You can flood your bathroom in a major way, plus damage expensive and impossible to get parts. In extreme cases you can even cut yourself badly on broken porcelain. A trip to the home center to get a new toilet, or a trip to emergency department at the local hospital are not fun. Read the directions that should come with ANY parts you buy. This article is no substitute for common sense and plumbing knowledge. When in doubt, consult the services of a licensed plumber or handyperson, depending on your locality and codes. Speaking of codes, it is the sole responsibility of the installer to make sure that all parts and methods comply with all applicable plumbing and building codes. Anti siphon fill valves are code mandated everywhere to prevent contamination of drinking water. Do NOT use any in tank toilet bowl cleaners as these WILL damage the parts installed and make servicing them MUCH more difficult. It is also the installer's responsibility to install these to prevent wasting water. Perform this fix and all other fixes at your own risk.
The Enviro Flush toilet was sold at Menards a few years back and they were and still are a great value for the money. It was this writer's first experience with a 3 inch flush valve as well as a anti slam seat, an elongated bowl, and low water usage for a flush that rivals and exceeds the old toilets in many respects. I spent about $110 for this, plus tax in 2013 and I've had a few issues with it starting in late 2015.
In the interest of saving water, the fill valve has a slight delay. After you flush the handle, the fill valve is timed until the flush valve closes before it lets water into the tank. The problem is when the flush valve hangs up, by the time there would a realization there's a problem the user has already left the room. Christmas Eve of 2015, this wasted 1500 gallons of water when the flush valve hung up.
This writer has tried to adjust the chain numerous times to no avail. Having mixed success replacing just the flapper, it was time to replace the valve in its entire. Since this was going to be apart, it was also a good idea to change out the fill valve and bolts to the tank as well. The handle was a bit rusty, but still in good shape and since there wasn't a viable replacement, this is going to have to be saved.
I bought a Fluidmaster 3 inch flush valve for $15 (kind of pricey, but 3 inch valves are few and far between, and to be fair this is an excellent product once it's installed) and a Plumb Pak Universal Adjustable Fill Valve for $8. Both were from Lowe's. Both of these will fit through the insulation without difficulty and will give you plenty of room to work.
I installed both according to directions. The flapper valve is a bit tricky as you need to install the chain on the outermost bracket, or it may not flush right. This needs to placed in the tank the same way as the old one, and you can raise the overflow tube if you want, As it is, this valve sits lower than the old one by 1/2 an inch; keep this in mind when adjusting the tube and the water level to save same. Raising the level more than this is unnecessary and wasteful without providing much more force when you flush.
The handle can stay in the tank, as the new fill valve is much smaller than the original. I used a large pair of gripping pliers to carefully snug the flush valve and secured the fill valve hand tight (do NOT USE tools on this flush valve as you WILL damage the valve or the tank. The new bolts with the flush valve kit need to be installed in the same way as the old ones were, carefully snugging them down. Over-tightening them will crack the tank and necessitate the purchase of a new toilet as these tanks are probably made of unobtainium at this point. Install the tank to flush valve
At this point, install the tank to the bowl with the wing nuts and washers you saved from the old bolts (you did save them, right?). Tighten them down enough to keep the tank from rocking, but no more as you will wreck the bowl. The length of the chain is probably the second most difficult thing to do on this fix. It needs to be fairly taut when hooked to the flush handle, but not so taut that it opens the valve and wastes water.
The proper way for this to operate is for the valve to open fully when you press the flush handle and then close by itself. Once you have the chain installed, then hook up the water line to the fill valve and turn on the water slowly checking for leaks. Following the directions for the fill valve, adjust this so the water level is about 1/2 inch below where it was before. Make sure the tank fills, the water level is right and the fill valve shuts off. Then flush this to make sure the flush is correct. Test and retest this until you're satisfied it works. Put the lid back on the tank, clean up your mess and you are done and have saved yourself lots of time, money, water and aggravation. Maranatha!
P.S. The flush valve lets the water drain into the bowl, while the fill valve fills the tank.