gas furnace duct changes

Hello,
Our house currently has one older gas furnace (40 yrs old) with one set of ducts that run between the first and second floors. The ducts branch such that our suite (upstairs) has registers on the floor and the tenant's suite (downstairs) has registers on the walls. I was interested in dividing the heating for the house by installing a gas fireplace upstairs, along with electric heaters in the bedrooms upstairs. We would then close off the furnace registers such that the furnace only heats the downstairs suite.
I'm wondering if by closing off our registers (either by closing the dampers or installing something to block the ducts) will we cause problems for the furnace or for the downstairs suite? By halving the space that the furnace feeds, will the furnace cycle frequency increase (more on/off cycles) and thereby cause more wear on the furnace? The cold air return is in our suite, is this a problem (obviously we won't block off the cold air return)? What potential issues should I be aware of?
Any other suggestions for how I can create two separate heating zones for our house? While the above scenario would probably work, it's likely quite inefficient for the downstairs suite. Probably the best solution would be to get a new more efficient furnace and install a second run of ducts. This would be extremely expensive though, I think. If I were to go this route, are there any required features for the furnace (variable speed fan, dual stage, etc.)
thanks Doug
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You do not provide any information on why this is the course that you want to take. Call a couple of pros and ask them. The gas fireplace upstairs might be a problem. Does your local area allow for non vented gas appliances?
Heat rises and damping down the upstairs registers to an point will not affect the heater. However if you restrict the air to much you can create other problems.
Electric heat will cost more than the main heater doing the job.
Zoning the system with automatic dampers could help or be exactly what you need. I doubt that you would be interested in the cost.
Might be time to look at an new system and have some bids done for your needs.
We would then close off the

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We have two separate suites with tenants downstairs. We should have separate heating in order to meet code. I'm not sure if individual furnace registers are considered to be separate controls. There's only one thermostat which pretty much guarantees only one suite will have enough heat or not too much heat. In order to ensure that no-one is ever freezing, the furnace has to be on all the time.

the gas fireplace is okay, we have an existing fireplace. we'd just get a gas insert.

I've replied to the m.ramsley about this, but one of my big problems is actually finding a pro to look into this for me. It may sound dumb, but I don't know where to go for pro advice/bids. I agree that this is what I should do though.
thanks Doug
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Electricity is Double the cost of gas for most of the US. Research your KWH cost first. Closing registers can overheat your furnace. You need a Pro to help you on this
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This might be a stupid question, but where do I look for pros on heating? I've tried to tap into three resources so far and frankly none of them seemed terribly pro. 1) yellow pages - Furnaces, Heating: almost all of these listings are for cleaning and repairing furnaces 2) Furnace retailers: seem to know a bit about heating but interested mostly in selling me their most expensive systems 3) yellow pages - contractors: general contractors seem to know a little bit about everything but they're not heating pros
It's been kind of a frustrating experience for me trying to even talk to someone that really knows their stuff.
thanks Doug

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DON'T go to alt.hvac and ask there. They don't want to hear it.
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