Looking at Honeywell mixing valve

[PDubs]

Wife and I completed building our house a few months ago (4bd/3.5ba) and noticed we are running out of hot water when taking a shower or filling the tub. Water heater is an AOSmith 50gallon dual thermostat. Here's what I know so far:

- Running 3 shower heads at 2 gpm each gives us just about 9 minutes of hot water
- We have the Kohler DTV set at 102 degrees during the shower
- Water heater thermostats are set at 140 degrees each
- Water heater tech did the "tub test" and it seems to be accurate. Hot water runs out around 43 gallons.

He mentioned that a thermostatic mixing valve would help us increase the duration of hot water. The product he had in the truck was a Honeywell AMX300 but his price was about 2x vs online. Given our 50 gallon tank, will this valve really help that much? If it's cheaper than a second tank and recirculating pump, and given how easy it is to install, then I want to get it. Ideally, we want to get at least 15 minutes of shower time.

 

[LiQuId]

you're not going to get more hot water out of the tank but a mixing valve would temper the water. Technically the shower valve IS a mixing valve as it lets you regulate ( although manually ) the temperature of the water by mixing hot and cold together to achieve the desired output temperature.

in Short, the only way you're going to achieve longer lasting water is by reducing the temperature of your shower, why not just turn the hwt temperature up ? it would allow for more useablle hot water, but be aware that the water supplyed ( hot only on ) will be the temp you have the hwt set for.

Try going to 150 ' and see if that helps. let everybody in your house know that the temperature of the hot water will be far greater. I suppose this is what your plumber had in mind with the mixing valve.

Increase the tanks internal temperature and then achieve safe use water by means of a mixing valve.

 

[PDubs]

I agree that the shower valve acts as a mixing valve but it just keeps running out too fast. Wife needs to take a shower at no less than 102 due to health reasons, so we can't adjust the temperature any lower. The plumber that came by yesterday replaced both thermostats but it seems to have no impact. Currently the termostats are set at 140 (settings available: 120, 140, 160 F) and increasing the temp yields marginal gains, maybe an additional minute or two at most.

I timed the shower last night and running two Kohler body sprays (2 gpm each) and two fixed shower heads (I assume 2 gpm each as well) we had 102 degrees for 7 minutes before the temperature stops dropping. Obviously if I reduce the number of heads while the temperature drops, it delays the rate at which the water drops. The weird part is that we did not have this issue at first move-in (this was a custom house).

Back in September/October 2011, we could run the shower significantly longer at same settings. Outside temperatures were generally around as low as 50 but as high as 90 or even the occasional 100. I noticed that once it cooled down outside in November, we started having this issue. I don't know if the two are related, but it seems that the cold weather is affecting the hot water for some reason.

 

[havasu]

Are your hot water pipes well insulated?

 

[PDubs]

Not sure how to answer so I'll give you everything I know...

- Water heater is located in the garage. It's not in a closet, just open area on a stand.
- Garage doors are of the insulated variety with wood trim on front
- Pipes leading to and from water heater have the foam wrapped around it
- Entire house is spray foamed insulated on exterior facing walls
- Distance between water heater and master shower area is roughly 50 feet and master is located on upper floor
- House was plumbed using flex lines (red and blue plastic lines) if that's what you call them
- We're in central texas, so it's always warmer here than most places.

 

[LiQuId]

You say "both thermostats", I understand this to be an electric hot water tank... correct?

if So then this is part of the problem, they have very slow recovery rates.

2 shower heads and 2 body sprays.... Lots of water consumed + electric HWT = very short hot water duration.

The reason this worked when you moved in was because the supply water temperature was higher and so the hot water tank did not have to work as hard to maintain the water temperature. We call this the Delta T, and it is a critical Step in deciding the Application of any heating vessel, be it a hot water tank, on demand tank or a boiler.... this is a Critical step... even more critical where heating sizing is concerned.

you can turn the elements up and it will give you as you say longer showers by a little, but the only real solution is to get a faster recovery hot water tank, You would have better success with a gas Hot water tank as the recovery rate is much higher. ( expressed on the rating plate on the hot water tank, look for hourly recovery rate. )

 

[LiQuId]

Knowing what tank you have installed would help also, look on the rating plate and note the
Make
Model
Serial number

 

[PDubs]

It is electric. The house was designed for two water heaters with a pump in between, but since it's only me and my wife, we decided to just get one water heater. I wouldn't think that source temperature would cause this dramatic decline in performance, but maybe it can. Outside temps have reached freezing or below maybe seven times at most between Dec and today. The reason why I asked about the mixing valve was due to the plumber suggesting that it can turn a 50gal water heater into the performance equal to that of an 80gal model.

AO Smith, ECT 52210, 1122J005923

 

[LiQuId]

The delta T Math is as such.


Desired water temperature - Supply water temperature = Delta T ( temerature differance ) this changes as the season does, increasing the Delta T means that a tank that might work with a 40' delta T wont hold the same performance curve when the delta T is 60 '

we then tank this temperature differential and Determine the amount of water consumed at peak demand ( in your case a shower, in commercial kitchen the buisy times with full occupancy, as with in hotels we average the water used at peak demand when all rooms are rented and ALL showers are going ) we take the output btu's of the hot water tank and times the delta T

so if water is coming in at 40' F
then you have a Delta T of 100 'F ( as your tank is set for 140 )

since it takes 1 btu to raise the temperature of 1 Lb of water 1 'F then you have

Capacity of hwt ( 50 Gallon )

then 50 gallons x 10 lb per gallon ( imperial gallons not usg at 8.33 lbs ) = 500 lbs of water in the tank

500 lbs of water times the delta T = number of btus required to raise the temperature of the water to the desired application temperature.

