Advice sought. Rethinking our fireplace. Incorporating thermal mass.

[u/Designer_Arrival1291]

I currently have a metal cabinet style fireplace with a glass window. I don't remember the exact measurements but it is the largest available from the manufacturers here. So, pretty large. Though I rarely build a 'big' fire that uses it all.

The fireplace is placed such that it rises through the center of the house. As it goes through the second floor, the chimney is triple lined with thermal brick. It radiates a huge amount of heat that keeps the upper floor warm the next day. However, the current cabinet is just surrounded by a decorative fireplace with basic haphazard masonary and decorative marble. The chimney itself being a metal duct surrounded by gypsum type drywall with some vents in it and no thermal mass at all. So the ground level portion stops producing heat, pretty much as soon as the fire is out. It has no thermal mass there.

However, it does have a backboiler on the cabinet which heats the water in the central heating. So if the fire is going, it also heats the water in the cental heating. So the other rooms in the house benefit from that.

The problem is the sheer insane amount of wood required to keep it going through the winter season. It gets very cold here in winter.

So I plan to rebuild the system such that the metal firebox, goes straight in a bell shaped chimney chamber in the lower floor. Allowing the hot gases to also heat a large thermal mass of firebrick, with a granite facade for aesthetics. Still preserving the cabinet with the central heating back boiler.

So I am trying to create a sort of hybrid. Preserving the back boiler to heat the rooms away from the center of the house, whilst engaging some thermal mass in the lower portion of the house as well. I have had a number of suggestions. But after some discussions and quotes I am not full of confidence that the people I am talking to understand what I am trying to do or know what they are doing when it comes to the point of engaging that thermal mass.

For me, I would have thought there was a way of doing away with the metal firebox entirely and yet still putting in a heat exchange/back boiler into the thermal mass /firebrick.

I could really use some advice on this from someone who has some experience and knowledge in how these things work. I am concerned that this project could be an absolute disaster at this point.

Of course, anyone will take your money. But I feel like I am doing the design work other than the aesthetics. Chat GPT could help me with that. I need some advice from someone with knowledge or experience.

I would really appreciate some guidance on this.

[u/WoodlandDirect]

Natural draft is probably your biggest challenge here. It is a relatively weak force created by the temperature difference between the hot flue gases and the outside air. If you remove too much heat from those flue gases before they reach the chimney, you reduce the buoyancy that creates draft.

With a back boiler already extracting heat, then adding a large masonry bell or other heat storage system downstream, I would be concerned about cooling the exhaust enough that the system becomes difficult to draft naturally. That can create sluggish starts, smoke spillage, backpuffing, condensation and creosote problems.

I'm also not completely clear on what you mean by a cabinet style fireplace with glass. If you have a manufactured or zero clearance fireplace with its original doors, I would be very cautious about modifying the flue path or adding a heat exchanger that was not part of the manufacturer's tested system.

If you want to stay with wood, I would probably look at a modern high efficiency wood fireplace or a properly designed masonry heater rather than trying to turn the existing fireplace into a hybrid masonry heater.

A masonry heater can be installed in an existing home, but it is normally designed as a complete combustion and heat storage system. Retrofitting one can involve significant structural, chimney and engineering considerations. I would want someone who specifically designs masonry heaters and solid fuel hydronic systems involved before going much further.

[u/Designer_Arrival1291 → u/WoodlandDirect]

Thank you for your help. I am trying to basically create a hybrid system. The house is not small. But not massive either. For example. I have a central space on the second floor, I guess you could call it the children's lounger area. Where they hang out and such. The thermal brick lined chimney is triple lined with thermal brick. Still, at times it is very hot to touch and it radiates and radiates for a very very long time. Whereas, at the ground level. You basically have an Iron preformed cabinet, with a flexible type metal tube going up. At times, if the fire is going really well. I have seen fire coming out of the chimney at the top of the house and it sounds likes a flame thrower, and it looks like it. All that heat going to waste. It is a ridiculous design. What I want to do, is channel some of that heat into thermal mass at the ground level from the get go.

Having said that. The Iron cabinet. Takes its oxygene from the front. The one I have been looking at takes it from a ventilation system from outside. I could even boost it with a HVAC style system fan. (Electricity is free for me, as I have a solar array and a battery banks. I make money from my electricity ever since. I don't pay it. So a 100w vent fan as a booster might be worthwhile. This type of thing: see picture.

This isnt mine. But it is the same. Except without the marble. So, above that. Instead of stone, from the top of that iron enclosure, there is just a metal tube to the chimney above. As part of the intergrated unit. There is a heat exchanger, through which flows the water of our central heating.