I installed the following in a 90sq bathroom on a conventional, with a solid and sound 3/4" plywood sub floor:

12"X12" porcelain tiles, on 1/4" hardy board, using latex modified mortar for porcelain with a unsanded grout.  

Now, there is a hairline crack down a motor joint that is center of the door way and the main entry way about 6' long with a few smaller places cracking.  They are very small cracks and you pretty much have to get on your hands and knees to see them, except for one spot that is chipping out pretty bad. I gave everything about four hours to set up before getting on it and I'm pretty sure that is what caused the problem.

I have to fix this.  I want to just dig out the bad places and replac it with new grout.  Is this possible or even a good idea?  If so how do I make the joint where the new grout will join up the existing grout?  I really DO NOT want to have to dig out all the grout and start over or tare the entire floor up and redo it.

I need some professional advise.

I have been in the tile installation trade for over 40 years. Your first problem is you do do not have enough plywood as a base. the industry standard is 1 1/4" min. your 1/4" hardy backer adds only thickness not structural strength.
Why did you use unsanded grout?
Unsanded grout is for grout joints up to 1/16" any thing larger should be sanded.
When you said you got on the floor after four hours that could be a problem, 24 hours is the recommended cure time,if the tiles are not loose the cracking of the grout joints is from flexing of the sub floor.

Did you follow the instructions for screwing the hardi boards down?

Should have used sanded grout.  

James Hardie board?  Did you use thinset under it?  How was it attached?  Did you wet the underlayment before applying thinset, then applying the tile?  I just finished tearing out about 1050 sqft. for a client that every grout joint popped after 6 months, said they had the floor done in a weekend, lmfao.  Btw every tile from the old install came up clean and was reused.

Double tap

Do you have any other recommendations?

The only thing you might be able to do is if you have access to the floor from below is to put some blocking between the joists in the problem areas. One problem with the hardi board is that it draws the water from the thinset very fast giving the appearance of being set but not fully cured.All I can say is what ever comes loose, clean off the old thinset then re set with a quality latex modified thinset morter. Make sure to back butter the tiles before setting to insure a good bond.
When i lived in NJ I had my own ceramic tile setting company for 26 years ( learned it from my father, my son is now in the business I have many uncles, cousins and brothers who are tile setters)
I think the tile backers that are being sold today do have a place but do not make up for a proper sub floor. It is all about flex between the joists. When I was in business all of the contractors I worked for always used two layers of plywood 3/4"+ 1/2". I would always scrape off all of the crud left form the sheetrockers, then wash the floor and screw every 6" in all directions. I would set the tile with a high quality Latex modified thinset  "Tec Super Flex" $48.00 a bag not some cheap home depot crap.
I know your position, the people who are selling you the materials should know what they are talking about.
The sad thing is I have had customers who after me telling them how I was going to do the installation would say "Well the guy at Lowe's, Home Depot etc. said do it this way.
My answer was let Lowe's,Home depot etc. do it.

Ditra is your friend here.

Too late for Ditra. Good product used a lot of it.

when i did my floors in my ranch, my subfloor was solid. 3/4, no flex.  one spot was soft due to water damage from the old linoleum tiles that were down before. replaced.  used 1/2 hardi backer pro grid, screwed it ever 4".  used HUNDREDS of screws across 600sqft.  used mapei thin set and their grout, set my tile. grouted.  no issues.  i think you had a subfloor issue.

My understanding is that Ditra only helps on subfloors where expansion moves laterally across floor, allowing the tiles to "float" on top of the ditra which can expand and contract with the subfloor. If the OP didn't put enough support between joints and it is indeed flexing up and down from weight being exerted on it, I am not sure Ditra would be the answer.

Always throught of ditra more for cement subfloors that expand and contract over cracks with the change in temps etc

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Ditra can support some flex in the joists...more than any other tile product I know of.  But, there are limits on what Ditra can do.  Their literature specifies how much deflection the product can handle.  I think it would take care of the OP's issues.

I know the OP doesn't want to rip out the tile but I'm afraid that's the proper fix here.  Been there myself.  I tore it all out and re-installed tile with Ditra.  Now, I wouldn't put down tile on anything else for a floor.

I


Good to know. Learn something everyday.

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I've done two tile jobs in the house.  First was a bathroom, and I had a problem with grout and one tile coming loose, and it was due to lippage, and not enough mud on the tile.  I ended up busting it out and got it reseated and grouted without issue.  Used 1/4" backer board mudded/screwed down to the tounge-groove subfloor and mudded/taped the seams.  The walls were a giant pain, I used Redguard as my waterproofing, painted four coats, horizontal, vertical and both diagonals.  It's held up well for the 6 years since I did it.

