mixing valve or ball valve, or leave it alone

evansNovember 24, 2006

Hello to my favorite forum. I've been reading for months with glee. I have consulted with two plumbers and got different answers to solve my problem. I'd love to hear feedback on this.

I am having put in a gas tankless water (controversial on this board) heater to supply a very large tub in our house (already have LP for the furnaces). Our electric tank heater is 50 gallons but the tub will hold over 75. Even though the tank is new and will recover fast, no one wants to wait for that. Both plumbers agreed that mixing a little hot water from the tank will assist the tankless (mixing BEFORE the cold water enters the tankless heater).. Well water is cold, probably 50 degrees and we want 105 degrees or even a little more (my wife has skin, I don't know how it doesn't burn). One plumber suggested a mixer (25-30$), like one for a boiler. The other suggested just a ball valve, only 12$. Does anyone think this is neccessary? Should the tankless be left to do its own thing and blast 125,000 btu, or does a little mixing help it run at less than max? If it is useful to mix, which valve is better?

Thanks for any feedback on this topic.


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i would put in a whole house tempering valve, then turn the water heater up to 180 and you will have enough water directly from the tank.

    Bookmark   November 24, 2006 at 11:25PM
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Thanks for your advice. I will research tempering valves. Google is my friend. Would you have any insight about the plumbers' suggestions, you see the heater is already here!

    Bookmark   November 24, 2006 at 11:52PM
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If you insist on installing the tankless install it in series with the tank type first and set the thermostat of the tankless slightly below the tank thermostat. In this manner all the water would pass through the tankless but the tankless heater will not come on until the temperature drops below the set point and you do not need any valves.

    Bookmark   November 25, 2006 at 12:02AM
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That sounds good. I think it will not work for this situation. Maybe I'm wrong about that though. This is not a whole house tankless, but rather one for a single fixture. It has only a 1/2 inch outlet, but there are many feet of 3/4 inch runs including through the water heater tank, and up further up until the 1/2" branches. Will the diminished size of pipe have an effect on pressure or quantity of water to supply the 30 fixture units down the way? Or will it have a negligible consequence and therefore your suggestion is really good?

    Bookmark   November 25, 2006 at 12:18AM
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If you have 3/4" piping at the water heater you will not be able to use that tankless at that position but, you can install it in series on the Hot water line at a point closer to the demand where the line is 1/2". The problem here is that you may not install a gas burner in a bathroom or a utility room that opens into a bathroom.

    Bookmark   November 25, 2006 at 12:05PM
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Sorry, I screwed up my last post. Wrong fixture units, and I misread lazypups post before that.
Anyway, I looked at a nice valve today at my local plumbing supply, and a good mixing valve is almost $100.00. It has temperature a indicator on top ranging from 80 to 120 so I could leave it at 80 and that would help. Reason it would help is that the tankless will provide more gallon/min if it doesn't have to heat the water so much; the range of this particular one is .5 to 4.0 gpm depending on the temp of the supply, essentially. Or at least that's what I think. But again I ask to anyone out there who has experience with this...would a ball valve off a T work, or is the mixer the way to go? Obviously the ball valve deosn't have the gauge on it. I'm leaning towards the valve at this point because it seems made for the job of ...mixing with control.

    Bookmark   November 25, 2006 at 4:47PM
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I fail to see how they intend to use a ball valve as that would require a manual operation to use it, on the other hand, if you will install a whole house tempering valve on the output side of that 50gal water heater, then turn the water heater up to 180 you will have more than ample reserve BTU energy in the water heater to supply the 75gal @105degF without even using the tankless.

    Bookmark   November 25, 2006 at 6:06PM
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The idea is that the ball valve would be left open a little I suppose(that plumber who said ball valve is not available at this time for follow up).

