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Hot water in shower then goes to cold

When we first get in the shower the water is hot then after 2 minutes it goes to lukewarm. We had the coil in the furnace replaced, with no change. What should we do next.

Thank You,

Re: Hot water in shower then goes to cold


Call the guy back who installed the new coil & stomp on his foot for installing the wrong fix.

Have you talked to him since the new coil didn't solve the problem, what did he say??

Typically, these coils hold very little hot water; most of them are rated for 3 gallons per minute, but in practice supply even less than this due to the mixing that occurs with the cold water already in the piping.

Inadequate dhw from a domestic coil is one of the most common complaints for those who own a boiler.

The only other factors I can think of is that there is often a TEMPERING VALVE located in the dhw (domestic hot water) piping that controls the water temp in order to prevent scalding of someone taking a shower.

Quite often these tempering valves fail, or are simply out of adjustment, or have a small piece of crud inside, especially if you have hard water.

Another easy adjustment is with what is known as the triple aquastat attached to the boiler.

The TA is usually a small gray box 6" long X 3" high; the cover can be removed by loosening one or 2 small screws.

There is usually 2 dials; one for "low temp", often labeled "lo" and one for "high temp" often labeled "hi"

Often the Lo is set to 140 degrees & the Hi set to 160 degrees.

You can set the Lo to 160 degrees and the Hi to 180 degrees (there should always be 20 degrees difference between the lo and the hi).

This will raise the boiler temp 20 degrees, and subsequently the shower water will be hotter, and last longer.

Caution: a scalding hazard is more likely with any HW over 130 degrees.

If you have a tempering valve, get it adjusted or replaced to avoid scalding in the shower.

Another issue is how much water does the family use, and how big is the family.

If there is only one or 2 people in the house and no clothes washer or dishwasher is used, then a "domestic coil", like you have now may be enough to supply your dhw needs.

On the other hand, if you have a larger family, domestic coils in the boiler are an inadequate, poor way of supplying dhw.

Since you've paid the price to have a new coil in, try working with it for the time being & make sure nothing else like a tempering valve is defeating its purpose.

In the long run, you might consider installing a 40 gallon indirect HWH.

This type of unit burns no fuel on its own, gets its heat from the circulated boiler water, and you'll never run out of hot water.

They DO cost ~$900 installed, & you would have to have enough room beside the boiler to accomodate this type of unit.

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