Thread: Sudden Flow Drop To Faucet With Bosch Water Heater

I recently inspected a home with a Bosch AQ125BNG tankless water heater. I turned the shower faucet all the way on to hot and after a few seconds the pressure dropped off significantly....about to the point where it would be hard to get the shampoo out of your hair.

The owner of the home explained that this was a characteristic of this particular water heater. He explained that as the demand for hot water increases, the flow drops...by design as it attempts to regulate how much hot water it can deliver at a given pressure.

This make sense, but I only had a single fixture running, and I have never observed this type of drastic drop off in flow while inspecting homes with a tankless water heater.

I'd be more inclined to think that this might have something to do with debris obstructed the flow, or something to do with the valve for this shower having an internal pressure balancing technology that has a compatibility issue with how this water heater is delivering hot water to this valve.

Anyone else ever observe such a drop off in flow, and know what the cause was? Does the home owner's explanation make sense?

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Hey David,

Sounds like the WH is not getting enough pressure and needs adjustment.
* take from another site
**
proper fix is to call the plumber and have the pressure tested if you can't do this yourself - you need gauges to read it - the book that came with the unit tells how much it needs for proper working conditions - then a riser and/or a collection tank needs to be installed if the pressure can't be regulated properly from the water source.

Thanks Billy -

A rarely see tankless water heaters installed properly, and the Bosch units are sold at Lowe's, and therefore often homeowner installed. Water & gas pressure checks are probably rarely completed. The homeowner in this case must be an engineer, because he went into fairly good detail about how this is "how they all work" and threw the theoretic principals of just why this is so as the final say on the matter.

I feel the same was about theoretical principals as I do about statistics.

"There are three kinds of lies; Lies, Damn Lies, and Statistics" - Benjamin Disraeli

He probably should have stopped right there, because then he went on about how great these are because they are "maintenance free". (hard to respond respectfully when you hear something like this) I gave him the link to Terry Love's Tankless Water Heater Forum as a rebuttal to the this maintenance free claim - there most be a thousand posts there about trouble with tankless units, and most of the issues are either deferred maintenance, improper installation, or issues with valves or fixtures down stream. This was a 9-year old unit that apparently has never had its recommended annual maintenance preformed.

Bottom line, I agreed with him on his basic theory, but respectfully disagreed with him on his "they all do that" stand. I stood behind my original report that this was not functioning as designed and that it required troubleshooting by someone that was familiar with this particular unit. I provided a PDF of the installation manual, along with highlighted excerpts out of it on troubleshooting and maintenance, plus a link to the Terry Love forum referenced above.

Oh...and this is good. He offered to hire a plumber to take a look at this, as long as I agreed to reimburse him for the plumber's fee, if the plumber echoed is opinion on this. Oh yea....sign me up for this....I inspect homes in this area, I know the skill set of some of the plumbers in this area....that's not going to happen.

I'd love to go back and troubleshoot this myself, as I'm betting that this could be resolved quite easily, but that's not my job, and who has time for that anyways?

Thanks again for reviewing my post....this forum is a great resource.

Originally Posted by David Tontarski

Oh...and this is good. He offered to hire a plumber to take a look at this, as long as I agreed to reimburse him for the plumber's fee, if the plumber echoed is opinion on this.

David,

That is where you reply with ... 'Let's do this one better, let's get the MANUFACTURER'S REPRESENTATIVE here to tell what what the MANUFACTURER says, you pay me for my trip when the manufacturer's representative agrees with me. I'll cover my own cost if the manufacturer's representative does not agree with me.'

They don't all behave that way, but this particular model does. It's a "flow modulated" version and the output is based on the selected temperature rise.

This time of year a 90F rise is about right, which means a 1.8gpm output at most.

Installation and Operation Manual

Thanks for your input Corn Walker. Well it still makes sense to me that this unit receive service, as it doesn't sound like it has received any since it was installed in 2004, and who knows, service might improve the flow a little.

I explained a bit more to my client about how the water flow drop off will be more pronounced in the winter, and suggested that they might want to install a high flow showerhead to get the most enjoyable shower possible, given the reduced flow that occurs (by design) when a higher temperature rise is required.

I don't think that either the current homeowner or I had previously explained that clearly and in practical terms.

The manual you attached is the same one that I sent them previously. Thanks for posting this. This could benefit the needs of other forum participants....good idea.

And Jerry - thanks for you input as well...I've learned a lot from you over the years.

Originally Posted by David Tontarski

I explained a bit more to my client about how the water flow drop off will be more pronounced in the winter, and suggested that they might want to install a high flow showerhead to get the most enjoyable shower possible, given the reduced flow that occurs (by design) when a higher temperature rise is required.

