Question about Rheem Tankless Hot Water Heater Rtg-74pvp-2 7.4 Gpm

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Inadequate hot water supply

When taking shower hot water is eraatic and consistant

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Tankless Water heater tempature is regulated by the rate(amount) of water flowing through the device. To get warmer water for the hot side, slow down the elbo stop leading into the device. Since it is a shower,and you did not say one handle or two.You will have to slow the rate for the cold as well , if one, unless it is an ajustable valve. The drawbacks to the tankless heaters.

Posted on Jan 18, 2009

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1 Answer

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Check for crossover.
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http://waterheatertimer.org/Crossover.html

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A fault often found with these units is due to insufficient water flow. Usually this causes specifically the shower to run hot and cold. Try turning on more than one hot supply; if this fixxes the fault you will need to try and increase water flow. Most new tapware/showerware have water flow restictors built in. These are the cause of many problems with continous flow heaters such as the Rinnai. In taps they are usually located in the spout if it unscrews. In showers try the shower hose. Find them and take them out. Also make sure your mains supply to the house is on full.
There are more technical faults but this is something you can easily try first.

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Try flowing more hot water with your shower. During the summer you end up mixing alot of cold water and very little hot water (because your cold ground water isn't so cold in the summer). When you get close the minimum flow rate of the appliance, it will start to short cycle (fire on and off). Here's some recommended things you can do:

1) Clean the shower head - remove scale deposits and or clogging
2) Widden flow restrictor - most shower heads have a flow restrictor on the back of the showerhead; It consists of a rubber washer with a hole in the center. You can widen the hole using a drill bit.
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1 Answer

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your gas supply may be to low to keep the unit firing while you are showering...especially if you have other gas appliances running while you are using the unit.

If this is happening after someone else has recently used the unit....then it may be a problem that some call stacking. The unit works on flow....not sure about that unit but they usually take between 1/2 - 3/4 gallon to turn the burner on. If someone recently used it and then you get in the shower...you will get the hot that is already in the line but then it takes that initial amount to fire again, thus loading the pipe with cold water.....it then comes out on you followed by the hot that has finally entered the line. Once it is past that point there should be no issue, unless it it a gas problem like I mentioned.

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1 Answer

When i turn on the cold water to adjust the temperature of water - the hot water turns completly off. I cannot turn on the cold water at all. It is either all hot water or all cold.


It might be a question of inadequate flow. All tankless water heaters have a minimum flow rate that must be met, typically .5-.9 gpm, before the heater will operate. In the summer time the incoming ground water temperature is higher, causing you to mix more cold water and less hot water to get a lukewarm temperature. This can reduce your flow rate near or below the minimum flow rate.
Here's a test for you, try running running a sink while taking your next shower. If the hot water remains mostly constant throughout, then your problem is with flow at the showerhead. The flow at a shower head can be reduced by buildup of scale and sediment. Also all showerheads have a flow restrictor that limits flow to 2.5gpm (some even less). Removing the restrictor will increase the showerhead flow rate as well.
Also look for any error code displayed on the digital read out of the heater. Some heaters will flash on their burner indicate light as well.

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1 Answer

My thermostatic concealed shower valve only gives either boiling hot water or cold water, there is no inbetween temperature? the cold is fed from mains and the hot from a combi boiler. the plumber has said...


Hi,

If the hot and cold are fitted the wrong way round then the thermostat on the shower unit cannot operate properly!

Most thermostatic mixer showers have a 'bi-metallic' thermostat, if you supply the hot to the cold and vice versa IT WILL NOT WORK.

It is like wearing the left shoe on the right foot and the right on the left! It doesn't work.

Re-install the hot and cold to the correct supply, and this will sort it.

I hope this helps.

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1 Answer

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Nothing wrong with waterheater. Did you perhaps reverse hot/supply lines on new shut off valves?? try reversing lines. Maybe

Please rate. Thanks jj505

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3 Answers

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Someone has not sized the system correctly. The water heater will heat a certain GPM a certain temperature rise. Most of those "generalizations" from the manufacturers are referencing the lowest of all the flow requirements like 2 lavatories. The truth is, don't use other fixtures while showering or get a bigger system. Lots of information related to sizing the tankless system at http://www.mechanicalengineeronline.com/Mini_Sites/Tankless_Water_Heaters_Ebook/Tankless_Water_Heaters.htm Have a good one. PS:Properly sized On-Demand water heaters are actually really good. Be prepared for the day your water heater quits.

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1 Answer

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