Gravel Bed Question

[benzo]

I've seen mixed reviews and water softener systems w/ and w/out gravel beds. The folks selling resin tanks w/ gravel beds argue you need it, the ones who don't say its not necessary for softening alone. Care to comment? I'm looking at a 48k grain system for my new home.

[biermech]

Underbedding is put in the tank for several reasons. It helps to prevent channeling, keeps resin out of the slats and prevents the baskets from expanding during backwash. I prefer to use underbedding with all my units. I"ve seen unit without it and I've seen units with it. So it will work with it or without.

[Gary Slusser]

There is no disadvantage to a gravel underbed except the small cost. The primary advantage is a reduction in pressure loss across the softener.

There is no advantage to not having a gravel underbed.

[nhmaster]

Three for gravel under bed.

[pspitael]

Aside from the cost - does adding the 4-6 inches of gravel diminish the required freeboard area above the resin? How does that impact backwash efficiency?

[ditttohead]

The gravel cost is minimal, average less than $1 per system. We buy it in bulk, 40,000 pounds a couple times a month.

As to calculating water softeners capacities and freeboards etc, industry standards state that only the non domed areas are to be calculated for resin capacity and freeboards. The domed sections are not part of the calculation. The gravel should only fill the domed section in the bottom of the tank. I have attached a picture that shows this.

Many companies use a smaller tank to save a few dollars, and they do not use a gravel underbed. This is improper, but in all reality, it will work fine. it is bad practice. Other system designs, Vortech tanks, and Turbulator do not use gravel underbedding.

There is no disadvatage to gravel, other than the slight weight increase for shipping. The units that do not use gravel and claim "incredible performance advantages to systems with gravel" are mostly marketing hype. If the advantages were that amazing, then why is it only used in the most common tank sizes, and not on huge commercial equipment where the advantages would be worth while? These designs are not bad, they are primarily just marketing.

[pspitael]

Thanks Dittohead - your picture illustration is helpful

So a quick follow-up. You state that companies tend to not use gravel and undersize their tanks. From your illustration, seems like the 50% freeboard is 50% of the RESIN volume (not the total TANK volume?). So for instance, a 1.25 cu ft system seems to commonly come in a 10x44" tank - which has a volume of about 2.0 cubic foot. Add 1.25 cu ft resin, and 0.75 cubic foot freeboard - is that sized about right?

And a related follow-up. If I were to take that same tank (10x44) and add just enough gravel to fill the bottom dome (and most of the distributor?) - that really shouldn't effect the amount of resin needed and the available freeboard, correct? Or would you upsize the tank to a 10x54 (commonly sold for 1.5 cu ft resin?).

Thanks!

[Gary Slusser]

A 1.25' softener is an oddball size and many web sites and local dealers do not sell them. Other than big box brands most softeners are sold in whole and half cuft volumes of resin.

A 1.5 cuft has a larger constant SFR.

[pspitael]

Thanks Gary. I was waffling between a 1.25 cu ft and 1.5 cu ft unit, so may be better off going with the 1.5 cu ft. If I do go with gravel, I'll have a bit more room to work with in the taller tank.

Per my question above, am I correct in assuming that the 50% freeboard volume is based on 50% of the resin volume (1.5 cu ft resin would require 0.75 cu ft freeboard?)

Dittohead - must be nice buying gravel in bulk - I see prices of $40 or more per 15 lbs online - that's a bit more than $1 per unit . I'll have to try and get some locally.

[ditttohead]

LOL, yeah, our gravel comes by the truck load, and they fill our super sacks on site. I have recalculated, and the cost per 1.5 cu. ft. unit is actually much lower than I said. We charge for repacking, shipping, handling, etc. You can get gravel locally, just be sure to put it in a bucket, and rinse it well with a bleach and water solution before putting it into your system.

You assumption on the freeboard is correct, the freeboard is based on the resin height. Our calculation are / .66 to get your resin/freeboard height. Remeber to remove the domes portion of the tank away from the equation. All this being said, it is not that crititcal, it is just the standard practice that allows for a lot of variance in water temperatures, pressures, etc, and the systems will still work almost anywhere in the world without problems.

[pspitael]

Is there any 'magic' about the type of gravel needed? I'm assuming somewhat small and uniform would be ideal - but do I have to go to great lengths to get the perfect gravel? I'm wondering if a pet supply place might have something that would work.

[ditttohead]

Just use 1/8 - 1/4" gravel, do not use the colored types from a pet store. Standard, irregular gravel is preffered, but gravel that is slightly "rounded" will work well too. Home Depot, Lowes, etc. should have generic gravel for underlayment. Be sure to clean and sanitize it prior to putting it into your softener. I would highly recommend buying a bag of NSF certified gravel from online, but it will be very expensive due to thehandling, shipping, etc. We get our gravel from an NSF certified supplier, they sell the exact same gravel to the road buuilding companies.

My only real concern is the contaminants that can be present in an unknown gravel source.

[Gary Slusser]

Thanks Gary. I was waffling between a 1.25 cu ft and 1.5 cu ft unit, so may be better off going with the 1.5 cu ft. If I do go with gravel, I'll have a bit more room to work with in the taller tank.

Per my question above, am I correct in assuming that the 50% freeboard volume is based on 50% of the resin volume (1.5 cu ft resin would require 0.75 cu ft freeboard?)

Dittohead - must be nice buying gravel in bulk - I see prices of $40 or more per 15 lbs online - that's a bit more than $1 per unit . I'll have to try and get some locally.

The best is to get the correct size based on your peak demand flow rate. And using water treatment gravel from an online or local dealer than playing around with gravel from somewhere else.

[Tom Sawyer]

Yea, what does get some locally mean? Are you going to head to the back yard with a bucket and a shovel? Look at the expense this way. That's 40 bucks over 20 plus years. Pretty cheap really. And go with the 1.5 cu. ft. while you are at it.

[Gary Slusser]

Hey. I've been meaning to ask you.... what's wrong with plumbers that they require a license when rocket science doesn't?