GRM Plumbing Lesson Requested: Hot Water Edition

Our house has been having the annoying problem of low hot water pressure and temperature and plumbers are expensive.

Setup: Well pump, reasonable length run to the house, inline hot water heater somehow integrated into the boiler furnace (oil fired), thermostatic mixing valve, then out to the fixtures.

Symptoms: Cold water works great, heat works, but hot water is lukewarm and low pressure.  Water heater display thing indicates good temperature at the furnace and so does burning my hand on the pipe.  All fixtures have roughly equal temperature/pressure, pipe on other side of mixing valve is just warm.

Attempts so far: Replaced mixing valve, minimal change.  Adjusted heater settings, minimal change.

What next?  The pipes here are old and get scaly crap and deposits in them, some sort of backflush?  Bring out the wallet?  Eliminate the mixing valve?  Replace the tiny copper pipes in the hot water side with something else?  Bypass the various shutoff valves?  I know nothing.

I've got a similar problem on the pressure side of things, although my hot water is still very hot.  I've been told that it's probably my water heater full of crud and needing a flush.

So how does one flush these things?  If I can shut off the house water but keep the well pump going I could potentially pull some caps off some lines and push water through it backwards.

Well, see, that's kind of where I got stuck, too.

Scaled up water  heater is one possibility as noted already

Another possibility is the pipes scaled up.  Next thing that happens is the pipes start springing leaks when the corrosion breeches the pipe..  An acquaintances' house had this happen and it ruined the first floor.

If your wallet can take it get it re-piped with PEX pipe.  There are three types of PEX.  A, B, & C.   I don't remember which on is the right one to use in a house so a little research is required.  

An added bonus of PEX is freezing does not hurt it.   We had pipes freeze solid during an unusual 3 day freeze in Houston which had the temps at 8 degrees F.   When it warmed up my pipes thawed out and kept on working.   Neighbors with galvanized and copper had leaks to repair plus water damage..  

PEX has one drawback which I unfortunately discovered.  When the house was being re-piped one of the hose-like PEX pipes was touching the bottom of the roof where the pipe was routed over an attic wall.  When I had my roof done a roofer unknowingly put a nail right through that PEX.   Center punched it.   This wasn't immediately apparent because the self sealing characteristics of PEX only closed a slow leak.   We found out when the bathroom ceiling started sagging with water dripping.   Careful inspection of the installation is required.

Anybody want to warn me about the perils of reverse flushing this before I order some sort of pump arrangement?  Looks like there are a number of options out there along with descaler to mix in.

You inspired me to finally try to flush mine out.  It's draining now, so I'll let you know how it goes/how expensive things get around here....

Good news is that it didn't get expensive at all.  Bad news is that it didn't accomplish E36 M3.  I went with the standard method of draining the tank then giving it some cold water, draining that, and repeating the process a few times.  I didn't get much out of it at all, but I did notice that it seems very slow to fill.  I've also noticed that when I run the hot water, the tank continues to fill for a handful of seconds after I turn off the faucet.  I'm thinking that my problem might be a clog in the supply line valve or something.

I ordered a pump and some descaler solution, will report back after I try that.

Here in the South I have seen customer's electric or gas hot water heaters fill up with calcium over a foot thick.  At that point it is a R & R.    Sometimes if one drains them every 6 months they can flush out the scale as it starts.  Nobody hardly ever does it, including me.

I think the descaler will only work if you drain the system,  fill it with the descaler, and let it work a while.   

Have you considered plumbing in a hot water heater separate of the home's heat system?

Does the water temp change when the thermostat calls for heat, compared to when there is no call for heat?

When we had the integrated boiler and hot water, we would end up with the hot water going cold when the system received a call for heat. 

Any photos of the system so we can see what you're dealing with?

How old is the furnace?  Our last one was 30 and had no build up but that can depend on water quality.  We have no pressure difference between hot and cold but there is quite a temperature drop from the furnace setting to the upstairs shower.  I never measured the drop but would guess about 50 degrees.  We have no mixing valve, that is accomplished by mixing via the faucets.

The furnace runs pretty minimally as we only use it as backup for the wood stove- the hot water has priority (little panel give you the option) so it's exceedingly rare that they try to run at the same time.

Does it have a plate heat exchanger like one of these?

It may be clogged with scaling which would likely give you the conditions you're seeing.  Although it may require draining the system if there aren't isolation valves, I would try disconnecting the pipes and then back flushing each circuit with the descaling solution.  Separated from the rest of the system so you don't pump any loosened debris through valves and whatnot.   In the end, it may need to be replaced but brand ones aren't cheap, so cleaning is worth a try. 

Try putting a sharkbite ball valve on the cold water feed to the mixing valve.  Throttle it back so you get less cold and more hot fed to the mixing valve.  Some mixing valves(Symmons) have special spindles for use with tankless water heaters.  They have less cold holes in the mixing chamber than hot holes in order to trick the valve into giving more hot water to the equation.  Those valves don't care about flow rates, they only care about equalizing the pressure on both the hot and cold sides so nobody gets scolded.  Cut one side to zero pressure and they aren't even suppose to let any water come out.   All single handle pressure balance mixing valves have temperature adjustment mechanisms.  As a plumber, I have to give you between 120 and 130 on the hot water, but I can only give you 114 on a mixing valve and no more than 110 on anything handicap.  You can also try a lower flow shower head.  New ones are down to 1.75GPM.  

Update- circulated descaler in two separate batches with my handy new bucket pump.  Water is now a little hotter but the pressure is still garbage.

Looks like you are on the right track.  I would continue until the purged "water" is clean.

Update: 2hrs of circulating this DrainX stuff with a 1/4hp pump has made the hot water temperature and pressure better than it was when we moved in years ago.   I bought more in case I can't get it next time things get plugged up, I'm amazed at what a huge improvement this is. 

¯\_(ツ)_/¯ said:

Update: 2hrs of circulating this DrainX stuff with a 1/4hp pump has made the hot water temperature and pressure better than it was when we moved in years ago.   I bought more in case I can't get it next time things get plugged up, I'm amazed at what a huge improvement this is. 

When we had an tankless water heater I would do an annual descaling run.  Pond pump, 5 gallon bucket, 5 gallons of white vinegar, and some hoses and that kept it in good shape.

Consider that the hot water has a different path and therefore potentially a different pressure drop. Instantaneous water heaters have a larger pressure drop compared to a tank.

Have you cleaned the little filter basket on the water heater inlet?

The two tankless water heaters I've had had them.