David, If this water heater is limited to 1.6 GPM then it is too small for the house. Water heater are sized based on fixtures served. You should advise your client as such and recommend a licensed plumber to review the system for recommendation on improvements and or repair.

I don't recommend that any home inspector get involved in any negotiations with the seller. That is a recipe for disaster (and potential liability). It is not an inspector's responsibility or job to negotiate anything between buyer or seller and it is not a professional way to conduct business. In addition, you can do your client a major injustice by providing any verbal communication regarding the inspection or condition of the home with the seller or trying to broker solutions between the two parties. It can only end up in "he said-she said" problems later on.

Inspectors should observe and report directly to their clients - only.

Also, as a side note - I would be careful in providing advise to replace a low flow shower head with a high flow device. It may be illegal to use in your state.

I second Ken's post. One reason we decided not to go with the tankless water heater was that in the winter the temp rise is pretty high here, and we were going to have to get a much larger water heater than we initially thought, hence more money. We just didn't use enough hot water to make it worthwhile. If my daughter still lived with us, that would have changed the equation a little, because she took looonnng showers.

If the seller really thinks that that is okay, his tolerance is much higher than mine would be. It's likely not the proper size for the situation.

I appreciate the concern for my liability.

I was pulled into dialogue with the seller by the buyer and their agent over the flow drop to an individual fixture. All communication has been by Email, with everyone involved in the transaction copied in.

Previously to being called into this dialogue I had called out for further evaluation of the installation, maintenance, and the correction of several serious issues, including a reduced PRV discharge line, a long run of 1/2" black pipe supplying natural gas to this unit, and B-vent in direct contact with fiberglass batt insulation. I included a link to the installation manual in my report to back up my opinions on improper installation issues, and maintenance requirements.

The fact that this water heater would not likely satisfactorily meet the needs of the new homeowner was discussed at length with the buyer at the time of the inspection, and I would have to assume that any professional plumber called in to review my report/recommendations would second that opinion...after all...they would likely see this as an opportunity to sell them a more suitable water heater.

Again - thanks for your concern. In nearly 10-years of home inspections, this is one of the few times I have communicated directly with a seller...and as reported above, at the request of my client. I'm just not seeing how I am not professionally conducting my business. Actually....I find this quite insulting.

David,

I own a Bosch Tankless and never have any issues like this. Had it for 6 years and only issue is sometimes getting an error code and having to press a button to reset it.

Originally Posted by David Tontarski

Thanks Billy -
...original report that this was not functioning as designed and that it required troubleshooting by someone that was familiar with this particular unit.,,,,
.

Was your report statement " not functioning as designed " from your report program?

I think you went wrong with saying "as designed", unless you are familiar with that brand and model and all of its specifications. You observed that there was an event when you operated the system. Offering an opinion that the buyer should take note of this event and determine if it is acceptable to them. Otherwise they should consider the need to make a correction that would better meet their expectations. Why bother to get into a argument or a debate about the design. If the seller says this is how it works, then he is acknowledging your observation. If the buyer wants to live with the design it is their decision. You nor I would want to live with the design, but we are not buying the problem either. Someone saying that the design is universal (they all work that way) would only beg the retort of " are you an expert with all manufactures and all of their models offered" ?

Some people love thankless no mater what argument you offer or evidence you present or how they function. It is sometimes like a religion or political position. You have to first believe (absolute) then you will understand.

I had this happen in the past I found a bad washer in the faucet, Check others if O.K. try looking into this..

Garry -

Good point regarding observing an event. Thanks!

There can be a few issues with this installation.

Undersized heater will drop the pressure when the demand in GPM exceeds the heating capacity this way the heater will always put out the set temperature. Running one fixture, even if the unit can only give 1.6 GPM you should not see a noticeable drop since faucets are rated at 2 GPM or less, and showers are supposed to be 2.5 GPM or less. So if the heater is only putting out 1.6 GPM, the shower should not be that much of a drop till another fixture is turned on. The solution is to install a properly sized tankless heater that can handle the total amount of fixtures. In larger homes this means more than one tankless unit with a system controller. To many buy a tankless heater on how many live in the home currently, which you all know is wrong way to go about it due to the next family in the home might want to run two showers at once.

Let’s assume it is the right size for the house. There can be two more issues. First are improper installation, undersized gas line, and or a undersized gas meter. If the unit does not get enough gas it will cut the flow down to the rate needed to give desired temperature. so if the unit supposed to run at 199K BTU and is only getting 50K, the GPM will drop to less than 1 GPM. Meter sizing is the most overlooked thing. In the summer months when most installs are done, no one notices anything. But once that furnace kicks there is nothing but trouble. The other issues is proper maintenance, these heaters must be descaled yearly. If not scale build up will restrict the flow in the heater and as you can guess low flow rates.


Best to right up the issue and recommend a professional installer to look it over. Not all plumbers are tankless installers.