The 2nd tile job was just a floor in the foyer.  Spent more time cutting tile than anything else as I laid it in a diamond pattern.  Have one area of lippage I can feel, its one of the highest traffic areas of the house, and has held up just fine for 4+ years.

Getting ready to do the kitchen, and I'm more worried about it than anything I've done before.  Part of me wants to lay some thin marine or pressure treated plywood down on the subfloor, before I put down backer board, and even use some self-leveling compound before I set the tile.  I plan on using Redguard around the sink/dishwasher area, and where the fridge is moving to, including the areas where the backsplash (tile) is going.  Laundry closet is right off the kitchen, and thinking of extending the floor into there, and doing the same waterproofing there.  

Our subfloor is tongue-groove lumber, and is about 2" thick. I don't anticipate this is a problem like the OP's?

I hate to be a broken record but look into Ditra.  It will provide a measure of waterproofing and also give you a nice base for your tile.  It's a lot easier than what you are proposing to do and will give you as good, if not better, floor.

I'm just under 200 sf, that is going to cost twice as much as hardiebacker, if not more once I factor in putting down 1/2" OSB or plywood.  I'll have to watch some videos and check out the john bridge forum.  CHanging plans so close to kick-off is

I did about 90 sqft of 12x24 tiles in my bathroom.

The subfloor was rotted and i ripped it out. Did 3x4 ply base. Glued and screwed the floor to the joists.

Then did thinset skim coat for ditra

Then laid my tile. I have heard over an inch for subfloor tile. But didnt want to add that much to the floor. Tried to match the existing hardwood in the hallway.

Any way. Ditra ditra ditra. I wont use anything else. The floor has been down for over a year. 5 kids and it still looks good. No cracks.

Not a tile guy. So take it for what its worth.

And youtube is your friend. Not those guys at the big box stores

I am thinking I need to start my own thread with the questions I am going to ask....

That being said, I am assuming that where I was planning on hardiebacker wall-to-wall, having the cabinets set without toe-kicks installed, then tiling to the base legs/feet, and having the toe-kicks installed when done that I would instead put OSB or plywood down, set the cabinets, then install ditra, tile, then toe-kicks?

If I understand what you are asking and what you have to work with, you shouldn't need more OSB or plywood.  You should just put down the Ditra and tile.  

Ditra isn't cheap....although it's not all THAT expensive but if your subfloor is in good shape, Ditra will ensure that you will never have a cracked tile floor.  It's really that good.  After using it, I'm a believer.  I'm hoping to be building a house later this year or next year and we are buying stuff for it and storing it.  I will need 6-7 rolls of Ditra for the house.  Yes it's pricey but it's worth it IMO.

So, would I use Kerdi-board for the walls?  I've wasted a metric ton of time at work looking at the videos, and products trying to figure out exactly what I am going to do.

Did mock this up in Visio/Photoshop from actual measurements, bid/contract, and the Snipping tool.

are the cracks just in the grout joints?

If so, it is likely you used unsanded grout in too large of grout joint, and it shrank/cracked upon drying/curing.

If the grout joints are larger than 1/16-1/8" MAX  you need to scrape out all that unsanded grout and re grout with a quality sanded grout like quartzlock, it doesn't need to be sealed and is a bit flexible.

*note* if you use Quartzlock, only grout a small area before wiping it up and moving on, that shit sets up quick like glue.

If the tiles are cracked, you're subfloor is flexing.

Tile repair is no fun, good luck

Since you guys are talking tile setting,
Excellent condition 1950's 1X4 plank subfloor, supported by 2X8 joist 16" on center (joist are supported @ 10ft),
Will ditra over the 1X4 planks work or should a layer of 1/2" (or even 3/4") plywood be added before ditra?

In the past I've used 1-1/4" of subfloor covered with 1/2" wonder board ( or better yet, where weight is not an issue skip the plywood & use felt paper/lathe +1-1/4" of mud). After all the Ditra KoolAid I've been hearing from lately,  I'd like to give it a try
Thanks in advance

You can put wall tiles on drywall and for a kitchen, you don't even need waterproofing behind the tile.  You can use mastic to put wall tiles on (but I would still use latex modified mortar) in a kitchen.  Kerdi board isn't needed in a kitchen.  

If you are completely demoing the kitchen, I would just put up drywall, paint, install cabinets, then Ditra and tile on the floor, then tile the backsplash right on the drywall.

Here's the manual for Ditra.  

Ditra manual

Based on the manual, looks like you need to put down 1/2" plywood before putting down the Ditra.

For Ditra you have to lay solid subfloor over t&g or planks.  Backsplash can go directly on drywall, rule of thumb is if you remove a kitchen splash with drywall, you replace it with drywall.