Helpful? 0

Hot water runs out when I take a shower, and registers blow cold air

Hydronic air handler connected to a tank hot water heater.

I have a hydronic forced air heating system (just recently installed), is it normal for my hot water to not last now when I take a shower, and is it normal for the air coming from my registers to be blowing cold air when I am taking a shower?

Thank you!

Asked by Dana Whitlock
Posted Sat, 01/11/2014 - 23:20
Edited Sun, 01/12/2014 - 08:16


11 Answers

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You have a hydro-air combo system using a water heater to supply both space heating and domestic hot water. This type of system only works if the water heater is properly sized, and has a fast enough recovery time to handle both loads.

Did your installer do a heating load calculation? Is the water heater properly sized to handle both loads?

What type and size of water heater do you have? How big is your house? Where are you located?

Answered by Martin Holladay, GBA Advisor
Posted Sun, 01/12/2014 - 15:33

Helpful? 0

Hey Martin! Thank you so much for your response!. I live in Oakland CA. My house is roughly 1200 sq feet. I have an Aqua therm 24VHBQB hydronic air handler currently hooked up to my old gas hot water tank which is 40 gallons. We are planning to replace the hot water heater in a couple weeks to a HTP Phoenix Light Duty 50 gallon (96% efficient). I'm wondering if I would be better off going to a tankless model so I don't have to worry about the hot water issue. Although I am currently a one person household, I have frequent guests and would like to know that the house can easily handle 2 people (ie - 2 showers) in the winter time. The company I am working with prefers the high efficiency tank water heater over the tankless if there is room in the house... but they have only installed one other Phoenix previously. (they originally recommended a AO smith vertex - but I don't have the head space in the crawl space for that one).
Am I correct in thinking that no matter what hot water unit I end up with - if the shower is on - the hot water will be diverted to the shower and so I will have cold air blowing through the registers? Also the part that I do not quite understand is that within 5 (no more than 10) minutes of the shower, the air coming from the register is hot again - so I don't feel like I'm truly running out of hot water..., but perhaps I'm missing something....

Thanks for any thoughts you might have!

Answered by Dana Whitlock
Posted Sun, 01/12/2014 - 16:03

Helpful? 0

If you're in Oakland, see if you can find Alan Forbes Plumbing. He's an expert in plumbing/heating applications like yours and can help you get the right equipment, controls, etc.

Answered by David Meiland
Posted Sun, 01/12/2014 - 16:10

Helpful? 0

It sounds like you know the answer to your question, so I'm not sure why you posted it. You wrote that you have an "air handler currently hooked up to my old gas hot water tank which is 40 gallons." You also wrote that you plan to replace it with a new 76,000 Btuh water heater -- presumably to solve the problem you are complaining about ("hot water runs out when I take a shower, and registers blow cold air").

Whether or not you are choosing the right water heater depends on your design heat loss and your hot water usage pattern.

You wrote that "the company I am working with prefers the high efficiency tank water heater over the tankless." On that point, you are certainly getting good advice. For a combo system, you really need a tank.

You are planning to install an HTD Phoenix Light-Duty water heater, but you should be aware that HTD also makes equipment specifically designed for combo systems. These are the HTD Versa-Hydro and Versa-Flame models. These might be overkill, however; it depends on your needs.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 01/13/2014 - 07:20
Edited Mon, 01/13/2014 - 07:22.

Helpful? 0

Hi Martin,
thank you so much! I posted the question, because I have found very little information on the hydronic forced air system and before 2 mos ago, I had never even heard of it, so I am trying to learn as I go and do the right thing for my house and my comfort while being environmentally conscious. We installed the hydronic air handler first as my old gas furnace had died and I needed heat and the company was able to squeeze that part in and are coming back to finish the rest at the end of the month. I became concerned when I discovered the issues that I posted, and wasn't sure a tank system with 10 more gallons of water would solve the problem - or was the right approach... It sounds like it has the potential to do so, but thanks for the info on the other HTD products. I really appreciate your expertise!!
Thank you for your time!

