Domestic Lukewarm Water

saikosis

I have a Weil-McLain PEG-50 gas-fired steam boiler and an HTP SSU-45 indirect hot water tank. It was installed eight years ago. Unless the boiler is really cooking, our hot water has never really been hot. Now, our hot water isn't really ever hot.

Here's my understanding of how it's supposed to work.

There's an aquastat on the tank. It is set to 140. When the water in the tank falls below that temperature, it turns on the pump and calls for heat.

The pump pulls hot water from the boiler, sends it into the inlet port of the tank, circulates it through a coil in the tank, and sends it out the outlet port back to the boiler.

There's an aquastat on the boiler. It is set to 180. If the temperature of the boiler water goes above that, it shuts off the burner, but not the pump. This is to prevent making steam in the summer. The pump will keep going until the water in the tank is at 140. If the boiler comes back below 180, the boiler will fire up again.

The hot water from the tank goes into a mixing valve to bring it down to a user-friendly level. The mixing valve is cranked all the way to the hot side.

The mixed water goes through a pipe to my shower. A mixing valve in the shower might further reduce the water temperature.

As I said, our water isn't hot. I don't mean that the tank can't keep up with a long shower.
I mean the water starts cold, gets lukewarm after the standing water in the pipes is flushed out, and then stays lukewarm until you finally give up on a nice hot shower.

To rule out a bad shower mixing valve or heat loss through the long run of uninsulated pipe from the tank to the shower, I've tested a sink faucet close to the tank with the cold supply turned off. The hot water just isn't hot.

I have some questions and some theories.

My first theory is that the mixing valve at the tank is bad. It's very hard to turn. The plastic knob doesn't work at all. You need to clamp down with pliers and pull hard to turn the metal knob inside. I assume that means it's caked with minerals inside and isn't doing a good job anymore. Or, maybe it is doing a fine job and it and I just have different ideas of what's an appropriate water temperature.

My second theory is that the the coil in the tank is filled with boiler gunk, or the coil is caked with minerals, or both. This would mean heat transfer from the boiler to the tank is slowed. However, the pump doesn't run all the time. That tells me the tank thinks it is hot enough. Also, inefficient heat transfer I think would result in a shower that starts hot but gets cold because the tank can't keep up, not with water that's just always not quite hot.

My last theory is that the aquastats are bad and that the tank isn't actually getting up to 140 F.

And here are the questions.

There's a small copper pipe connecting the pipe from the boiler to the pipe going back to the boiler. It's bypassing the pump. What's the purpose of this pipe? Should the valve be open or closed?

How can I rule out a bad mixing valve or bad aquastats?

Does the boiler gunk inside the tank's coil need to be flushed? If so, how is that done? Do the mineral deposits on the coil need to be removed? If so, how is that done? I've never had the water tested.

What's the life expectancy of these indirect tanks? (For that matter, what's the life expectancy of my boiler?) Would I be better served by getting a traditional water heater?

Thanks.

Jon_blaney

Start here: Drain some water from the tank and take a temperature. Does the pump run and does the supply pipe get hot and the return less so? Run water in the bath tub will get the biggest and hottest draw.

saikosis

The only way to drain water off the tank is through the relief valve. I measured 130 with a cheapo meat thermometer. At the faucet, with the same thermometer, I get 100 from the hot supply and 50 from the cold.

130 isn't quite 140, but it's pretty close considering my thermometer and the aquastat aren't exactly precision instruments. And, it's plenty hot for domestic hot water, right? 100 is not hot enough for me.
So, bad mixing valve?

Maybe this is a dumb question, but is it possible that my cold water is too cold? The mixing valve is a Honeywell AM-1. The knob says it goes from 70 to 145. It's cranked all the way toward 145, but that's clearly not happening. If it passed the 130 degree water through by itself, our showers would be nice and toasty. (We'd have to count on the mixing valve in the shower to not get scalded.) But 100 at the tap means it is sending roughly two parts 130 to one part 50. So either the valve is bad or the valve is always sending through some amount of cold water, and our cold water temperature is too low for the valve to keep up? Again, that's maybe a dumb question. I have no idea what's going on inside the valve.

Jamie Hall

This ^^^. If you can get water from the tank before the mixing valve, check the temperature. If it isn't up to spec., then the problem may be clogging in the heat exchanger or scaling on the outside -- domestic water side. If the water direct from the tank, well... then it's the mixing valve, and that wouldn't surprise me one little bit. They are supposed to be very free to move with changes in flow and temperature, and it sounds to me like yours may be pretty well jammed up.

Jon_blaney

Start by replacing or rebuild the mixing valve.

saikosis

If I wanted to replace the mixing valve, are there any models I should look at or avoid?

