Heat transfer of cold water pipe system

[bornagainmtnbiker]

I have a client who lives in a very hot climate and has un-insulated cold water pipes running in the attic. As the summer temp's run between 100 - 116 during the day and only as low as the 80's at night her cold water is always hot in the summer. She has asked us to insulate her cold water pipes in conjunction with a current structural project, so she can have cold water in the summer. She does not believe me that insulation will not help this situation. The insulation will only slow down the heating of her copper pipes and ultimately only make the problem worse because it would delay them cooling down in the evening because of the insulation. This is a large house with several cold water runs in excess of 60'. All 1" copper pipes. The water does not get used that often during the day, so most of the time it is stationary in the pipes. I've indicated that a recirculation chiller system is really the solution to the problem. However she has a somewhat educated friend who insists that I show her some heat transfer calcs to prove my point. I'm a hard core ME who works primarily in structural analysis and haven't looked at these types of equations in years. Any help would be great. I don't want to spend any more time in her attic than I have to.

 

[Montemayor]

bornagain:

First, you are wrong in your statement - if what you describe is true. You haven't said that the client wants instantaneous cold water the moment the tap is opened in the house. You said: "so she can have cold water in the summer". Since she just wants the water cold, then insulation is definitely the answer. Another problem you or the client have not mentioned is a bad one (that is probably already happening): cold copper pipes in a hot, humid environment will condense the moisture in the air and cause liquid water dripping down on her ceiling dry wall and ruin it - plus all the wood rot that it can cause.

If you truly want cold water, you need to insulate the cold lines. 1" copper lines are no sweat (pardon the pun) to insulate. Depending on the hotest temperature around the pipes, 1" to 1-1/4" thick pipe insulation should do the job. I can understand your reluctance to work in an attic; but that doesn't contribute anything to resolving the heat transfer problem. Your client should have had the insulation done during the winter or fall months. Since she obviously hasn't, the price of insulation goes up during the hot summer months. Recirculating cold water will give her the product she wants, but at a stiff price: higher electrical bill, waste of good refrigerated water, and worse: condensation in the attic, leading to wood rot and water damage. I would not apply a recirculation system in this application without at least warning the owner of what she'll have as trade-offs. To do otherwise is unprofessional and shoddy engineering. That may be OK for a plumber; it doesn't make for a professional engineer.

 

[pennpoint]

Montemayor
You are right on. My first inclination to reply was "Aw heck give her what she wants and get out. But I knew that was too shallow.
Iam glad I waited a bit. I really missed the condensation aspect, "bornagainmtnbiker" has a real opportunity to do a good job here and make some money with potential new customers in the area. She'll talk to her friends.

Best Regards

pennpoint

 

[bornagainmtnbiker]

I've looked at the pipes and condensation is not a problem as the humidity here is usually < 12. So in this situation where the water lines are in the attic with no movement of water during the day that the pipes will not heat up to the surrounding temp (around 150 deg) if they have insulation around them. Thus in the evening when she comes home and runs the water for a few minutes the water will run cold (as possible)?

 

[moltenmetal]

The insulation will reduce the amount of time she needs to run cold water through the pipes in the evening to get cold water. It will lower the peak temperature the water reaches during the day as you've stated, but how much it does so will depend on a lot of factors- it may reduce the peak temperature by ten degrees or one degree but probably not by sixty, unless there's a leaky toilet or faucet somewhere or you put a very thick insulating layer on there with a good radiant barrier. Insulation will also permit her to obtain a lower cold water temperature while the water is running, even at night, since the attic temperature in the evening is higher than the cold water temperature entering the house.

Split sleeve foam insulation is cheap and easy to install.

 

[PZas]

I actually ran these calcs somewhere around a year ago and IIRC a 1" copper pipe with a delta T around 60F and a 30' run results in a fluid temp increase of less than 1 degree in free standing air. So this should not be a problem for her without insulation if she is willing to run the water in the evenings.

