Temp differences in city supplied water

Unknown

in this abnormally cold winter , all for not enough hot water . They all were new cast iron boilers with Megastor indirects . One job - a Burnham V84 and a 40 gal indirect on priority , had 80 psi in the house , so I went back to install a PRV for the heater , set at 50 psi . With the heater set at 125 degrees and off on temp I ran the 3 gpm shower for 5 minutes before it went cold . Seemed mighty fast to run out . The only variable I can figure is the temp of the water coming in , and the heater just cant keep up ? It was right in the middle of the sustained arctic temp days around here , but how low can the temp of the water get coming in the house ? Some customers are pretty ticked off , saying they never had a problem with hot water using their old tin can Federal with a rusty coil . The water itself is stored in town owned towers over here .

Dave Palmer

Ron

sounds like the supply is too cold,can you turn up the aquastaton the tank or the boiler for a couple of weeks? We had that and this past summer just the reverse.Lady called that the toilets were filling w/ warm water and the w/h MUST been broken.She would not believe that the 2 weeks of 85-90* weather and the fact the city cut down the trees on half the street could heat the supply into the house.I told her at least the toilets wouldn't sweat anymore,of course I also asked why she was feeling the toilet water but she wouldn't say.

Tom Meyer

Cold water

A couple of thoughts...

Most DHW systems have charts which show the available flow rate is directly related to the delta T between the supply water temperature and the desired water temp (in your case, 125 deg.) Simply said, the colder the city (supply) water the lower the flow rate of heated water.

Also, all a shower is a mixing valve. People adjust hot water (125 deg again) with cold water (frigid city water again) to the desired temperature. Unfortunately, this means using more hot water and less cold water when the city water is super cold.

The double whammy. You have less hot water available and need more hot water to get the desired mix at the fixture.

One way to tackle this problem is to measure the temperature of the incoming city water and adjust the boiler water temperature accordingly.

The reason they think the old tin can worked better might be you lose some flow going through the heat exchanger (or equivalent) and I'm not sure how much standby storage you have. The tin can probably had 40 gals with a quick recovery.

We had the same problem in a design for northern Michigan. Measuring the cold water temp and adjusting the boiler water temp along with some standby storage seemed to solve the problem.

Good luck. Nature loves to play with us Wetheads.


Senior Designer/Trainer
Precision Hydronics
www.precisionhydronics.com

Art Pittaway

Booster for Megastor

If you find the coil doesn't have enough poop to keep up, size up a brazed plate HX to pre-heat the cold water. Split the pumped boiler flow between the Megastor coil and the plate HX, with the added surface area your customer could fill the basement with hot water and still have a hot shower. Takes up almost no extra room and very little piping added. Flat Plate has a nice sizing program so you can determine the temp rise you want.

Dave Yates (PAH)

new product!

Flat Plate also has their "Hot Water Generator" for instantainous domestic hot water generation. Stand alone or couple up to a storage tank. Very slick.

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[Deleted User]

Those dedicated

boilers we install for our indirects normally do the job w/ 180 degree aquastat settings. We had to set most of them to 200 degrees in January. Measured incoming cold water as low as 38 degrees. That cold wind also found each and every uninsulated hot water line. Resulted in some pretty cool water in the recirc. coming back to the heaters. Have been talking to some owners about insulating those lines.

Your new system surely has a higher flow thru than the old tankless heaters. If a higher stat setting does not do the trick, you might consider a flow restrictor.

Unknown

Thanks for all the responses guys

I'm very intrigued by the flat plate heat exchanger Dave Yates brought up . Sounds like a viable alternative on a system in a tight spot . Hows the recovery on them ?

In the past we used Peerless and Phase 3 together , with great results . Now its Burnham and Megastor . Maybe its just that this problem came up because of the long cold winter alone , at least I hope so .

I just found out we had another problem job - removed a 50 gal aquabooster and installed a Bock 51 oil fired . The customer told us they run out of hot water alot faster now . We sent someone back to use the boiler's coil to preheat the tank .

Bob Cat_3

Ratings

Ron this is a problem that I have seen happen to many tankless hot water heaters. They only have troube in the cold months. You have me very intereste in this phenomenon so I went out to burnhams web site and looked at their storage tank(I don't know if this is what you used)and found that they base their ratings on a 90 degree chang in temp(delta T). The interesting thing about that info is their temp high temp is based on 140 degrees and low temp of in comming water at 50 degrees. Now if you only want 125 in tank I would seem that you could get away with a 35 degree input. There are probaly some laws of physics that prevent this relation from being true but what I did notice was they say you would only get 7.2 min of 140 hot water. So with the laws of physics working against you, your 5 minutes of hot water seems right. Especially if it is mixing with a real cold supply temp at shower.

[Deleted User]

Very interesting

specs on the 40 Alliance. Minimum circulator flow is 11.4 GPM @ 6.1 ft of head. The Taco 007, which is what I see most often on residential apps., gives 9 GPM @ 6 ft. That is a 20% drop. I wonder?

The Weil 40 is 8 GPM @ 1 ft. Once again, I wonder?

Unknown

Youre right

We did have a 007 on the indirect . Never thought of bumping it up to the right size circ , what with my atrocious math skills I would get it wrong anyways . Im gonna pass that info on , thanks Ron .

Bob Cat_2

Flow Chart

Hey Ron I was wondering where you got the info on the 007. I'm looking at a flow chart I got off the web and it looks like the flow rate at 6' is 16gpm. I'm wondering if I'm reading the chart right.

Bob Cat