You are a good...pup. No one else has responded. Both of the plumbers I consulted admitted only installing a handful of tankless water heaters. I am feeling on my own here. It says in the owners manual for my kenmore 50 gal power miser 12 water heater that the max temp is 160. Recovery rate I think is around 17 gpHour, pretty good IMHO. It seems the tank would be spent filling my tub, and no laundry, dishes, or showers could take place in the house simultaneously. Once the 50 deg well water and 160 deg heated water fill the tub, (averaging 105) I'm about out. We have young kids so I have it set low now. With a mixer that could change. Anyway, Since it does take time to fill a large tub, it makes sense to fill it at a hotter temp than you want because of the waiting. Now my wife says 105 is not hot enough. Lucky me!!!!!! It's gonna take time to fill it, depending on the fixture at the tub (not chosen yet). This is for her dream tub, the champagne bubbler! It better not run out of hot water filling up!

I digress

I am trying to avoid catching the tank on recovery after two or three long baths/showers, a load of dishes, and a load of white wash on a Sunday afternoon. Tankless is my answer, my ga-run-tee.

Thanks again for the dialouge Lazypup.

    Bookmark   November 25, 2006 at 11:01PM
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You tub requires 75gal of water @ 110degF and your incoming water supply is 50degF.

Let us begin by determining how many BTU's of energy you need to add to the water to meet the demand when filling the tub.

The incoming temperature is 50degF and for the purpose of illustration let us say the desired temp is 110degF for a differential of 60degF.

1BTU will raise 1lb of water 1degF and water weighs 8.34lbs/gal so the total energy required per gallon is 8.34BTU x 60degF = 500.4BTU/gal.

500.4BTU/gal x 75gal = 37,530BTU.

Now let us consider your 50gal water heater set at 160degF.

The water heater is 160degF and the supply is 50degF so the differential is now 160 - 50 = 110degF.

8.34BTU/gal x 50gal x 110degF differential = 45,870BTU thus from the start of flow a 50 gal water heater at a temperature of 160degF contains 8,340BTU more energy than what is required by the tub demand.

Years ago all water heaters were operated at 160 to 180degF but there were two problems associated with that.

First, in those days water heaters only had a light 3/4" fiberglass insulation blanket and there was a tremendious standby loss of energy. Initially they encouraged people to lower the temperature and add insulation blankets to the water heaters but in some instances the blankets were improperly installed and caused problems by restricting combustion air or trapping heat in the thermostats causing them to fail. To offset that problem the plumbing codes were rewritten requiring all water heaters to have a minimum or R-12 insulation. In order to meet the new specifications almost all water heater manufacturers switched from fiberglass blankets to high efficiency cast in foam insulation so standby losses are now nearly no-existant, especially on electric heaters which have no flue loss.

The second problem was that when the water heaters were set at 180 many children were injured by scalding accidents. To counter that problem the plumbing codes were revised now stating that the maximum allowable temperature in the domestic water distribution system is 120degF. From the building contractors viewpoint it was cheaper to set the water heater at 120degF meet the new spec than what it was to install a $50 whole house tempering valve. The problem here is that with the water heater set to 120degF we are now we are now delivering water to the shower or tub at 120degF and nearly all the water consumed by the tub or shower must go through the water heater. This then results in an insufficient supply of hot water to meet the demands of consecutive showers or large tubs such as yours and most people resort to adding a second water heater or a tankless .

Now consider the tankless.

Your tankless has a flow rate of .5 to 4gpm. which is determined by the differential between the incoming cold water temp and the demand temp.

At best most tankless water heaters can only provide a 70degF differential which means if your incoming water is 50degF the maximum output is 50 + 70= 120degF so here again, you are heating all the water going to the tub, however it must be understood that as the differential increases the rate of flow decreases and at 70degF differential you can expect the .5 to perhaps 1.5gmp flow, hardly sufficient to meet the needs of a shower which requires 2.5gpm.

    Bookmark   November 26, 2006 at 2:28AM
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You think we could get by without the tankless by running the tank at max. Does running the tank at max have a more taxing effect on the tank? Will it shorten the life...?
PS I reread the manual gpm range is 2.3 @90 deg rise up to 4.6 gpm a@ 45 deg rise.

    Bookmark   November 26, 2006 at 7:46AM
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