Unlimited hot water

[tkappeler]

In your case I don't think a manifold would work with the loop. What do you actually save running a PEX line from the manifold to each faucet. Seems to me like you would use more PEX tubing + the cost of the manifold. Why not just run a 3/4" line from one end of the house (from water heater) to the 1st faucet then a T to the next faucet and so on down the line. When you get to the farthest faucet use another T but this time come back to the cold water inlet on the water heater. The return loop line can be 1/2" if you like. You will need a couple of spring check valves and a pump but you'll always have instant hot water when you need it. The pump installs in the loop back to the water heater.

The cost of Pex is really pretty cheap. We have 14 cold and 8 hot water end points, about evenly split on opposite ends of the house. One thing I like about the manifold approach is that each one is a home run back and no crimps except at that end point. An added benefit is that each line has a shutoff at the manifold.

 

[Depmandog]

On a hot water recirculating loop, does one really need a circulating pump?

The reason I ask, a few years back, we had plans to build. I had all my sub numbers but we never pulled the trigger to start the build. Anyways, my plumber was going to install a recirculating line for the hot water, but I recall him specifically stating a pump wouldn't be necessary. He said the hot water would migrate towards the colder water, and the end result would be a slow recirculating flow.

A lesson in thermodynamics.

The fact that atoms and molecules of solids vibrate with greater amplitude as temperature rises means that each atom or molecule takes up more room. It is not surprising then that the volume of a solid, or a liquid for that matter, will increase with rising temperature and decrease with falling temperature, even though the molecules remain in virtual contact throughout the temperature range up to the boiling point.

Based on this as I picture it - Hot water is slightly expanded (less dense) than the cooler water. As the water cools, it contracts (more dense), taking up slightly less space within the pipe, allowing room for the hotter water to flow towards the cooler water - thus providing a natural movement of the water.

The same principle when you heat up one end of a pipe, the heat travels towards the cold end.

Folks, I am pretty sure you don't need the recirculating pump.

Thoughts??

 

[schweizer]

depmandog, look up the phenomenon of thermosiphoning. Older engines used it. I have a lister engine powering our off-grid house that does not have a water pump because it circulates water through thermosiphon effect. Laws of physics state that cold water is denser than hot water, meaning the cold water will fall to the bottom. If it's in a loop and there's a one-way valve controlling the direction of flow, the water will self-circulate. If you have a woodstove downstairs and water tank upstairs you can heat the water without a pump, but only if there are not many bends/corners in the pipe. The thermosiphon effect can be broken easily if there is too much resistance. Also, think about water stratification by temperature in a hot water tank. That's why the hot water comes out the top and cold water goes in the bottom.

Marcus

 

[PineRidge]

On a hot water recirculating loop, does one really need a circulating pump?

The reason I ask, a few years back, we had plans to build. I had all my sub numbers but we never pulled the trigger to start the build. Anyways, my plumber was going to install a recirculating line for the hot water, but I recall him specifically stating a pump wouldn't be necessary. He said the hot water would migrate towards the colder water, and the end result would be a slow recirculating flow.

A lesson in thermodynamics.

The fact that atoms and molecules of solids vibrate with greater amplitude as temperature rises means that each atom or molecule takes up more room. It is not surprising then that the volume of a solid, or a liquid for that matter, will increase with rising temperature and decrease with falling temperature, even though the molecules remain in virtual contact throughout the temperature range up to the boiling point.

Based on this as I picture it - Hot water is slightly expanded (less dense) than the cooler water. As the water cools, it contracts (more dense), taking up slightly less space within the pipe, allowing room for the hotter water to flow towards the cooler water - thus providing a natural movement of the water.

The same principle when you heat up one end of a pipe, the heat travels towards the cold end.

Folks, I am pretty sure you don't need the recirculating pump.

Thoughts??

A gravity loop won't work for a on-demand water heater there is just too much resistance in the heater and lines to overcome. On my last conventional tank I used the smallest Grundfoss pump w/timer that I could find and it worked well. The manufacturer's tech recommended a Taco 009 pump for the tankless heater. The Taco 009 1/8 HP 0-8 GPM and 0-34' head. That pump cost to me was $320. I opted for a Wilo Star 32-BF bronze pump rated at 1/12 HP 0-10 GPM 0-32' head. The Wilo Star is sold new on eBay for $150. w/free shipping.

