Not much room for mass and a long vertical exhaust

I am in planning stages and have not bought the Book yet. I only have room for 12' of horizonal mass pipe with 3 each  90* and 2 each 45*  then 6-7' of vertical to the 19' of 6" vertical triple wall stove pipe in a prefab chimney.
The chimney worked great for a regular wood stove .
Before I hurt my brain over this.
Assuming I build the RMH correctly with 6" heat riser and 55 gal bell/barrel ( i know there is a lot more to it)
My questions are :
Because of the lower exhaust temps is the long vertical chimney going to cause stawl problems or will it create more draw?
Since I don't have much room can't I build a taller mass "bench" for more mass? ( my area is 5' x 5' )
This is for supplemental heat in a well insulated 1500SF house on a basement the same size RMH would be in basement of course.

I am sure these answers are on here I just have not found them yet.
Thanks Joe B

First, a basement is not a good location for an RMH unless it is regularly used as a living space. You need to be able to see or hear the fire to know when it needs tending, for the hour or few that it is running depending on heat required. Also, it functions more as a radiator and warm contact surface than as an air heater, so it will not heat the upstairs that well unless the levels are interconnected.

If your floor space is limited, you can make a taller "bell", or a tall one followed by a bench-height one. Bells have a simple cavity in the masonry mass, rather than a winding path of ductwork, so have much less friction while functioning at least as well. See [ulr]http://batchrocket.eu/en[/url].

The tall chimney will be beneficial, but an RMH in a basement has a risk that the upper floor will draw more strongly ("whole-house stack effect") and possibly cause the RMH chimney to backdraft. It won't have as strong a draft as the woodstove had because it will not be wasting tons of heat up the chimney.

Joe,
We find ourselves in a pretty similar situation.  I first started looking at Rocket Mass Heaters and decided I'd see about doing one in my basement.  Glenn is right, doing a traditional rocket stove is going to chain you to the area for as long as you intend to keep it going.  Fortunately, there is the Batch Box Rocket Mass heater which might be a solution to your specific build site.

 After consulting with Glenn and Peter Van Den Berg I have decided to go with the Masonry Batch Rocket for my basement dragon due to the fact that I have only 7 feet from slab floor to wood joist ceiling.  The brick, with an additional heat shield will spread the heat out more evenly and prevent the ceiling from too much heat.  The Batch Box will allow me the luxury of filling it up and letting it burn with the door closed so i can get a compete burn without having to tend the fire constantly.  BBR's are said to have a burn cycle of about an hour.

I'm not an expert, but I also have a tall chimney and my suggestion for you is to build your rocket close to your chimney pipe so you can use that heat source to warm that pipe to induce a positive flow.  I am also going to use a bell bench whereby the hot gases will flow through a hollow cavity in the bench structure that will be built with concrete block, patio slabs, cob, and topped with sandstone slabs of reclaimed old city sidewalk from here in Toledo, Oh.  Bench Bells are said to be much less restrictive to flow and still do a very good job of distributing and storing the heat.  I might add that I replaced the door to my basement steps with a louvered door to allow the supplemental heat that is presently supplied by ventless Nat Gas heaters to flow upstairs.  You might want to do the same.

Now if all this doesn't seem complicated enough, I'm going to embed some copper tubing under those sandstone slabs in the cob to provide a way to regulate the bench temperature and capture any excess heat that might otherwise make the bench uncomfortable and divert it to my Drainback Solar Thermal hot water system.  But that's another story.

I hope this provides some food for thought.