The first concern I would have is the temperature of the water entering the house in the evenings. Here in texas there are months when you CAN NOT get cold water no matter what you do because the water exiting the ground is 80+. I would first test the first tap in the system to find out what the minimum temperature of the cold water is.

If there is cold water you need to speak with the end user and find out exactly what they want. If they want cold water on demand insulation will not satisfy them. If they will be unhappy make sure they know first...this is the only ethical thing to do.

In the event that the water coming out of the ground is not cool enough for the customer you may want to look into an evaporative water cooler (similar to a swamp cooler). This is only possible because of the extremely low humidity.

If you would like I can dig up the basic calcs on the heat transfer to the water in the pipe...however during flow it really is a minor concern. Your biggest problems are water volume in the pipes and the pipe wall thickness (only true because of the very low convective heat transfer coeff).

For any of us to approach this without knowing if the customer would like the cold water on demand or "eventually" along with the incomming water temp is somewhat academic.

 

[StoneCold]

bornagainmtbiker
Wouldn't it be cheaper to buy a refridgerator with a cold water tap on the front?

StoneCold

 

[bornagainmtnbiker]

My client really does not want to spend the money on a chiller to have cold water on demand. She just thinks that insulation will help with the cold water situation. And I get the idea from talking with her that someone has told her that insulation will make her water cold. I’ve spent a few days at the house now and this is what I’ve found.

The water entering the house is from a well buried municipal source and enters the house right from the ground. I didn’t take an actual temp measurement but the water is comfortably cool, not cold, at 9:30am when the outside temp is 97 and the overnight low was 89. It enters the house and runs through the attic approx 40’ under normal attic insulation with a radiant barrier on top. It then takes a vertical run to the upstairs attic and does another run of about 100’ before descending back down to the first level and then does another 50’ run to the last facet which is used by her son. Yesterday evening I sat at that facet for more than 10 minutes and never got close to the temperature of the incoming water. In fact when I turned the water on (I’m guessing) was 110+ as it burnt my hand and I could not hold my hand under the facet for 10 to 20 seconds. I’m guessing that the attic temp is around 140 and that during the day the copper pipes heat up to the surrounding temp and then transfers that heat to the cold standing water. In the evening the pipes are still hot and any cold energy from the new running water is used up trying to “cool” the pipes and thus the water does not get cool in any reasonable amount of time. I’m guessing that insulation will help this situation I’m just not sure how much. If the pipes are not allowed to heat up by being exposed to the heat, I’m guessing that it would remedy the evening situation of not getting any cool water from the tap. How cool is the question. I don’t want this client to have any unrealistic expectations of just how much the insulation will help. I’m being as up front with her as possible as a professional.

 

[PZas]

I see a couple of problems with your assumptions at this point that may cause us difficulty. I will try and talk through them so I can be corrected where needed.

Municipal water systems are notorious for having poor water temperature regulation. Even though the lines may be well burried the temperatures in the afternoon/evening are often not based on the temperature of the ground. During the day when use is up the water in the main lines is constantly flowing often from a water tower which is exposed to the ambient air and worse off the sun. This often results in hotter water entering the pipes. Because of this you need to check the incomming water at the same time you are checking the water at the far faucet. Also keep in mind that the line to the house can hold enough water for a few minutes and will be cool as the water has been static in this line all day. To check this water I would open both the close and far faucts at the same time in the evening and log the temperatures of the water at both points. You may find that the incomming water temp starts low and increases because you start pulling from the main line. This goes against normal thoughts but may be the case in this particular location.

Once the incomming water temps are established and are acceptable the problem becomes much simplier. The first approach I would take is to assume that the time is so long during the day that no ammount of insulation will keep the water in the pipes cool. Run a simple capacitance system to find out how much water would have to run though the pipes before they reach a reasonable temperature. Once you have an approximate ammount of water that should cool the pipes to the point where the exiting water is reasonable find the flow rate of the faucet and estimate a time. This will help you estimate what the max water temp in the lines during the day can be to achieve the desired results in the evening.