If you want to add a small circulater to your conventional water tank I have a used Grundfoss w/timer for sale cheap.

 

[WayneB]

Here are a few sites that might be helpful for household owners considering On Demand Hot Water systems:

Point of use Hot Water Heater
Point-of-Use Water Heaters

Whole house or point of use:

Tankless Water Heater - Point of Use

Water use energy savings:

http://michaelbluejay.com/electricity/waterheaters.html

 

[ptgdigger]

For those who are writing that on-demand water heaters can't keep up with 2 showers, or 1 shower and 1 dishwasher, etc., maybe their water heaters are undersized. Like I wrote a few posts back, we have a Rheem RTG-84DVP. We often have 2 shower heads on simultaneously -- no problems.

You have to look at the Delta-T flow rate curves for the heaters before buying to calculate it that heater will handle the flow of 2 faucets at XX delta-T. Example: incoming water temp is 45 deg and output goal is 120 deg, therefore delta-T is 75 deg. 2 faucets at 1.5 gpm = 3 gpm minimum. Our water heater does 4 gpm for that delta-T on the graph.

Could it be that some unsatisfied customers just bought what the store had on the shelf and now it underperforms?

Marcus

If you read my post earlier on the subject I used the flow rates for the largest heater on the spec sheet in the original post, 199,000btu, by the numbers it should work IF everything remains "optimal" after the install, however every install can't be "optimal" and the chances of it remaining so after some inefficiencies like scale and corrosion creep in are low.

I didn't say demand heaters are a bad choice , I think the average person doesn't look deep enough to know if it is right for their particular situation and what it will take to make it work. Areas with very low input temps require the most thought, one could always put in a holding tank in the basement to allow the water to "temper" before being fed to the heater, but you need to plan for that not put it in as an afterthought because your install isn't working. Same with propane supply, many of these units will probably run fine off a single 100 gal upright tank at 30 degrees outdoor temp, but take the outdoor temp down to 5 deg for a week and the vapor producing capacity of your propane tank drops dramatically, can you fix that ? absolutely by going to a bigger tank but that could be an expensive surprise to install a 250 gal tank and fill it because you didn't plan it well enough. Now if a local plumber makes these mistakes it doesn't take long before everyone in the area says "those demand heaters are bad".

By the way the standard flow for a shower head in the US is 2.5 gpm not 1.5gpm , so your calculation above is 1gpm short on hot water for 2 "discrete" showers , I realize that in a 1 shower install with multiple heads that lower flow heads are used so 1 multiple head shower will be fine.



Ray

 

[schweizer]

Good points.

My calc would actually be 2 gpm short for 2 separate showers if you calculate with 2.5 gpm shower heads.

Maybe the reason we never have problems is that we have a large propane tank (250 gal or 500 gal) and a large feed line to the house, and a full 3/4" propane line to the water heater. Also, in winter the water is usually preheated quite a bit.

 

[schweizer]

Here's how my system is set up, if anyone is interested. The plumber was skeptical, but no problems yet. Because my storage tank is under the house, I don't have to worry about incoming water temps in the 30's like some have mentioned. The tank temp never get's below the 50's even without heat input.

 

[dave1949]

Here's how my system is set up, if anyone is interested. The plumber was skeptical, but no problems yet. Because my storage tank is under the house, I don't have to worry about incoming water temps in the 30's like some have mentioned. The tank temp never get's below the 50's even without heat input.

That's a nice setup! I can see from your posts, you have really worked-out a way to reduce your energy footprint. There is no reason systems like yours shouldn't be common place.

 

[sdkubota]

A little off topic but an interesting fact. Water is the most dense at 39 degrees. Because of that as it gets colder (or hotter) it becomes less dense and rises. This is why lakes freeze on the top. Otherwise the freezing and ice formation would continue to the bottom essentially making a solid block of ice. This is the same reason lakes/sloughs turn over and smell terrible 2 times a year as the water at the top hits 39 and tumbles to the bottom displacing bottom water witch often contain sulfide gases from decaying vegetation.