Here's mine (with the plastic cap removed). Is it really as easy as unscrewing the three nuts? I found a rebuilt kit online. Is it worth soaking the valve in vinegar first to remove scale?

mattmia2

or citric acid. The soaking is worth a shot vs $100 or so for the cartridge.

Make sure the valve body is clear too, they sometimes clog up with sediment.

JUGHNE

Does anyone know the function of the 1/2" bypass??

He could close the valve to see if it changes anything.

mattmia2

The bypass mixes some return water in to the suction side of the circulator to keep it a little cooler so that it doesn't cavitate especially when the boiler is steaming since the water is close to boiling and the circulator is operating at atmospheric pressure.

saikosis

I'll try vinegar first and see what happens. What are the odds I take it apart and can't get it back together because something is broken inside?

One weird thing about this setup (other than lukewarm water) is that sometimes the relief valve on the tank pops and dumps a bunch of hot water on my basement floor. Fortunately, it's a dirt floor, so there's no harm done.

It happened a lot when the boiler and tank were first installed, but something was fixed (not sure what) soon after the initial installation and now it happens a lot less. "A lot less" meaning just a handful of times or so in the last eight years.

I haven't noticed anything that we do that could be causing it. It's happened during both the winter and the summer, so it's not a simple case of the tank temperature getting too high while making steam. But, the temperature is set high on the aquastat, so maybe I'm just near the limit of the relief valve and occasionally get unlucky. I'm not sure what our water pressure is, but I think it's pretty good. I can fill a five gallon bucket in a minute from a the tub on the second floor. There's no expansion tank and I don't see a pressure reducing valve or backflow preventer on the cold supply, just a shutoff valve.

mattmia2

You need to have an expansion tank on the domestic hot water. Those mixing valves usually have check valves in it so there needs to be an expansion tank to allow the hot water to expand as it is heated. If you use the water frequently enough it may not build enough pressure to open the relief valve so that might explain why the problem diminished. Usually you will get a burst of very forceful water when you first open the hot water after it hasn't been used in a while if lack of expansion tank is the problem.

saikosis

There's no expansion tank.

Jamie Hall

saikosis said:

There's no expansion tank.

That's as it may be -- but you need one.

saikosis

I think I posted my last response while mattmia2 was editing his. There was originally a question about whether I had an expansion tank, which is what I was responding to.

What does the expansion tank do? Does it just give some additional buffer before the pressure relief valve pops? Because there's no backflow preventer, some amount of extra pressure can go back out to the municipal supply, right?

Unless it's going to improve our personal safety or significantly prolong the life of the equipment, I wonder if it's worth the cost of having one installed. As I said, the valve has only popped a few times in the last eight years. The spilled water isn't causing damage because it's going onto a dirt floor. It's not ideal, but it's not a big deal. If something is going to explode though or if I'm going to damage something costly, then maybe it's worth it.

bburd

mattmia2 said:

The bypass mixes some return water in to the suction side of the circulator to keep it a little cooler so that it doesn't cavitate especially when the boiler is steaming since the water is close to boiling and the circulator is operating at atmospheric pressure.

Most likely the ball valve on the bypass should be throttled, not fully open. This may be a large part of the problem.

Jamie Hall

As noted above, most mixing valves (all?) have check valves in them. What this means is that as water heats in the system and expands, the pressure must rise if there is nowhere for it to expand to. The expansion performs that function. Otherwise, the pressure in the hot water piping must increase. Is this really a problem when the only water which is trapped is the small amount between the mixing valves and the faucets? Perhaps not. The pressure rise may not be that great. Are you going to damage something costly when something breaks? That rather depends on what breaks and how quick you are to turn off the water -- and that's not a call I'm going to make.

I don't think it is a code requirement, however (it is, if there is a check valve or backflow preventer anywhere on the feed to the hot water tank).

rick in Alaska

I would start by taking the mixing valve out and seeing if it is gunked up on the intake. There could be a lot of debris in it.
Since you can't turn the cartridge without a lot of force, then most likely it is just scaled up and needs cleaned or replaced. But, it does sound most like a mixing valve issue.
Rick

saikosis

When the pressure in the tank builds, it has three places to go.

1. Out the relief valve.
2. To the mixing valve.
3. Back toward the cold water supply.

I think what you're saying about the check valve is that, for #2, it will be able to push out the mix side of the valve but it won't be able to push out through the cold side. Is that right? That would put pressure on my hot water piping from the tank through the valve and all the way to my hot taps.

For #3, it would put pressure on my cold water piping from the tank to my taps.

The risk from #2 and #3 then is that a pipe bursts in the wall and causes all sorts of damage. Right?

But, because there's no backflow preventer, for #3, can't it also push back toward the municipal supply?
Isn't that the way it'd go to relieve pressure before blowing the relieve valve?

Trying to understand, not trying to be stubborn...

