Gravity Recirc - How bad is it?

iced98lx

Hey folks, thanks in advance for advice.

JAH (Just a homeowner) here so apologies for having to ask more questions than normal.

We recently (october) moved into a substantially bigger house than we were in previously (from ~1000sqft to ~4500sqft) and in the process went from an electric 70 gallon "marathon" heater (which are popular here in SE SD due to our low electric rates) to a 40 gallon Rheem propane heater.

Everything in the house that can be propane is... Boiler for baseboard water heat, stove/oven, water heater, garage unit heater.

We're burning propane at an astonishing rate. I knew it'd be much more expensive but $500/month is a system shock.

We're in the process of converting from propane to NG service, and eventually we'd like to upgrade the 1978 utica boiler to modern mod/con with an indirect water heater attached.

In the mean time (and probably after it's installed) I am looking at this very effective gravity fed recirc loop (3 of the 4 bathrooms in the house are served by the recirc and it is VERY effective) and wondering how much propane I'm burning letting it do it's thing.... Should I put a timed pump somewhere? A temperature sensitive pump instead? Turn it off completely?

It's a very loved feature by everyone... (we've got 4 adults and 5 kids living here thanks to COVID) but I'm willing to put up with grumbling if it stretches the propane out a bit...

All options considered, frankly at this point. We could even just forego the indirect and plop a big o'l electric water heater in, we have 400A of total service to the house, using very little (electric dryer).

TIA!

Youngplumber

So there are charts that tell you how effective insulation is at different thicknesses. If you look at one of those you will probably decide on your own to keep the free circulation and insulate the pipes throughly. You won't spend much money heating those little pipes of you keep the heat in. The money spent heating is mostly when you emit the heat off of whatever is being heated.

JUGHNE

I have gravity recir in my house, it give hot water almost immediately at the farthest point of maybe 60' of pipe away. The return pipe has a ball valve, check valve and connects into the bottom of WH tank. All is insulated with 1/2 to 5/8 foam.

The ball valve is cracked almost closed to slow down the flow.
The WH is set up to provide nearly 140 degree water, the returning water is then heated up to that temp in the tank before reuse.

I am sure it costs something to have this constant circulation.
But given the cost of a bronze pump, aquastat, and electricity and future replacement of such I believe I am ahead without the pump.
All the water you run down the drain to get the hot water to your location used to be hot water that you heated anyway.
Can you throttle the return flow on your system.

iced98lx

I do have a ball valve on it, basically same system as you with it returning to the bottom of the water heater.. I'll shut it down some and see if we notice a difference and look at insulating the lines @Youngplumber that is something that I have total access to do thanks to the construction of the house but hadn't really prioritized until now.

iced98lx

Do you use a Tempering valve @JUGHNE ? the heater isn't really big enough (4 showers, 1 90 gallon whirlpool tub) and I've contemplated turning the heater up and using a tempering valve to extend the draw a bit...

JUGHNE

No tempering valve here, I do have a 120 gallon tank heated with tube in shell heat exchanger and 80,000 buth Mod Con boiler.
I did not insulate the drop pipe to the bottom of the tank, thinking the cooling "leg" would induce more gravity flow.
We have just about the same fixture load you have and pretty well never run out of hot water.
The tank was a close out bargain, the heat exchanger is home made 6" x 36" steel pipe pulled out of the iron pile, it has 20' of 3/4" copper for the bundle inside.
When the tank leaks I will have to get some factory indirect tank.
That tank has been in use since 1995. I am in northcentral NE sandhills with great water.

Youngplumber

The screenshot is an example from stream piping which is more dramatic but your hot pipes with no insulation are giving out a lot of constant heat. You will save a decent amount insulating. Also @JUGHNE is right. Don't insulate the last 10 feet or so. It's helps the thermosyphon. 

JUGHNE

Obviously one loses heat with an uninsulated hot pipe.
However, steam is at about 215 degrees, your return would be 110-120 degrees. You consider the temp difference between the pipe surface and the surrounding air temp.
That example is for 2" pipe you may have only 1/2-3/4". The surface area of the pipe is an important factor.

Still you should insulate your hot and return lines.

Youngplumber

@JUGHNE just the first decent illistration of my point that I found. 

Larry Weingarten

Hello @iced98lx , Being an energy nerd, I'm going to suggest the most efficient approach I know of for your situation. That would be to insulate all the pipe you can get to with at least 3/4" foam. Make it as air tight as you can. Put a quiet spring check on the return so there is no convective flow and put a demand controlled pump on the system. This will do more to lower heat losses than anything, other than keeping the lines cold.

Things I do know... an always on recirc pump roughly triples your water heating bill. If the gravity system works well, it almost certainly isn't that bad, but could be close... Demand control uses less that 10% of the energy of other control systems as it's putting hot water in the lines for only minutes a day instead of hours... Energy prices are not going down. Hope that helps

Yours, Larry

EBEBRATT-Ed

@iced98lx

The most effective thing you can do is replace windows and add insulation if needed. Do this before you replace the boiler. Natural gas is probably less expensive than propane

iced98lx

Thanks for all the input folks. I had always had insulating all the hydronic lines (a lot of 1 1/4 inch copper) on the "Someday" list but perhaps I need to move it and the hot water lines (a lot of 3/4 and 1/2) up the list of things to do as an "easy" way to optimize what we have. Luckily we have easy access to 99% of them. I've adjusted the ball valve on the recirc, and will continue to monitor. Just glad that isn't something "We should get rid of" as the wife factor is big on that one.