I worry that if the pipes are already under the house insulation adding additional insulation to the pipes will not help significantly. It may be much easier to cool the entire attic than to keep these static pipes cool.

Alternately because this sounds like an area where the temps will not freeze often you may be able to replace some very large runs with a material other than copper. PVC or similar would make very good sense here and would help more than insulation as I suspect that the pipes will get hot during the day no matter how much insulation you have. You might try taking a lunch cooler up there in the morning full of ice and see how much melts by the evening. This is a completely insulated box filled with ice and likely in the evening you will have hot water (depending on the cooler size and such).

It sounds like you want to run some calcs. To do these we will need the icomming water temps, (after it has run for say 10 minutes) the pipe od and wall thickness, flow rate, and exit steady state temps (for seelf checking).


One last thing is that water is very funny to the touch. A difference as small as 10 degrees can be the difference from feeling hot to feeling cool.

Do you have a reference for the formulas? If not I can post them up for you tonight.

 

[bornagainmtnbiker]

If you look at my original post you can review my original assumption or hunch as an engineer very familiar with the local conditions and somewhat of an understanding of heat transfer. I studied it in school like everyone else but specialized in structural analysis and have spent most of my career in structural FEA or Code development, so I’m pretty rusty in the heat transfer area. I could probably model this situation if I had the right heat transfer FEA modules for my code, but I don’t.
I personally feel the following is the case.
Because it never really cools off here at night and the days are extremely hot, even if you where to insulate up to the critical insulation thickness the pipes would heat up and transfer that heat energy to the water. It’s 85 to 90 degree’s at 5:00am and regularly gets up to 112 – 116 during the day. By 10:00pm the temp might be back down in the 90’s. So that’s 16 or 17 hours of time dedicated to heating pipes. That’s a huge amount of time to control a temperature delta that is not that big in comparison, with just insulation. Most in this area put their plumbing under the concrete slab for this reason. I don’t know why this was done different. I also think that if you insulate the pipes that the problem will be further compounded by the fact that once the heat cycle reverses in the evening, all that heat energy that has been accumulated during the day will now be insulated thus delaying or hampering “giving up” that energy allowing the water to cool down. As new cold water enters the insulated hot pipes it will give up its temp trying to cool the hot pipes that are now insulated. Others on this post disagree with this assumption.
I want to do what is best for this situation, and for my client, not just make a buck or avoid working in her attic. I want to present her with the real facts. Ultimately I feel that I’ll insulate just to meet her demand. I just want her to have a realist idea of how much it will really help if at all.

Note: The incoming municipal water is from a well. There are no water towers here. Only wells or buried tanks on the hillside, nothing exposed, due to the heat. The water is cold in the winter months but only cool in the summer months which run from early May through early October. The joke here is that we have 9 months of summer and 3 months of really hot weather. We bury water lines here to keep them cool, not from freezing.

 

[PZas]

If I might ask where is this location in the world. Sounds like a place that isn't real nice to bike in.

Looking at the boundary conditions through all this it appears that insulation will help in this case. If you have cool water entering and hot water leaving the faucet at steady state (long run time) then somehow heat is transferring to the pipes. There might be some very large currents over the pipes due to the large thermal gradients.

Realistically if you have a good insulation on the pipes and she is willing to let it run for a long time then she should get water that is within a few degrees of the incomming line. For giving her information to keep things realistic I would just calculate out a run time to get the pipes cool.

I don't think the insulation holding heat in will be a problem as she is probably concerned with the temperature from 4-7pm which is still a very hot part of the day.

 

[dicksewerrat]

Why are the water lines in the attic? If you bury lines to keep them cool, why not bury the supply lines to the different parts of the house.

 

[49078]

How big is the attic? Could you insulate that better to keep the temps lower?

good luck