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keith mccree
Tankless water heaters

Q After installing unit. Water takes tine to get hot. But when it finnaly reaches temp, And your takeing that shower, There is a suprise waitting. It turns back and forth from cold to hot. I did'nt install unit , But would like to find reason as to why this happends. Signed COLD SHOWERS.
PS Is there a step they overlooked ???

JLMCDANIEL
Re: Tankless water heaters

I don't own one but as I understand it, the instant hot water heaters shut off if the call for hot water stops and this will allow cold water back into the system. I wonder if you have and anti scold or temp control valve on the shower that could be temporarily cutting the hot just long enough to cause the problem.
Jack

hvhehcca
Tankless

Tankless heaters need .5gpm flow rate to activate them. The cold water sandwich is the water that has been sitting in the line all day. In order to trouble shoot this there are some questions to ask.

What's in this shower? Is this just your standard pressure balanced shower valve with a standard 2.5gpm head? Is there anything other hot water being used in the house at the same time your taking a shower?

What is the brand and model of the tankless? What is the setpoint or target temp the heater is making? Where are you located? Asking this because I would like to know the incoming cold water temperature during the winter months.

Flucuation is water temp is a sign of a couple of things. One may be that another fixture is being used at the same time and the heater can't keep up. I would say the other would be the mixing valve in the shower but that would tend to give luke warm water not dead cold water. The valve would have to be seeing some serious hot water for it to close off the hot water side and give just cold water.

I always start from scratch when troubleshooting these units and it starts with making sure the unit is properly sized to delivery what you need it to deliver.

bestguesser
Re: Tankless water heaters
keith mccree wrote:

Q After installing unit. Water takes tine to get hot. But when it finnaly reaches temp, And your takeing that shower, There is a suprise waitting. It turns back and forth from cold to hot. I did'nt install unit , But would like to find reason as to why this happends. Signed COLD SHOWERS.
PS Is there a step they overlooked ???

I have discussed this with a plumber. He says this is the problem with owning a tanless water heater and that he has uninstalled most that he has installed because of this. It is cycling on and off because the water that is runnign through it is quickly reaching it's desired temp and then turning "off" then back on and so forth. Cycling the water as you feel it through the shower head.

hvhehcca
Don't think so

A tankless does not shut off when it reaches setpoint. Its on demand. It will run the set point all day long as long as there is flow activating the flow switch. Sorry but your plumber friend is not provding you correct information.

Cold spikes are a sign of an undersized unit or there is an issue with flow through the flow switch.

johnjh2o
Re: Tankless water heaters

What are we talking about here a gas or electric tankless heater? If it's gas I don't see him over drawing it with just a shower running. It may be that he has a shower head that doesn't have enough of a flow rate to keep the unit firing.

John

HoustonRemodeler
Re: Tankless water heaters

hvhehcca is correct. Many times there is an adjustment to the flow sensor which turns the unit on and off. Call the manufacturer tech support to find out. The adjustments are fairly simple.

Or your unit is too large for your house, but that is a rarity.

hvhehcca
Re: Tankless water heaters
johnjh2o wrote:

What are we talking about here a gas or electric tankless heater? If it's gas I don't see him over drawing it with just a shower running. It may be that he has a shower head that doesn't have enough of a flow rate to keep the unit firing.

John

A shower is a loose term. What's in the shower? Body spray, rain head. You must be careful in the definition before ruling the shower a cause or not. The other issue is, your in the shower, is the wife or another running a load of wash, the dishwasher or kitchen sink? You must start from scratch and look at the size of the unit before moving on to the next step.

johnjh2o
Re: Tankless water heaters
hvhehcca wrote:

A shower is a loose term. What's in the shower? Body spray, rain head. You must be careful in the definition before ruling the shower a cause or not. The other issue is, your in the shower, is the wife or another running a load of wash, the dishwasher or kitchen sink? You must start from scratch and look at the size of the unit before moving on to the next step.

The first step being is it gas or electric?

John

hvhehcca
Not Really

Fuel source has no bearing. A btu is a btu whether produced by a kw or a therm.

gpm = Temp Rise x 500 divided by btu

If electric and the unit is 20kw then it can produce roughly 68,000 btu's. 1 kw = 3,418 btu's. Temp rise is relevant to the climate location and set point. If I use my area the math would be

gpm = (70 x 500) divided by 68,000

This unit can only produce 1.9gpm

johnjh2o
Re: Not Really
hvhehcca wrote:

Fuel source has no bearing. A btu is a btu whether produced by a kw or a therm.

gpm = Temp Rise x 500 divided by btu

If electric and the unit is 20kw then it can produce roughly 68,000 btu's. 1 kw = 3,418 btu's. Temp rise is relevant to the climate location and set point. If I use my area the math would be

gpm = (70 x 500) divided by 68,000

This unit can only produce 1.9gpm

That is my point if it's gas it can produce a higher BTU rating then most residential electric tankless heaters. Giving it a higher GPM rate. How can we tell what flow rate the heater can produce without knowing it's BTU rating?

John

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