As for the rest of the rest of the house, we're making plans for some additional insulation but that ties in with other projects and things aren't too bad... 2x6 construction, modern double pane windows.

mattmia2

You are losing heat from the re circulation loop 24/7. You only lose heat from the hydronic piping when there is a heat call. The re circulation is a much higher priority. The loop essentially becomes an extension of the tank itself and the insulation of the tank in effect is only as good as the insulation of the loop.

iced98lx

mattmia2 said:

You are losing heat from the re circulation loop 24/7. You only lose heat from the hydronic piping when there is a heat call. The re circulation is a much higher priority. The loop essentially becomes an extension of the tank itself and the insulation of the tank in effect is only as good as the insulation of the loop.

That is a good point, and a good reason to do it first. Is everyone just using something like this?



or a rubber based one? or...?

mattmia2

You can get a much higher r-value in the rubber unless anyone here sees a reason not to use it.

hot_rod

Your uncontrolled recirc loop is essentially a hydronic heating loop running through the building. I agree it should be well insulated and temperature an or demand controlled.

Here are some graphs showing heat loss in uninsulated and insulated copper.

For the LP consumption in general, see if there are any Energy Audits offered in your area. Sometimes the fuel provider offers them, or the local govt program. there may be $$ available for upgrades.
An energy audit should include a blower door test and an infrared camera look at the home. Around my area realtors really encourage energy audits on properties they list or sell.
This site will show you what is available.
www.dsireusa.org

JUGHNE

Hot Rod, thank you for that. Now I am curious about that heat loss.
So I have 60' of 1/2" insulated (some better than 1/2" foam) for my recirc line.
130-70=60 TD = 420 BTU loss (however at the return point the temp is less than 100)
Does the flow rate matter, as the pipe is always the same temp?

A small pump pulling .5 amp x 120v = 60 watt x 3.4 = about 200 btu of waste heat generated by it's motor. (lost to the air around it).

I just completed a recir project for a school.
So the cost for a bronze pump, flanges and aquastat was right at $500, my cost.

My NG bill for summer months averages less than $40/month, this is water heating and cooktop usage. $480 per year.
IF a controlled pump would save 10% a year = 48/year x 10 years = 480.00savings over 10 years? What is the longevity of pump and control?
Also I already have an exp tank for WH, that would be an additional cost if needed.
Plus some electricity for pump operation.

Also any waste heat is lost into the heated basement. We need some heat down there for at least 5 months of the year. These btu's come from the same source-Mod Con boiler.

The same conditions would apply to the HO who posted initially, he is in South Dakota, IIRC.

So please post any comments if my logic is flawed somewhere, Thank You.

mattmia2

Put an ammeter on the pump. I bet it is drawing far less than the rating at the actual operating point. The heat will flow from hot to cold at the same rate for a given delta t regardless of the flow rate. Probably will get your payback on more insulation faster than on the intermittent pump control and it won't really degrade for decades. How much of the heat that is lost goes to something useful vs just causing more air to be drawn in/out around the sill is maybe a little questionable.

JUGHNE

We could just take the pump motor heat generation out of the equation, it is not a major factor in the whole scheme of things. I was just crunching numbers.

I consider all of the heat loss useful in the heating season, it is contained in the heated envelope of the structure.

This house is super insulated with air/vapor barrier wrapped around sill plates and continuous from that point, comes in to inside walls, ceiling over to the other sill plate.
So I believe my infiltration is at a minimum.
Any heat dissipated contributes to heating the floor above, some of which are plated and tubing run. Or part runs down the center of the house in basement soffit, this will heat the center portion of the house floor.

True this adds to the AC load in the summer as does the multitude of bulbs in any house.

iced98lx

mattmia2 said:

You can get a much higher r-value in the rubber unless anyone here sees a reason not to use it.

Something like this from Amazon?

Armaflex IPAPC05812 1/2" x 1/2" x 95' Continuous Coil Pipe Insulation, Rubber

I'd have to split it to put it on but that's not the end of the world with a sharp utility knife.

hot_rod said:

Your uncontrolled recirc loop is essentially a hydronic heating loop running through the building. I agree it should be well insulated and temperature an or demand controlled.

Here are some graphs showing heat loss in uninsulated and insulated copper.

For the LP consumption in general, see if there are any Energy Audits offered in your area. Sometimes the fuel provider offers them, or the local govt program. there may be $$ available for upgrades.
An energy audit should include a blower door test and an infrared camera look at the home. Around my area realtors really encourage energy audits on properties they list or sell.
This site will show you what is available.
www.dsireusa.org

Thanks for the Tips Hot Rod, I appreciate it. I will insulate it right away as it is accessible with a small amount of work. I also appreciate the graphs, I've nestled them away in the "good things to know" folder.

As for the rest of the house, we did thermal imaging and have a few things to fix, a blower door test is in the works for the summer, we know we have plenty to do there. Those come with bigger attached items as the house was built in 78 and while it was built to a high level of quality most of the house is untouched from 78. Thankfully 2x6 walls, quality double pane anderson windows etc.

mattmia2

You can get Armaflex with adhesive and a flap in 6' sections that is a little easier in a retrofit although I wish I could have just slipped solid over some of the stuff i was doing before i connected the pipe.