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Question DetailsAsked on 4/23/2018

how to remove a blockage from the hot water line on a water heater

The hot water heater is heating water, but no hot water comes out of faucets throughout house. Cold water is flowing perfectly, so I'm figuring a blockage

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1 Answer


Steps I would take to debug this problem - in order till you find the problem source.

1) First, check this is true at all hot water demand points - bathroom sinks and tub/shower, kitchen sink, clothes washer, etc and not just a blockage (like with iron algae buildup) in just one pipe or at one fixture's fittings.

2) Second, check no one has turned off the valve at the heater (usually on incoming cold pipe). Also cycle that valve - sometimes they crud up and will free up (at least partly) if you close and reopen them. Do this with one or more hot water faucets open so if something is there the flowing water will flush it out and through the pipes to the faucet (hopefully).

Though if a solid and it makes it past the tank, it will then probably block off the faucet strainer or diffuser - so best to run the hot water at a faucet without a diffuser or stainer or aerator - maybe a laundry tub faucet or such. I do NOT recommend using dishwahser, clothes washer, or shower/tub to do this because the valves in them can then get blocked - best to unscre4w the aerator/screen on a sink faucet and open it full open.

3) run hands along hot pipes, checking for a significant change point from hot or quite warm to luke warm or cold as you go past the blockage point. How well this works depends on pipe accessibility of course.

4) Then drain off some water (be careful - hot) using the bottom water heater tank drain - if the pressure drops off almost immediately to low pressure (though water would still run or trickle out until tank was emptied) would mean incoming cold water is shut/ blocked off somewhere, so you need to be tracking the cold side. If it comes out real dirty drain a lot out - until it flows clean. In that case, could be the hot water dip tube (which leads to the hot outlete from the tank) was immersed in crud and blocked. If so, drain out the tank to clean water, and if hot water does not then flow in the pipes, may need to backflow cold water through the hot pipes back to the hot water heater with its outlet open, to force the blockage out of the dip tube and into the hot water tank.

Another alternative way to do this test, if the overtemp/ overpressure valve drains to a place you can control the water safely (and not drown out the bottom of the heater), is open that for a few seconds (be clear of the hot water and any splash) - if water initially sploshes out forcefully but immediately drops off to a trickle or low flow, your incoming water supply is blocked or interrupted - line blockage, failed valve, or possibly blocked flow at an expansion tank fitting. If water continues to flow forcefully (as much as a garden hose or open faucet would flow) then inlet water is good and hot side is blocked.

5) Now you are into the point where some plumbing is involved - so may not be a DIY job from here on, depending on your home repair skill level. And realize, once you start taking piping apart, there is the risk it will break or is so corroded that it needs replacement, so you may get into a plumbing job anyway. I am assuming here this is a standard water heater, not one with a heat pump on top - if heat pump equipped, probably a plumber chore to check out if that is blocked internally.

So, next, shutting off the incoming cold water valve, with some sink faucets open to hot (to let air in so the hot water in the pipes can drain out) drain some water out of the tank (to relieve pressure and drain hot lines). Then (realizing there may still be some water trickling out of the hot pipe when you do this, so be prepared to divert away from front of heater and have power shut off to it) open up the dielectric coupling (hot or cold depending on above test results) located near the top of the tank - holding the bottom pipe securely so it does not start to twist out of the tank fitting.

Offsetting the outgoing pipe, check for blockage at the tank outlet or up in the dielectric coupling using a long pipe cleaner or piece of wire or such. The outlet of the tank is a common place for such corrosion buildup to occur. If you have an integral hot water dip tube and cathodic protection anode or a slip-in anode, you may have to remove the anode to check for blockage there - some have a solid bottom and let water in through openings in the side so probing with a wire would feel (a foot or two down into the tank) like it was hitting a blockage.

6) Just to confirm it is not a dip tube blockage inside the tank, I would hook a temporary hose connection (leading outside or to a drain) to the hot water outlet on top of the tank and turn the cold water valve back on to be sure it flows through the tank freely - some tanks have dip tubes which are plastic and can get blocked by broken pieces of it. Also, sediment buildup in the bottom of the tank can potentially (if real deep) block outflow.

