Tankless Hot water systems. Design criteria.

[flyinhigh2]

Maybe a little OT but I installed a tankless system in our office area a while back and has been nothing but trouble. To top it of, NO ONE, even the MFG techies know how they are designed so I can make sure not to make another mistake in a purchase.

From what I have learned, the inlet water pressure goes through some pressure reduction and that ^P is what moves the reg diagram to turn on the gas. I really thought they should throttle the gas to match the flow rate but I guess better systems use multi staging of burners for changes in demand.

Problem I have right now is I can turn hot water on all day, no problem, as soon as cold is introduced, because the hot has a lower pressure, it causes further restriction in the mixing valving and shuts the heater off.......&*$% Been fighting this for some time.

As I understand it, the incoming water flow and temp rise are calculated to determine when it can turn. IE, each burner must run with a certain amount of gas flow and I guess is NOT throttled (I would think that would be smart. Each burner creates a certain amount of BTUs so they flow and temp rise setting have to be close to the BTU value of a single burner to kick in.

If anyone has first hand experience with these, I would really like to learn more and see if I am even on target so far. I thought I bought a unit that was .6-1.6GPM, what I got was a system that was "either" .6 OR 1.6gpm with nothing in between. Mine is a single burner system .

 

[dcasto]

I just hung one on thw wall beside my tank heater. I've been a little nervous about low flow. I'm on a water well with a 30 to 50 psi switch, so I'm changing that to a 40 to 60 psi switch. The water pump is 220 foot down and I don't have the specs, but it looks like the pump should do 5.5 to 6 gpm and the heater is rated to 6 gpm.

I opened the Rheem unit up and there is what looks like a ultrasonic flow meter strapped to the water inlet. I'm guessing that that is the low flow sensor and a feed forward device to the burner.

Without name or model, we can not help you.

 

[rmw]

What you are doing is reducing the hot water flow to below the minimum when you introduce the cold water. As long as you run the hot water full blast, you are operating the heater above its minimum flow requirement to maintain operation of the heater. Once the water flow through it goes below the minimum flow, it trips the burner.

Try this: turn on the water in two or more faucets and then mix the cold with the one you want to use.

I made the classic engineer's mistake of sizing up my home unit to be able to support two showers, a dish washer and a washing machine plus a couple of sinks all at once. I don't think it has ever happened. Maybe any two of those happened in combination at any time, but even at that, less than 1% of the total "on" time of the heater. The wife and I live alone and the children come to visit only a few days a year.

I have the same problem when only one shower (my shower) is running. At times of the year (when hot water demand is low, like summers when just running water through the pipes in the attic would heat the water enough - as opposed to winter when the pipes running through cold air lose heat to the surroundings and means I have lower hot water flow) I have to run a little hot water in a bathroom sink just to make sure the heater doesn't trip off on me while I am in the shower. Especially when a filter I have in the shower head is getting a little clogged and the overall flow is reduced. I hate this routine.

Now, in the "learned too late" department, my advice to anyone considering a tankless is to undersize it. What would undersizing mean? It would mean that instead of delivering hot water at 118F, for example, when multiple users are putting demand on it, it would only deliver water at, say 116F or 114F. Either will still require the addition of cold water to temper the water to ~110F so you can stand under it.

I'd still buy a tankless again, but I'd buy a much smaller capacity unit if I did. And I'd give this one to an orphan's home that could use all that capacity.

rmw

 

[moltenmetal]

Can't help you other than to say that the Navien high efficiency condensing tankless unit I installed a few months ago is working marvellously. Unlike rmw I did buy a unit which only good for one shower at a time, or a shower and the dishwasher combined etc. Aside from a modestly longer delay in getting hot water at the sink, I've had no complaints or problems with it. The low flow cut-off seems to be set pretty low. I can blend hot and cold at a sink without losing the hot supply, but I tend not to do that much any more. Instead I tend to fill the sink using the hot only, such that the cold water originally in the hot line goes to some productive use.

Unlike the mid efficiency units, it uses plastic venting. A co-worker both a Bosch and ended up spending as much money for the stainless B vent as for the unit. My venting materials cost $50.

 

[zdas04]

I bought a Bosch about 8 years ago. It was installed by the absolute dumbest plumbers on the planet. The gas line was too small, the vent was too small, the water shut off was reduced port valves, the water pipe was too small. All of these things are clearly prohibited in the installation instructions. We hated it. Especially when winter hit and it had to raise the water temp from 40F to 112F and couldn't get enough gas.

After the first winter I called a competent plumber and stood over him with the installation manual. He repiped the gas, raised the pressure in the gas system from 7 inH2O to 11 inH2O (which is the design point for my gas fireplace, central heater, and water heater, gave me about 35% more flow), replaced the 1/2 inch inlet piping on the cold water side to 3/4 and put in full port ball valves, and replaced the vent with the size called for. The thing has been a dream ever since. All the hot water you could ever want, hot water for one sink or three showers and the dishwasher.

I just can't say enough bad about the bad plumber or enough good about the unit when it was installed according to directions.

David

 

[dcasto]

I have a Rheem 199,000 BTU/hr unit with a min flow of about .5 gpm and it will do up to 5.7 gpm at 70oF rise in the winter and 6.5 in the summer. I haven't turned it on yet. I ran a new 1" line from the gas meter to it, then swedged down to the 3/4" with an 18" flex line, I had heard about under supplying these things. I informed my gas company and they installed a 420 CFM meter, about twice the normal.

I am adding an 800 sqft heated area and I plan on using another of these units to hydronicly heat the area, hence the need for a 600,000 BTU/hr meter. The gas company came out and saw my new gas header with 2- 1" lines and the original 3/4" line with a side tap for a future outdoor BBQ running on good ol methane and were quite happy?? My header is nothing, a friend of mine put in a 500,000 BTU/hr boiler that he uses to melt the snow from his driveway. He has 2 gas fire pits, a gas hydronic greenhouse and all gas heating an appliances. I wonder what size meter he has.

 

[IRstuff]

You haven't seen anything until you see the gas meters attached to restaurants ;-) A typical small Chinese restaurant has probably 500 kBTU running into a single cooktop.

TTFN

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