Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
FPHX Thermo Pressure siphon, will this work?

This guy (in the attached video) installs FPHX on the cold water supply to a DHW tank (heat by outdoor wood boiler). He says it heats the whole tank (even when you're not using hot water) using the principal that heated water it expands slightly. I'm not sure how the hot water just doesn't get stuck in the HX?
Seems hard to get much thermosiphon flow without separate hot and cold pipes?
Has anyone done or seen something like this work? Avoiding a pump on the DHW side of the HX would be nice. The other idea is a side arm heat exchanger.
Seems hard to get much thermosiphon flow without separate hot and cold pipes?
Has anyone done or seen something like this work? Avoiding a pump on the DHW side of the HX would be nice. The other idea is a side arm heat exchanger.

0
Leave a Comment
Categories
- 80.9K THE MAIN WALL
- 2.5K A-C, Heat Pumps & Refrigeration
- 36 Biomass
- 394 Carbon Monoxide Awareness
- 2.6K Controls
- 1.4K Domestic Hot Water
- 3.7K Gas Heating
- 101 Geothermal
- 130 Indoor-Air Quality
- 2.3K Oil Heating
- 30 Pipe Deterioration
- 564 Plumbing
- 4.2K Radiant Heating
- 333 Solar
- 12.1K Strictly Steam
- 36 Water Quality
- 7 Industry Classes
- 51 Job Opportunities
- 7 Recall Announcements
Comments
Without circulation to the tank, no. Sorry.
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
Yours, Larry
I sure would like to see an anti scald valve on the outlet.
He sure doesn't lack confidence. I wouldn't want to try to convince him he is wrong
Albert Einstein
> Thermosiphon with a plate is near non-existent, especially on top of the WH like that. I've got a few out there in mobile homes that will creep a little bit because the plate is under the WH, but not enough to matter. I will say though, with a properly sized plate on the cold input like this, you can heat enough water during a shower to have enough for another shower several days later. I do all my OWB installs like this. Are you often gone several days at a time or why the hesitance? How much hot water would you say that you use in a typical day? Sidearms are nice if you're gone a lot and use very little water, but assuming you're home at least once a day to feed the boiler and shower or whatever, you'll never run out with a plate. I prefer a 30 plate 5x12 for those needing some storage or a 20 for those using a lot of water and keeping the tank hot. As an experiment last winter I shut the gas completely off to my 40 gallon WH and used a lot of hot water, bringing my domestic tank temperature up to 168 degrees. I left the next morning and was on vacation for 6 days with my dad filling the boiler for me, zero water usage in the house. upon my return, there was still enough hot water left to shower after 6 days. Another experiment was 2 showers, 3 sinks, a jacuzzi tub, washing machine and dishwasher all pulling at once and still preheating the tank input to ~120 degrees. I've never seen a sidearm that'll keep up with 2 showers much less everything else, but I don't use them either.
Agree with groundup,
I've used hundred of flat plates preheating the cold entering the water heater on owb installs. No need for it to thermo-syphon. It works a lot better than most folks can imagine.
trainer for Caleffi NA
The magic is in hydronics, and hydronics is in me
ME
ME
So the tank just "becomes" warm by adding a plate HX. With lead free copper and brass connections, of course
trainer for Caleffi NA
The magic is in hydronics, and hydronics is in me
April Fools!--Even if it would work as he says, it won't work as he described it. Without a check valve on the cold water inlet, the pressure would push out to the street main. AND...the pressure is the same everywhere. There isn't any pressure variation between the FPHX and water heater.
Funny tho.
If a check were installed the HX and tank pressure would raise at the same rate. If somehow the tank temperature increases, with a check installed, and no apparent exp tank the relief could pop off.
Those long tube side arm DHWHX sold with many outdoor wood boilers work somewhat as they connect at the top and bottom of the tank.
I suppose if the tank has electric elements in it the HO has no idea that the HX is doing nothing to add heat energy, other that an burst when cold water flows across it.
It would be interesting to see if he has one mocked up in his shop to demonstrate performance
trainer for Caleffi NA
The magic is in hydronics, and hydronics is in me
Wood boiler salesmen and log home salesmen are the same breed of BS artist.
I had a wood boiler guy insist that you could install an open wood boiler system 25' below the top floor (in a log home). He explained, with the same confidence as this knucklehead, that it was just like holding your finger over the end of a straw full of water. Just like the straw, it held for a few days then air locked the upper floor and flooded the boiler.
Albert Einstein
As for this clown on the video though, yes. I will agree wholly, as was stated in my last post
Having a FPHX connected to the cold water supply putting in heat energy to the W/H is the same as an Indirect tank with an internal HX. Only the Indirect water heater is more efficient in transfering and storing heat energy. Hahaha, sorry, I can't contain myself.
I am not saying it could never work. I don't think it is great idea. This home likely had the slightest of leaks on the upper floor, just like the finger on the straw.
Nature really dislikes a vacuum.
Albert Einstein
The bigger problem is lack of adequate pressure on the circulator coaxes them into cavitation. And it worsens as temperature increases. Locals around here keep a spare circ on hand and get a season or two out of them. The OWF salesmen just refuse to learn why that keeps happening
trainer for Caleffi NA
The magic is in hydronics, and hydronics is in me
I cut a cartridge apart once to see why it failed. It was filled with magnetite (black sand). Replaced it with a Grundfos and never had a problem since. I can clean a Grundfos.
I've never seen pitting or destruction of the impeller on a small circulator because of cavitation, but I suppose it can happen. But, I have seen it on large circulators where the NPSHA was less than the NPSHR.
I thought I recognized the producer. Pretty sure it is the same guy that did this one.
Albert Einstein
Here was the first video I watched, it seems fairly conventional.
Coincidentally, I just put one of those in my Subaru. I used mostly old vacuum parts
Albert Einstein