7) If no blockage at the tank, then tracking along the hot pipes to find the blockage is next. Note since all demand points are out of hot water, the blockage has to be an incoming cold water failure, in-tank blockage, or barring one of those - a blockage in the pipes from the tank to but not past the point where it divides (usually) to run to different bathrooms and kitchen and such, because if it was past the divide point some demand points would have hot water and only those on the blocked branch pipes would be out of water.

8) if you have a toilet tempering/mixing valve located near the hot water heater and leading from one tempering valve to all toilets, at least one type has the hot water line run through that rather than branching off the hot and cold lines to it - if the tempering valve is in-line rather than stubbed out off the hot and cold lines, a blockage in there could be causing the problem too.

9) I (having air compressor available) wouldthen hook up about 30 pounds pressure to the line and start checking at every accessible threaded connection for blockages by undoing the connections (then resealing and retightening afterwards) to check for airflow to that point - narrowing the length of run needing checking. Then back up and start checking along the way by opening up soldered joints from last threaded fitting which had flow - using a fiber optic scope to look each way from the opened connection for blockages. You can map out the piping and play the divide and conquer method - check at starting end, then open pipe up at a convenient point midway along run to check if blockage is before or after that point, then subdivide the portion determined to have the blockage, etc - hopefully within 2-4 openings up oyou will have it tied down to a short enough length that running a wire through the pipe will locate the blockage. I have also see plumbers use a long length of #10 or #12 electrical wire or an electrical snake - single strand - and shove that into the pipe as far as it will go to narrow down the location - but again, generally does not go around elbows worth a darn. Some plumbers in areas with pipe buildup problems keep a plumbers snake "clean" for this use - to snake through water lines hunting for blockages and breaking up any mineral or algae deposits which are not hardened yet. In one case (pretty high-end house with very high end finishes so opening up walls to break the pipe was not a good option) I went and bought a brand new 50 foot plumbers snake for about $25 and after bleaching it to disinfect it, ran it through the pipes to locate the blockage (which turned out to be a piece of water heater dip tube plastic which had finally turned crossways in the pipe when it got to an elbow, to essentially shut off flow.)

Sometimes if a physical item (rather than buildup of corrosion or algae) exists the air will blow it out - if not at an opened connection, then sometimes by putting air at an opened connection and flushing it back to the open dielectric fitting at the tank. Any valves along the line tend to be prime blockage suspects - usually if you have a corrosion or algae or lime or such buildup you will see if all along the lines, not just one spot, indicating you have lines pretty much at their lifespan. (Can rarely be cleaned with an acid treatment with some types of buildup - primarily with algal growth, not mineralization.

Lacking compressed air, you can get a hose to pipe threaded fitting and hook a garden hose up to the disconnected hot line at the heater (at a very low flow setting on the faucet), but makes it messy checking fittings for presence of water, and has to be removed and drained down every time you want to desolder/resolder a connection. On threaded fittings some plumbers would just open it enough to detect if there is flow there - but that leaves a weak link which may start leaking, so I undo connections all the way and reconnect as if a new connection with new soldering or teflon tape or plumbers dope as applicable to type of fitting.

10) There are some plumbers and sewer cleaning companies (though have to be careful about disinfecting the equipment if previously used in a sewer line) with small-diameter fiber optic inspection tools which can reach 50 or sometimes even 100 feet into a pipe - but finding one small enough to go into and through household piping (normally 3/4 or even 1/2" diameter) can be tough. Ditto with HVAC contractors and Duct cleaning companies - some have small diameter fiber optic probes, though not all will risk them in small diameter pipes. Basically has to be not more than about a third to half the pipe diameter to have any hope of going around elbows in the pipe - so would have to be a REAL small probe for normal 1/2-3/4" pipe.

11) Cost for a plumber do to this sort of search - plan on $500-1000 though you might get lucky and have him find it right off for minimum service charge of typically $75-350 (usually around $125-175 in most areas)- but don't count on it. And of course, if having to open up connections where accessible and the pipe is looking nasty (corroded or built up with minerals or algae) inside, you get into the question of whether to just start replacing pipe - at least till you get past the blockage point at a minimum.

BTW - if using an electricians fishing tool for this, the hook end has to be clipped off or have some shrink tubing put on it or it can get caught inside the pipe - needs to have an end which can move forward and back freely across joints. Most snakes are OK because they have a tapered end facing both ways to help ease it past any blockage or snagging point.


Answered 6 months ago by LCD

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