Frustration over buying an insert!

Insist on an blockoff plate. They increase safety, performance, and heat output, noticeably. They are readily available for sale online, and all you need to do is trim them to fit. If they don't want to install one, buy one yourself and put it in. It is a must-have for inserts. My installer did not install one originally, giving me the same information yours gives you. Let's face it, it's extra work, costs more, slows down installations, etc. That said, I had one installed in year two, and it was a completely different stove. Much better performance, better heat output, and less troublesome.

-- Mike

Too be honest, if you asked any the sales people at the place I work about a block off plate, they also would not know what you are talking about. Don't be surprised. We never put them in, there is no demand for it and it is not required. If we were to put them in every time we would probably have to raise the install price slightly and no one would like that.

Would we refuse to put one in? No. But we would charge extra up front if a customer wanted one.

Mike, where would you buy one online? I have searched and never been able to find one.

Here's one for a oval liner, and I'm sure they have 'em for round as well:

http://www.chimneylinerdepot.com/store/comersus_viewItem.asp?idProduct=54

Cocey, you may just have to end up doing it yourself. I'm not the handiest guy in the world, but I was able to make one work. Get some sheet metal and a tub of stove cement to seal it up. Here's a thread of mine, I insisted my "installer" make one for me, but it was too small to cover the entire opening from the poured liner I have, so I used some sheet metal in conjunction with it, and a WHOLE LOT of stove cement to get a good seal. Not pretty, but it got it done.

https://www.hearth.com/econtent/index.php/forums/viewthread/12405/P0/

We always fall back on the mfr's instructions, and NFPA-211. Most instruction booklets will detail the install with a "direct-connect", which does use the block-off plates described previously. I have never seen any with other than an oval of 7 " equivalent. Ovalizing a round liner more than a tiny amount changes flow enough that most professional installers will not use an oval take-off unless it is considerably upsized. I know the cross sectional area doesn't change, but nevertheless the flow does. On modern stoves (which are 6" vented) the better size is an 8" oval to pass thru the throat. I dont think I've seen any that size. For the best flow characteristics most will stick to the vent size and configuration (6" round) and remove just enough frame or plate to install. Sluggishness and turbulence due to size and shape changes can translate to smoke spillage for the customer opening the insert door.Which means call-backs and complaints

What is seldom addressed is the change in heat flow when a throat plate is installed behind an insert stoves surround. Modern inserts all have built in heat exchangers so the "efficiency" aspect of sealing this area is a little dubious. It's installation may increase the temperature in a confined space (behind the stove surround) but that doesn't mean that measurably more heat will now proceed into the room, at least not enough more to chance the performance issues. Aside from that , the geometry required usually makes a nightmare of a hook-up for no benefit. The big unknown is clearance issues. If you no longer allow that trapped heat to move are you setting the stage for pyrolysis at the fireboxface/breastwork/mantel? Original construction flaws aren't always visible except forensically, but I have seen charred headers where no headers should have been.



A "partial liner" or a "direct-connect" Must have a block off plate. under most other circumstances I see no need for an additional headache. Manufacturers instructions and AHJ can always veto that.

J.T. Black Goose

I did a search in this site and found some great pictures. I am going to insist on one, or they won't get the job

In my case the dealer used fiberglass insulation packed in the open space where the fireplace damper was!Not very pretty but does the job.The top of the full liner is also sealed from the rest of the fireplace flume.

woodsie8 said:

I did a search in this site and found some great pictures. I am going to insist on one, or they won't get the job

Click to expand...

You must refer to the manual. eg: my new insert calls for a block-off plate if installed in the US but not in Canada. Calls for a hearth extension of 16" usa and 18" canada.

I did not install one and find that the recroom where it is installed is "cooking hot" 86 degrees as I type this. I don`t have a view one way or the other on this one. Will say though, that a full liner to the top of the chimney even though not required in the US is the smart way to go. And that is my "canucks sense worth" :cheese:

I had my insert installed a month or two ago. When getting estimates everyone I asked about block off plates said it was not required as long at the liner goes to the roof. They also told me that the hot air around the liner keeps the liner hotter reducing creosote build up, I have no idea if that is true but it did sound logical.

Jotul Rockland, CT

blackgooseJT said:

What is seldom addressed is the change in heat flow when a throat plate is installed behind an insert stoves surround. Modern inserts all have built in heat exchangers so the "efficiency" aspect of sealing this area is a little dubious.

Aside from that , the geometry required usually makes a nightmare of a hook-up for no benefit. The big unknown is clearance issues.

If you no longer allow that trapped heat to move are you setting the stage for pyrolysis at the fireboxface/breastwork/mantel?
J.T. Black Goose

Click to expand...

Let me give my opinion on these three points. As to the "heat exchangers" built into modern inserts, keep in mind that eny insert efficiency claims do not take into account all the heat radiated and lost off the surfaces behind the panels.

As to the "geometry", let me say that I am not the world best mechanic, but it took me about 3 weeks back in 1980 to perfect my method of making and installing block off plates. Since that time, we have installed a few thousand. Any installation mechanic who cannot do this relatively easily is, IMHO, someone I would not hire to install inserts.

As to #3, the "danger" of too much heat buildup - well, this contradicts the first point 100% - if the heat exchangers are so efficient, it would be impossible for this "tiny" bit of excess heat to cause dangerous temps in the firebox and mantel areas! But the truth lies somewhere in-between...inserts heat exchangers are NOT efficient in many cases yet the amount of extra heat released is not the type or amount to endanger wood surfaces on the other side of masonry.

There are MANY reasons for a block-off plate, but here are just some:
1. Insert panels do not fit tightly against the fireplace front or floor, and also against the insert itself. This can cause everything from wasted heat when the unit is burning, to additional smells in the house (reversed chimney) when it is not.

2. When no block off is installed, the insert rear is effectively part of the chimney structure (in exterior chimneys), making it very cold. This makes it harder to warm the masonry from a cold start - meaning less effective heat. A block off plate provides a smaller chamber (around the insert) which heats quickly.

3. A block off allows the possibly of even removing the panels or fitting them extremely loose in order to gain some of the convection heat from the rear of the units.

There are always exceptions to the rule. An interior fireplace with a full liner and tight fitting front panels would be fine without a block off. Same may go for very temperate climates where the difference in efficiency is not noticed. Some inserts are designed better - some even have some insulation on top of the convection chamber to retain heat.

But let's not fool ourselves. A LOT of efficiency is being lost with many inserts placed into exterior masonry fireplaces. A customer will not know the difference between 45% and 60% efficiency, but that does not mean it does not exist! As a professional, for the 10 minutes and $10.00 worth of sheet metal required, I would do it whenever possible....at least north of the Mason Dixon line!