# low domestic hot water flow with tankless coil



## mike42 (Oct 26, 2009)

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.


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## beenthere (Oct 11, 2008)

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.


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## mike42 (Oct 26, 2009)

beenthere said:


> 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: http://www.inspect-ny.com/heat/MixingValves.htm

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


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## beenthere (Oct 11, 2008)

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.


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## mike42 (Oct 26, 2009)

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.


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## beenthere (Oct 11, 2008)

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


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## mike42 (Oct 26, 2009)

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.


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## beenthere (Oct 11, 2008)

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.


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## mike42 (Oct 26, 2009)

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.


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## beenthere (Oct 11, 2008)

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 140°F (60°C ), a tempering valve shall be provided to temper the water supplied to the potable hot water distribution system to a temperature of of 140°F(60°C) or less.


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## mike42 (Oct 26, 2009)

Thanks again.


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## skidrowpete (Nov 22, 2007)

*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


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## JohnH1 (Jan 9, 2009)

skidrowpete said:


> 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.


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## mike42 (Oct 26, 2009)

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.


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## beenthere (Oct 11, 2008)

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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## AllanJ (Nov 24, 2007)

Mineral deposits will both restrict the water flow and result in a drastic lowering of output temperature as you turn the water on higher. The minerals insulate the domestic water from the boiler water and in effect reduce the gallons per minute versus temperature rise heating capacity.

On some furnaces the tankless works worse in summer because the circulator (that sends FHW to the radiators) is not churning the boiler water about the tankless.

If you are lucky, the mineral deposits are not adhering to the tankless coil so alternately tapping the tankless unit (removed fromt he furnace) and flushing backwards with a garden hose might dislodge it all.


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## mike42 (Oct 26, 2009)

I was planning to drain my boiler, cut the domestic coil input/output pipes, and just replace the whole coil or flush it somehow, but a friend who does maintenance for apartment complexes warned me to never remove the coil because it may never go back in properly and could leak. Is there any truth to this? He probably works on 20yr old equipment while my boiler is ~3yrs old. 

I'm learning towards doing an electric water heater, by the way.


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## beenthere (Oct 11, 2008)

On older boilers. You can have trouble with a leak if you don't take some precautions.

As for going back in right. Only a problem on some larger coils(as in 10 GPM and larger). 

For what you can save in oil. The cost of switching to electric is well worth it.


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## mike42 (Oct 26, 2009)

okay I have another question... my water report says total dissolved solids is 375mg/L, which is very hard. I don't mind the effects on drinking water, faucets, etc., but it is clearly detrimental to appliances (hot water heaters, dishwasher, laundry machine). Would it be wise to hook up a water softener to the pipe that feeds the water heater, so all hot water is softened? From what I've been reading, even an electric tank heater will be quickly destroyed by hard water.

Of course, this site says that softened water _increases_ corrosion: http://www.waterheaterrescue.com/pa...vity/water-heater-preventive-maintenance.html


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## beenthere (Oct 11, 2008)

Best to have the softener piped that it does all the water the house uses.


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## mike42 (Oct 26, 2009)

Figured I'd update this. I bought a 80gallon electric hot water tank heater and a friend and I installed it over a couple beers. Been in a month or two months now and its an amazing difference. I actually get real water pressure now, no regrets at all. The boiler will just be used for winter heating of radiators now. I completely understand what the inside of the coil must look like, too. I got rid of my old drip coffee maker and started boiling water in a pot to use a french press. The water pot gets caked in minerals after only a couple uses. With the amount of water flowing through that coil for a few years I'm not surprised at all that it is clogged to hell. Thanks for all the advice.


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## beenthere (Oct 11, 2008)

Thanks for the update.


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## jerzeedivr (Apr 15, 2009)

*Low Flow Coil*

I have an old system with a hot coil and i installed the little shower head that saves water. Its a little stainless steel water saving shower head. It slows the water flow which gives a slow pass through the coil with hotter water. I also up the temp. in winter and lower in summer.


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## woodse (Aug 30, 2010)

*Tankless Coil Produces Only a Short Duration Hot Shot of Water*

Same setup as original post i.e. oil fired boiler/baseboard heat/tankless coil (all 20+ yr old). Problem is we only get a short duration shot of hot water delivered at same rate/pressure as the cold water(around 40#). Plumber installed a new tempering valve which seems to work since the temperature of the shot of hot water changes w/ adjustment of the tempering valve. Tried cranking the house thermostat up til the heat came on and water was circulating thru the baseboards, but this had no effect on the water from the tankless coil. Water supply is New Hampshire well water so probably pretty hard, but if flow is not an issue, seems to me that the coil should not be clogged. Bottom line, normal hot water pressure to faucets but only a quick shot of heated water at whatever temp the tempering valve is set for. Any suggestions would be welcomed.


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## beenthere (Oct 11, 2008)

May not be clogged. But it can still be insulated from the minerals.


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## woodse (Aug 30, 2010)

*Maybe a bad Aquastat?*

Is there a possiblity it's a bad or wrongly set aquastat?


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## beenthere (Oct 11, 2008)

Is the low limit set below 160?

Its very common for a coil to become insulated.


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## Netmouse (Apr 6, 2008)

I have the same issues - oil boiler with domestic coil. What worked last time, when the hot shower quickly turned to lukewarm, was having the coil cleaned. Oil company says the coil probably needs cleaning every 5 years or so due to such hard water, so that timing is right as the problem returned. I am holding off the cleaning as it is about $200, and if I turn up the house heat the domestic stays hot from the boiler.

I've not bought an electric water heater because when the boiler dies I would like to switch to natural gas overall. But then maybe an electric water heater now may make sense if I can turn off the boiler in summer and save on a tank of oil (that I use over the summer) which is about $800. Actually, I guess that one summer's oil would pay for the tank. Then, I like the endless hot water from the coil for long steamy showers.

I've not added a whole house water softner due to adding salt to the water and any health concerns. Not keen on adding any chemical in drinking water. I have filters for both kitchen faucet and also shower (that removes chlorine) both from Aquasana. My washer was replaced when I bought the house as it broke but I realized also that the hot water was plugged up. Had my new washer now for about 5 years and no issues (yet). Additional concern for water heater is if it bursts - it would be in the basement - and flood is a concern. I recall when I lived in NH that there was such super hard water, and everyone's water heater in the condo complex was bursting at 3 to 5 years which surprised all of us. I moved to NJ so did not do anything to resolve the situation in NH, and the hard water here in NJ seems a bit more tame.


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## ben's plumbing (Oct 6, 2011)

beenthere said:


> 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.


 yes it sure is did this for my son ...really saving ...great point beenthere......


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