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mike42 10-25-2009 09:48 PM

low domestic hot water flow with tankless coil
Hi, I have been investigating a hot water issue at my house and I think I have it figured but I wanted to ask around before I start cutting pipes.

I have an oil burning furnace with a domestic hot water coil, and water baseboard heat. The problem I've been having is lack of hot water in my fixtures all year 'round. Specifically, water temperature is very hot, but the flow is terrible. This is true of all fixtures (showers, faucets, washing machine, etc). This is not unbearable, because I can simply increase the oil furnace temperature and adjust the cold water faucet to reach the desired output temperature. It does make showers unpredictable, however, because very slight pressure changes DRASTICALLY affect water temperature. The large flow difference betwen hot and cold also means that adjusting the cold knob to the right temp is very difficult as the slightest turn bounces between hot and cold extremes.

I checked the manual for the furnace, it says that the model I have is rated to heat 3.25gpm of 40F water to 140F with a 200F internal boiler temperature. According to my well pressure tank gauge, the cold water pressure is at 43psi. The furnace's max cold water supply pressure is 50psi. I measured the flow rates manually with buckets from the very first fixture on the line. The cold water was flowing at over 5gpm, but the hot water only about three quarts in a minute.

I have hard water so my guess is that the hot water pipe, or the cold water pipe that leads to the furnace, is partially blocked or clogged up with scale. There are some old corroded compression valves in there, too. I was going to start replacing pipes and valves until flow improves, but I want to make sure I am not just wasting my time.

Shouldn't the hot and cold water pressures be about the same? Both lines are entirely 1/2" copper pipe from the well pressure tank onward. If the manual says it can do 3.25gpm at 50psi, shouldn't I be getting nearly that at 43psi? Certainly at least 1-2gpm.

By the way the furnace is 2yrs old so I doubt it is at fault unless it has a valve somewhere that isn't fully open. This issue existed since it was installed, which was when I bought the place, as the old one was failing.

beenthere 10-26-2009 03:50 AM

The tankless coil is restricting.
Also. You should have a flow control valve, and a mixing/tempering(code requirement) valve on the hot water line from the coil.

mike42 10-26-2009 10:22 AM


Originally Posted by beenthere (Post 345468)
The tankless coil is restricting.
Also. You should have a flow control valve, and a mixing/tempering(code requirement) valve on the hot water line from the coil.

Are you suggesting that it is restricting by design, or because it is improperly tuned or damaged somehow? If it's rated at 3.25 gallons per minute, shouldn't I get something close to that? I'm getting well under 1gpm.

Should I call the boiler installer or this just "how it is" with this kind of system. This is my first home and first oil furnace so I do not know what is normal.

There is a compression valve on the hot water line coming out of the boiler, and it is fully open though it looks very old. I'm not sure what a mixing valve looks like, I'll have to look it up. It should be up to code as it was installed in 2007.

I don't think I have a tempering valve. There isn't anything external to the boiler mixing the hot and cold pipes as shown here:

Either it isn't a code requirement in my area or I got screwed over and will have to install it myself.

beenthere 10-26-2009 01:51 PM

The tankless coil is soimewhat restrictive by design. But as mineral build up occurs. It becomes more restrictive.
So it could need cleaned out.

Also look for a device that almost looks like a coupling on the copper line that feeds water to the coil. It may have a flow control device on it. That restricts how much water you get through your coil.
Although they are usually sized close to what your coil rating is.

Code has required a mixing valve long before 2007.

Your coil most likely needs cleaned/cooked out.

mike42 10-26-2009 03:34 PM

I definitely don't see a mixing valve anywhere on there. I just went out and bought one for $30, I'll have to install it.

