How to plan gas pipe job to hook-up tankless hot water heaters

MIademarco · #1 Jan 3, 2005, 06:32 PM

Dear Ask Me Help Desk, I am installing two Paloma 200K BTU tankless hot water heaters in our L-Shaped 2900 square feet 1963-built rancher in Atlanta, GA. I am trying to decide if I should tackle the gas line part of the job. I have successfully done a number of small gas jobs which required purchasing black iron (low alloy steel for natural gas). The hardware guy cut it to length and thread as necessary. I have put in shut off values and made connections to distal flexible tubing attached to appliance. I put in traps. I check joints with soapy water, then call the gas company and ask them to come out with their sniffer for one final check. No problem worked well. For this job, each unit 3/4" nipple needs to be connected to 1.5 inch main (I laid out the entire house, matched BTU demand of all appliances, planned for possible future appliances, used the gas tables, confirmed with the hot-line engineers that sold heaters to come up with 1.5 inch backbone that must come up to the heaters.) The current pipe is 1.25 inch OD, starts at a union at the meter, looks galvanized, and shoots straight under a very accessible crawl space, and then enters the basement level into the furnace are. There along the ground (without any vertical trap in sight) it transitions to 1 inch OD "black iron." At the furnace it angles 90 degrees up (again without any vertical trap), transitions to 3/4 inch OD black iron, into a home made manifold feeding the furnace, current 40 gal hot water heater, and then branching off a flexible copper long run across the house for the dryer and outside barbeque. The arrangement certainly is not neat and without the various traps is probably not code? We will be adding the two hot water heaters near the furnace and will add a 60K BTU stove top out at the other end near the dryer and barbeque. So I want to redo the main trunk from the meter, bringing it off the floor with a riser, and then branching down or up from the basement ceiling level to the appliances, adding the necessary traps. I up for it, but the thought of handling those 20 foot lengths, cutting and threading, and the strength to uncouple the main gives me a pause. Two estimates are in at $2000. So, I have some leeway to get a tool to cut and thread. I read about CSST, but this takes special training. I will redo the pipe size calculations. Salute, Michael

labman · Jan 3, 2005, 07:12 PM

It sounds like you have done your homework. Nominal pipe sizes, 1'' will be a little over 1'' ID and about 1 1/4'' OD. In general, the OD is close to the next size up. 3/4'' fittings will screw on to pipe with about 1'' OD. Usually anything that calls out a pipe size means the nominal ID. Have you checked with the gas company to make sure your meter and service line can keep up to 2 of those monsters and everything else?

Do you have friends, neighbors, family you can call on? A 20' length of 1 1/2 black iron could be a handful. Go to Lowes etc. and try hefting one. I can remember struggling with a shorter length of 1'' pipe when I put my furnace in. I also remember one project I decided to get done while my teenage son was still at home.

As for the traps, I think what you need is a short drop leg just before each appliance.

Tom will have a different prospective on all this. He knows code much better than I do. Likely he will suggest getting rid of the copper. Any galvanized too. Zinc can flake off and plug gas jets.

I looked at the tankless water heaters. I was hoping to install something in my crawl space closer to where I use water. The specified clearance was a problem. A bigger problem seemed to be the minimum flow rate for hot water. I am not even sure my pump can deliver that much cold water. We don't use that much hot water, and would use even less if I didn't have to empty 25' of 3/4'' pipe every time I washed my hands.

Note, for maybe never use again, check rentals on pipe threaders.

MIademarco · Jan 3, 2005, 07:31 PM

Dear Labman, I will call the gas company and ask. My understanding from the plumbers who gave the estimates was that the street side of the gas has much excess pressure and and hence volume than the very low pressure on the house size. So even though the BTUs are relatively large, the supply should keep up as long as the pipe is the right size. The newer residential tankless can do up towards 8 gpm. One is almost enough, but given the caution from many people on this issue, I decided to oversize and go with two (there is a convenient "T" where I can break the house in half). You are right, I am concerend about water inlet flow (hence, my parallel thread/questionon how to increase water flow with which you have helped me). I am fortunate, clearance and access to air intake and hot exhaust are no problems in my configuration. I will check rental prices on cutters and threaders. Thank you, Michael

labman · Jan 4, 2005, 09:54 AM

Dressler. I have been trying to remember the name since my post this morning. Dressler makes couplings for pipe that do not require threading, good alignment, and not even a neat cut. The gas industry loves them. You have 2 flanges, a sleeve, bolts, and rubber gaskets. Tighten up the bolts and you have a good connection. I don't know if they come as elbows and tees. They are expensive, but you may be able to buy several for less than renting a threader. Still for short lengths, you can buy precut, threaded nipples or short lengths of pipe at the hardware store.

Also, you can buy pre threaded black pipe. I think it even comes with a coupling on one end. When I managed a small factory, we bought mostly unthreaded using mostly welded joints and a Victalic coupling to act as unions. They are much like the Dressler couplings but offer the security of fitting into a groove providing positive joints. You see a lot of them in large, exposed fire sprinkler piping.

MIademarco · Jan 5, 2005, 07:49 PM

Speedball, Let me show you my gas sizing calculations and ask about copper. Starting at the meter it is 27 feet to the “manifold” that will feed the furnace and two tankless water heaters. To get there, I will need five 90 degree elbows (equivalent to 25 feet where each 90 adds 5 feet) and 8 vertical feet. The furnace is 150K BTU, and the tankless heaters are 200K each. After the manifold there is an additional run of 60 feet and two 90 degree elbows with branches to stovetop at 90K and dryer at 50K and barbeque at 100K. The total BTU is 800K. If all were running, and realizing that they are not, this 800 BTU must make it to the equivalent of 60 feet at the manifold. In the tables from the tankless heaters, the 800K is equivalent to 727 cubic feet where there are 1100 BTU to each cubic feet. (It has been a while since my last physics class but it seems that “time” is not considered in this approach. In the table, 727 cubic feet to 60 feet falls closest to 1.5 inch pipe. From the manifold to the other appliances, assuming they are all at the end (which is close to true), we need to deliver 240 BTU or 218 cubic feet at an additional equivalent of 70 feet. Now here I am unsure what to do. Do I use 70 feet or the total 130 feet? In the tables, this would correspond to 3/4 inch or 1 inch. Another aspect is that these will not all be running together. However, I can imagine situations where it would be close. It’s winter time, the heat is on, the dryer is running, and I am cooking dinner when two guests fire up the showers. My current configuration runs furnace 40 gallon water heater on 1 inch and then dryer on 3/4. See exact layout in other email.

Since the first run is about 35 linear feet of pipe. Why cannot I do this in copper? I realize the pipe is more expensive but a) I am very comfortable with and good at soldering and b) compared to the rental fees, handling the heavier pipe, and relative to the cost of the plumber’s estimate, this may be a way to go. Can I use 1.5 inch copper? I assume that there is a connection solution at the meter. I very much appreciate your time. Thanks, Michael



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