Block off plate and insulation. Necessary or not?

[Spaceman]

I had a Supreme Lumis insert installed and my installer said that a block off plate is not necessary. He is WETT certified and has installed hundreds of inserts and says he never does it. Claims that as long as it is sealed at the top, which it is, there is no air flow, so no need. I was surprised as I thought that common practice is to put insulation above the insert and a block off plate to seal the chimney above the insert. In doing so, it will deflect more heat into the room and not up the chimney. Maybe that difference is incremental. Also I read that it may not be a bad thing t have more heat warm the liner, but the other thing is about whether cold air from the chimney will seep into the room when the insert is not in use. It is installed flush, so no big open space, but there are air vents of course.

My chimney is brick and on the outside and my liner is insulated. Also, I am in Vancouver which does get cold, but nothing like eastern cities.

 

[begreen]

With all due respect to the installer, a well installed block-off plate makes a big difference in heat output of insert. It keeps the fireplace firebox cavity and thus the insert's convection jacket warmer. Otherwise, a lot of heat is lost through the mass of the chimney. If proper insulation can fit behind the insert, the gain is even better.

 

[Spaceman]

Thanks for your reply. Supreme specifically say not to put any insulation behind or around the insert as it voids the warranty and can cause overheating, so I am really only concerned about the insulation which would be above the block off plate. Besides the heat loss issue, what do you think about the issue of cold air coming into the room when it is not in use? Is that a meaningful concern?

 

[begreen]

In our climate, this is not usually an issue unless there is an open air feed or ash trap inside of the fireplace cavity.

 

[stoveliker]

With an outside chimney I would put a block off plate in for sure.
Insulation is tricky given the warranty.

 

[Ctwoodtick]

The effect of no block off plate will depend on a number of things, including your weather, size of home, insulation of the house, etc.
I have no block off plate and house heats fine. I’m heating 2 floors with 1500 ft total. I have the Jotul 550 insert which is 2.1 cubic sq fr firebox I believe. Insulation is ok in house but not great. Chimney is split between living space and garage so I guessing that would be considered mostly an external chimney. Live in southern New England.
I would have made or asked for block off plate had I known at time of install. Depending on your variables you may be good. Yes, you will lose heat, but it may not be major.
Of course, my experience is with no block off plate so I have nothing to compare to.

 

[stovelark]

Block off plate is always a good idea, every chimney breathes. Insulating the fireplace back walls and sides is a good idea too, if room allows. Insulation around the stove's shell may not be necessarily good. Any way of losing air up the chimney is always a good thing, in my view. Good dry wood, stove ran at proper temps, house warm, all is grand.

 

[Spaceman]

Thanks for the comments. As for insulating around the insert, Supreme specifically state that this is a danger as the insert can overheat. They are clear about not doing that, but a block off plate is a different thing. Seems that some people do not have them and are fine, but most feel that it is a good thing and some feel it is quite important.

 

[stoveliker]

if you're concerned about the insulation on top, I'd just add a plate (and seal it with silicone). Having no convective air flow up the flue is going to be a large part of the improvement that an insulated block-off plate will have.
That way you get most of the benefits without risking warranty issues.

 

[begreen]

if you're concerned about the insulation on top, I'd just add a plate (and seal it with silicone). Having no convective air flow up the flue is going to be a large part of the improvement that an insulated block-off plate will have.
That way you get most of the benefits without risking warranty issues.

I think the concern is about placing insulation directly on top of the convection jacket surrounding the insert. Ceramic or rockwool insulation above the blockoff plate will be fine.

This is from the Supreme Fusion manual:
Fill the space between the liner and the baffle of the existing chimney with Stone Wool insulation to prevent heat from escaping.

 

[Spaceman]

In our climate, this is not usually an issue unless there is an open air feed or ash trap inside of the fireplace cavity.

Can I ask what you mean by an issue if there is an open air feed? I assume you mean an outside air feed to help with combustion. Isn't that a good thing?

 

[begreen]

We're talking about cold air seepage when the stove is not operating. Some fireplaces have an open ash trap or fresh air feed. That can possibly lead to a cold draft from the fireplace, especially if the outdoor intake is on the windward side of the house.

That said, yes, outside air for combustion can be a good thing in many cases. In others, maybe not so much.

Sweep's Library - Why burn outside air?

Should I burn outside combustion air in my stove or fireplace?

hearth.com

Alternative point of view:

A non-commercial service in support of responsible home heating with wood - Outdoor Air Supplies

The Wood Heat Organization was formed to support the public in the responsible use of wood energy in the home. We fulfill our mandate by providing reliable information, by conducting research into wood heating-related issues and by representing the public interest in discussions of policies that...

woodheat.org

 

[Spaceman]

Right. My installer (reinstaller, because the installation was so badly done that it had to be redone!) used the ash pit access to install a short pipe to the outside so that there is a fresh air intake directly into the insert cavity area. it is not on the windward side and is low down and well protected by fencing and the next house. He did it just to help startup as my house is well insulated and I was having to crack a window. This is meant to avoid needing to do this, but if it does end up causing cold, I can always block the intake from the outside.