As for the restrictive coils... is it reasonable for me to expect something near the 3.25gpm rating? I don't think corrosion/building in the coil could be the issue as it has pretty much been this way since it was installed. The inlet compression valves are all open. too, though they *are* really corroded as they were not replaced when the unit was installed.

beenthere 10-26-2009 03:36 PM

If you have the water pressure. Yes, you can expect 3.25GPM. What is your water pressure.

mike42 10-26-2009 03:52 PM

The cold water pressure is 40-45psi ~20ft from the boiler, all 1/2" copper. The boiler says max cold input is 50psi, and its rated for 3.25 gpm hot output. I'm only getting .69gpm of hot water output, something must be severely restricting flow.

It looks like a tempering valve will "fix" the issue, but really I think it will just hide it.

Thank you for the advice, by the way. I'm somewhat nearby, Norristown/King of Prussia area.

beenthere 10-26-2009 04:21 PM

About 45 minutes to an hour a way. Depending on traffic.

I would disconnect one of the valves bore the coil. And see if its getting good pressure. And then take one apart after the coil. And see if the pressure is the same.

If its not, something may have got in teh coil when they installed the boiler.

mike42 10-28-2009 01:35 PM

My buddy and I started taking off valves and cutting/rejoining the suspect sections of pipe yesterday. Nothing was found, the boiler itself is definitely the bottleneck. The cold water pipe has good pressure all the way to the domestic coil input, and the pressure of the water coming directly out of the domestic coil output pipe is very poor. I am going to call the boiler installers to find out why I'm not getting the rated capacity, and why no tempering valve was installed if that is a code requirement.

Can you give me any more info about the code specification that requires mixing/tempering valves? I know of, for example, the national electric code, which is referenced by local governments, but I do know of a plumbing equivalent.

beenthere 10-28-2009 02:01 PM

IMC 1002.2.2.2 Scald protection.
Where a combination potable water heater and space heating system requires water for space heating at temperatures above 140F (60C ), a tempering valve shall be provided to temper the water supplied to the potable hot water distribution system to a temperature of of 140F(60C) or less.

mike42 10-28-2009 03:15 PM

Thanks again.

skidrowpete 02-07-2010 01:32 PM

water pressure
hard to believe you have 43# water pressure supplying the boiler feed most boilers i have seen have a30# relief valve and an adjustable water feed and pressure regulater reducing the water pressure to the amount needed

JohnH1 02-07-2010 03:25 PM


Originally Posted by skidrowpete (Post 396211)
hard to believe you have 43# water pressure supplying the boiler feed most boilers i have seen have a30# relief valve and an adjustable water feed and pressure regulater reducing the water pressure to the amount needed

That is the water psi feeding the domestic water coil in the boiler. Not the boiler feed line for heating.

mike42 06-07-2010 08:21 PM

Wow, this post is top google result now. So its been months and I haven't gotten around to fixing the problem yet. It's gotten to the point where I can't even take a warm shower. From talking to various people and researching online, the coil is definitely clogged with scale/sediment. A water test reveals very hard water, to the point where if I boil down a pint on the stove there's a fine layer of white powder in the pot once the water evaporates.

One guy I talked to said tankless heaters generally suck as far as water pressure goes, and to just get an electic water heater and disconnect the domestic coil and only turn the boiler on for winter heat. I'm torn between getting the coil acid flushed (or replaced) and getting a water softener to fix the root cause, or buying an electric 50 or 80gal tank heater and calling an electrician to run a new 240v circuit.

I'm not even sure which will cost more to operate these days. Oil is really expensive and electric isn't too outrageous around here. Any advice on fixing the tankless coil vs just abandoning it? I can install the a new heater myself but I'd still have to buy it and pay an electrician to run the circuit. Heater is $400-600, plus another $100 or so for the electrical work I guess.

I have absolutely no clue how much an acid flush would cost, and I'd still have to buy and install a water softener anyway to keep it from happening again in a year or two.

beenthere 06-07-2010 09:38 PM

Cooking out the coil. Is a time and material job. Could take an hour and a half. Or it could take several hours. As in 4 or more.

An electric water heater, is often cheaper to use for hot water then a tankless coil in a boiler. Almost always.

You can then set your boilers min temp to a much lower temp after you install an electric water heater.

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