced into designing such a system I would take the supply and return of the fireplace or wood stove off the supply side of the geo unit down wind of the coil. This assumes tightly sealed duct work and a blower off the wood burning system big enough to over come the static pressure of the geo unit's duct work. I would protect the heat pump further with a pair of duct sensors, one in the supply for LAT and one in the return.

Besides finding, cutting, spiting, moving and cleaning up the mess, I do not like the fact that a wood burner can take all my hard earned conditioned air right up the flue. I design with huge amounts of make-up air for combustion.

My choice would be using a hot water coil down stream of the refrigerant coil.

Gabby:

I think such a system could mess with the geo cycle rate and end up with the rest of the conditioned space looking for more heat. That could be reduced using a LAT sensor and the duct work I described. It is very difficult to use combustion air to regulate a burn, it can be done but it makes a big mess out of the flue.

[Pine Mountain Energy]

I believe the question was regarding High efficiency Fire places that are readily available, Most of the EPA approved units will burn for 8,10,12 hrs on a single loading, they have out puts in the 50,000 to 90,000 Btu range, (mine is rated at 90K ) all of these units are designed to pull all the combustion air from outside. The problem is what to do with 50K + btu in a single room, in my system the Hot High return air is diluted with cool return air from the other rooms, so Im not exceeding the 80 degree EAT at the unit, What happens here, is the Geo stat will be satisfied ( usually within 30-45 min), turn off and remain off until the fire burns down, then starts up again, the damper, and fan control is to address an overtemp condition that occurs in the greatroom & kitchen,and will circulate the free heat throughout the home, If the ducts are properly sized, you wont exceed the EAT for the GEO. Im still toying with the idea of using some sort of EAT stat in the High return drop, to shut down the GEO early, to save energy, when the fireplace is in use.

Mark

[gabby]

If I understand your system, you did not duct for stand alone fireplace heat to other rooms, but used the cold air returns (damper controlled) to mix the cold air returns from the cooler rooms. I like that, especially when you answered the question that the geo stats called for shutdown, but you had excess heat to remove from the great room. The stat you used to control the dampers and bypass the geo controls to turn on the ecm blower addresses my concern of uncomfortable heat in the great room until the fire burn rate slows. It eliminates a bulkhead for the trunk line, and the extra drops on a per room basis. What is the estimated cost for the damper control (flat ceiling would only require one cold air return), sensor and the extra stat to override the ECM blower controls? I believe he is considering a 70K Btu/k wood burner.

Right Mark...way to messy to consider a conventional open wood fireplace. This is something like a Napoleon NZ-26 with external air input...but a different brand.

[Pine Mountain Energy]

Since we fab our own duct, I would guess that that trunk, with transition,& damper to return trunk is about $ 275.00 including the damper, the controls were quite easy (my area of expertise) just a digital cooling stat, 24 v trans, and a couple of RIB relays,
another $50.00 or so...

Mark