I have a Rinnai tankless. The temperature fluctuates wildly
I suffer occasional loss of heat in the sinks but the basement shower is the worst. Temperature will fluctuate from burning hot to icy cold withing seconds with touching the controls. This will last for months, and then not happen again for a while. First my serviceman thought our pipes might be clogged with sediment, but they are pretty new. Then he thought there wasn’t enough gas coming into the house, so now we have enough for an apartment building.
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Re: I have a Rinnai tankless. The temperature fluctuates...
I think the dip tube in your water heater has failed, this is a plastic tube that goes in the cold side of your tank and forces incoming cold water to the bottom of the tank allowing hot water out at the top. When this tube deteiorates the incoming cold water mixes at the top of the tank causing the problem you have. the dip tube runs about 5 bucks but takes someone with pluming skills to install.
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Have you ever tried to pump water for a fountain... It is so tough and you need larger and larger pumps to get it to go 7 feet high to let's say 10 feet high. Having the on demand water heater in the attic dramatically drops it's pressure. It's best to feed it with a 1 inch or at least 3/4 inch pipe downstairs before it splits the hot and the cold to get the best pressure everywhere then the least pressure will be when it reaches the 2nd floor. but the heater won't cut out because the pressure it even greater downstairs.
The Rinnai will shut off it can't maintain the temperature you have set to within +/- 3 degs so I'd guess that might be the problem. The cause can be a few options. Check water flow first; any filters you have in tapware, main water flow etc. get cleaned. The Rinnai has a water line filter in the incoming supply, middle pipe under the heater just after the isolating valve. Gas supply could be the other option mainly being a possible intermittent fault in the gas regulator. Hard to check this one short of just changing it. Final option is some sort of electronic malfunction, pretty rare. I'd start with the water flow issue as you can do this yourself. Checking the gas will need a good technician with digital gas pressure guage to check incoming and working gas pressures; it might just need to be recalibrated. Just hasd a thought about the heat exchanger being scaled up if you are in an arera where water scale is a problem but this would normally take far less than 8 years to be a problem. Sorry can't be of more help; hard when it might be a few different causes.
Ok i intall tankless water heaters and sell them and repair them . The trouble your are hving is more than likely ,the mixing valve on your shower has a ajustment on it to prevent you from burning your self . that is the number one reason in showers when water never get to temp . What it is doing is addng cold water to your shower for you bummer . Take handle off and adjust water temp to your hottest setting . Some times you have to remove the low flow washer in shower head to get the right flow . also every time you start up your hot water it shouldbe set all the way ot other wise withe the battery 2 D models the unit will not sence flow and shut off the unit ,with some units it is a fine balancing act to get them adjusted right. If your buying a tankless water heater always buy a blower type unit that has a computer that runs it . You will be happy . If the Battery operated unit is for a cottage or a smallcabin then your just going to have to fix the pumbing to make it work right many of the safety features of the newer showers are not made to work well with tankless water heaters .
lower the output temp on the unit. the tap water could be too cold coming in. The bosche units don't like a large temperature difference between tap water and the water at your shower head. Condensation becomes a real problem. I run my water thru my basement slab to heat it up and that made all the difference, but you probably didn't buy your house with a dirt basement did you?
this is usually a problem with all tankless water heaters.
the solution is to install a small buffer tank (5 or 10 gallon electric water heater) after the Rinnai. this will give the water something to mix in. it will solve your problem.
did you make sure to use a 3/4" gas line to the unit. not just 3/4" at the unit itself. sometimes when there is not enough gas, the unit will act funny and cycle from hot to warm or even cold. Make sure you check the gas line size, its very important.
what you have is called a hydronic heating system.
this heating system is completely isolated from your water heater I believe this as most hydronic systems I have installed circulate water through the furnace to heat the house. If this is true you need only to connect your old water heater cold water supply side to the tankless and, old hot water side to the house system: the showers/tubs/sinks etc. in your home.
my appologies if I have missunderstood your inquiry.
no the tub faucet is the only faucet that is wide open you might have to not run full blast on the faucet , your tankless water heater is under sized for the application, to many gallons per min going thru the unit, look at the rating chart for your unit, the r94 is only 9.4 gallons per min, and a tub faucet can be upto 5 gallons per min, depending on the faucet,,