Jamie Hall

The problem isn't the hot water in the tank, it is the small amount of water between the mixing valve and the taps. You are quite right in that anything on the tank side of the mixing valve will push back towards the municipal supply, and so it's not a problem. The water on the mixed output side of the mixing valve can't back to the pressure relief valve on the tank -- that check valve is in the way -- never mind into the tank and the municipal supply, so it's only a problem from the mixing valve to the tap. To be honest, I suspect it's a non-issue. Which also means, incidentally, that the opening of the temperature pressure relief valve isn't related -- that's a different problem altogether.

saikosis

Aha, that makes sense. Thanks, Jamie.

I took the mixing valve out. But, I can't remove the nut to get at the guts. (I maybe should have tried removing the nut before disconnecting the valve altogether. I don't have enough leverage now and don't have a vice handy.) I've got the whole thing soaking in vinegar now. We'll see what's what in an hour or so.

Thanks again.

mattmia2

given how much other hardware is there I would look very closely for a check valve on the cold water to the tank somewhere. Is there or was there ever a hot water recirculating loop?

saikosis

There is a backflow preventer on the boiler's supply, but nothing on the water heater's supply.  Unless a ball valve somewhere is doing double duty?

I managed to get the nut off the mixing valve.  The inside was totally white with scale and the magic gadget inside was totally seized up.  I got it cleaned up reasonably well and reinstalled it.  I now get 125 at the faucet where I was getting 100 this morning.

It's not all good news though.  The valve is weeping ever so slightly on the cold and mix sides.  I didn't use any tape on the threads when I reinstalled it because there wasn't any there when I took it off.  Should I have used tape?  I realized plumbers have bigger wrenches than I do, they're probably stronger than I am, and they might have magic sealing goop that's better than tape but doesn't leave any residue I would have noticed.  The plumber was also working with new washers eight years ago.

I still might replace the valve even though it's working again.  I couldn't get out all the scale.  I still can't turn the adjustment knob without really clamping down hard with a pair of pliers.  Also, it looked like some pieces inside the valve might be broken.

mattmia2

If those unions use gaskets you will probably need new gaskets. ground joint unions take a fair bit of force to tighten and you need a wrench that won't distort the fitting but gasketed unions shouldn't take a lot of force to seal.

rick in Alaska

At this point, seeing as you still can't get it to open up easily, and the gaskets are leaking, I would just bite the bullet and replace it and be done.
Rick

JeffM

I agree with Rick. If you buy the same model, you can leave the nuts attached to the existing piping and just put in the new valve (using new gaskets that came with it, but tossing out the new nuts) for a simple quick job. Soak the new gaskets for a while first in water if they are the green composite type.

saikosis

Thanks everyone. I'll get a new valve soon.

With a functioning mixing valve, what should I set my aquastats to? They're set to 140 on the tank and 180 on the boiler now. I don't remember what they were originally. I cranked them up to try to get hotter water. With a new valve, I'm wondering if I should go lower.

Jamie Hall

140 on the tank is a good value. Hot enough to keep things safe. And you need the 180 on the boiler to get decent recovery.

Kickstand55

There's a lot going on here. Running dirty water rusty through a coil can be a problem. This can create an insulation barrier and reduce heat transfer. Also, I see ferrous unions connected to the tank coil line. Not what I would do.
If the water is loaded with minerals, they will come out of solution in hot water and adhere to metals. (I tried to attach some pics of a fouled mixing valve innards, but wouldn't take.)
If there's chlorides (chlorine) in the water to disinfect, this also can cause issues including leaching out through cast brass. You'll see fuzzy white spots appearing in the finish.
Acidic water is another issue.
A water test will be your best friend in solving these headaches. Better than asprin!
I suggest triple filtration on the water main based on what I see. First, a 50/5 micron filter, next a charcoal filter and finally a water softening filter. These definitely need regular changes, but worth it in the long run.
Next, fit the boiler with washout fittings to clean out the muck. Add a boiler water cleaner and overfill the boiler, run off and on for heat but don't bring to steam. This loosens up the muck.
Then flush out all the water, clean the glass gauge, low water cutoff and pig tail.
One other thing you may try, as I found this effective. Place a large magnet,
available at a popular tool store, on the end of the washout fitting. You'd be amazed at what it picks up allowing trapped rust to be easily removed.
And finally, feed your boiler from the hot outlet of the hot water tank before the mixing valve. There's a port on the side of the valve for that.
Let us know what you decide and the results.

saikosis

Thanks for the link. I measured my pipes at 7/8" outer diameter, which I believe makes them 3/4" pipes. The soldered nuts are apparently called sweat unions. Putting it all together, I think this is one I want: https://www.supplyhouse.com/Resideo-Braukmann-AM101-US-1LF-3-4-Sweat-Union-Mixing-Valve-LF. I called a local plumbing supply and they gave me a price of $250 for the valve. I'm glad I didn't buy from them.