500lbs x 100'F ( delta t ) =50 000 btu's Required to raise the water in the tank to the desired temperature for the application

looking at the product specs your tank claims 21 Gallons per hour recovery with a 90' delta T ( called temperature rise in the chart )

Your tank says 4500 Watts is what goes into heating the water which is where this 21 gallons per hour recovery rate stems from.

So 4500 watts is Equal to 15364.9 Btu's per hour.

SO...... remember how we needed 50 000 btu's to heat the whole tank ?

50 000 btu - 15364 btu per hour = 34 635 < this is how many more theoretical Btus would be required to heat an un heated tank ( called first hour yeild ) if you used 50 gallon per HOUR..... see where i'm going with this?

3 shower heads at 2 gpm = 6gpm consumption ( this is for optimally hot water )

6 x 60 = 360 gph ( if you took an hour long shower with all 3 heads running you would need to recover 360 Gallons per hour RECOVERY (not first hour yeild) , to achieve this.

Installation of a mixing valve would allow you to set the tank higher, providing a higher btu usage of water initally but still would not affect the recovery rate. You would be able to get slightly longer showers ( as you have found ) but the differance would be fairly minimal. this could also be achieved by simply turning the tank up and noting that the supply water at the fixtures would be much hotter.

If you tell me how long a shower you are looking for I could do the math on what capacity and recovery would be needed, If electric tanks are all that you can use given the installation then there is likely a way to increase the capacity, but it would require another tank and a circulator or that you switch to a gas fired hot water tank.

hope i've not lost you on the math, I;ve a broken leg right now and it gives me something to do... normally doing this type of work would cost, but I've plenty of time ...... and everybody thinks us plumbers are just turd herders.... lol

 

[LiQuId]

What is the current temperature outside where you are?, and what is the coldest Temperature you would see ? also. these would help with the design .

 

[KULTULZ]

...and everybody thinks us plumbers are just turd herders.... lol

No Sir!

I have much respect for your craft. It has to take you guys years to gain the knowledge you must have. It's just the sloppy ones that bother me...

 

[LiQuId]

thank you good sir... the sloppy ones bother me too, but Do keep me buisy with repairs.

 

[PDubs]

Wow!! I asked for the math and sure got it! That was truly enlightening and I appreciate the time and effort you took to explain what's happening "behind the scenes." I knew some level of calculations had to go into designing the best system but obviously was unaware of all the intrinsic variables.

Your very first statement stuck out at me: Delta T will change as the season does. I've heard that source water impacts overall hot water but have never understood how much. This is usually conveyed from the manufacturers as basically saying if it's colder out, expect less hot water, which sounds like a standard, blanket statement to me. Or at least that's what AOSmith gave me as a response.

Current temperature forecast is below
- Monday: 70/47 F
- Tuesday: 62/53 F (precip 70%)
- Wednesday: 59/44 F (precip 90%)
- Thursday: 63/43 F (precip (40%)
- Friday: 71/43 F

Electric tanks are it. The house was originally designed to have two water tanks with a recirculating pump as you mentioned, but budget constraints and the fact it's only me and my wife indicated one should be enough. Desired time for shower is minimum 15 minutes, although if my wife could get 20 minutes so she could massage her back believe me, she would do it.

Again, I can't stress enough how much I appreciate your time with troubleshooting our issue or perceived issue.

 

[LiQuId]

15 mins x 6gpm = 90 gallons needed for a shower, are you sure its 2 gpm from each head? this is a large amount of water.

the obviouys and cheapest way to extend the hot water is to use less, either by using different heads with less flow or by use of flow restrictors.

21 gph recovery on your current tank
21 / 60 = 0.35 gallons per minute recovery.

so, in essence you are using 6 gallons per minute and recovering less than 1, which tells me another tank may not be enough alone to recover what you are using and the temperature would still drop off towards the end of your shower.

My recomendation would be to install a storage tank and a circulator, though I have never heard of this being done with an electric tank, though I suppose it is plausablle it would also cause the electric bill to be large and extend stress upon the current heaters components, namely the heating elements.

increasing the temperature would help to extend the shower but it may still be marginal.
I would consider a real world test just cranking the temps up on the hot water tank you have ( considering it sounds like this is not a system you;re looking to invest to much into ) take the top element up to 150 and either keep the lower one set for 140 or just sliightly more. be aware that the supply water has been increased to this now and the risk of scalding at the fixtures increases with it. this would be the reason for installing a thermostatic mixing valve on the hwt outlet.

if you say you are getting 9 mins with one tank then without even doing the math one could reason that you should yelid 18 mins with 2..... though gas tanks will always be a better value for both amount aof water availablle and cost of fuel to heat .

 

[LiQuId]

http://www.pexsupply.com/AO-Smith-G...covery-6-Yr-Warranty-Residential-Water-Heater

here is a link to a similar hwt that is natural gas, look at the recovery rate 67 gallons per hour

67 / 60 = 1.11 gallons per minute.. still not enough to recover as quick as you're using it, but your shower would stay much hotter for much longer without having to oveercome as much temperature loss as in the electric model.

 

[LiQuId]

http://www.pexsupply.com/AO-Smith-GDVT-50-50-Gallon-ProMax-Direct-Vent-Residential-Water-Heater

another link to a direct vented hot water tank...
I have looked into it and all electrics seem to have a 21 gallon per hour recovery rate.

 

[phishfood]

You ARE bored, Liquid!

 

[LiQuId]

broken leg.... yup, can only do so much around the house stuff.