Answered by Dana Whitlock
Posted Mon, 01/13/2014 - 11:50

Helpful? 1

For more information on combo systems, see Choosing HVAC Equipment for an Energy-Efficient Home.

That article notes:

"Combo systems are tricky to design. Moreover, it’s hard to find the right combination of contractors to install them, and the systems are often quirky to troubleshoot and maintain.

"If you want to install a combo system, [John] Straube advises installing a hydro-air system (that is, a system that distributes warm air through ductwork) rather than a system with hydronic distribution system (for example, one that uses fin-tube baseboards). The reason is simple: most new homes include air conditioning, so you’ll probably need ducts anyway.

"Fortunately, small air handlers with copper hydronic coils are readily available. Options include the Rinnai 37AHB series hydronic furnace or an air handler from Ecologix or First Company. Straube advises that low-temperature systems — for example, systems designed for 120 degree water rather than 160 degree water — are often best. “I’ve used an Ecologix air handler with 90 degree water,” Straube said.

"Combo systems based on tankless water heaters are tricky, Straube notes, unless the designer includes a buffer tank of some kind. “Instantaneous water heaters have lag times and minimum firing outputs,” said Straube. “The initial slug of water is cold. So systems with tankless water heaters should have tanks. A tank provides a buffer capacity.”

"Straube often recommends tank-style condensing water heaters, which represent “a very safe solution that is pretty affordable ($2,000) and pretty efficient (over 90%).” These systems work best if the flow rate to the space heating coil is under 2 gallons per minute.

"When I spoke to Straube on the phone, he elaborated. “With a tankless heater, I have to worry about coincident draws of domestic hot water and space heat. If you include a 40-gallon tank, there is so much storage capacity that you can’t really screw it up, even if you have a completely out-of-sync heating demand. The tank just smooths it all out.”

"However, even systems that use a storage water heater can be somewhat tricky to design. “Use a small pump so you don’t remove all the heat output of your water heater,” Straube advised. “Otherwise you overwhelm the heating capacity of the water heater.”

"Straube is aware that the history of combo systems is littered with design errors. “You need to choose a low-watt blower and a low-watt pump,” said Straube. “This requires design. That means it’s harder to get these systems into the mass market.”"

Answered by Martin Holladay, GBA Advisor
Posted Mon, 01/13/2014 - 12:00

Helpful? 0

For more technical tips on designing combo systems, see Stuff I Learned at Joe Lstiburek’s House.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 01/13/2014 - 12:09

Helpful? 0

Cool beans! Thanks!! I have to rely on the company I'm working with to be the ones to know... they highly recommended the hydronic air handler over gas furnace due to the size of my house - so hopefully the air handler they put in is the right one and it sounds like there is good potential if they tweak the system right, that with the new condensing hot water heater should solve the issues that I'm having (otherwise I've spent a lot of money for a short shower! :-)).
The above info is super helpful!! Thank you so much again for your time!

Answered by Dana Whitlock
Posted Mon, 01/13/2014 - 12:35

Helpful? 0

Your old gas heater probably has about a 30,000 BTU/hr output (at best) burner, and running it with the air handler would shorten it's life significantly with condensation inside the flue.

The HTP Phoenix Light Duty unit modulates between 23-76,000BTU/hr, and has more than 2x, probably close to 3x the output, and designed to take advantage of the condensing return water temps. A 76K burner is sufficient to run a full-flow 2.5gpm shower forever in a bay-area climate.

Most air-handlers used in hydroair systems (look it up in the manual on the one you have) control the blower with an aquastat on the plumbing, and can be set to turn off the blower when the incoming water temps drop to 110F. That allows 100% of the burner output to be delivered to the hot-water load, making it "share nicely" with the person in the shower. In your current situation if you set the air handler to only be enabled when the water is 120F or higher, and set the thermostat on the tank to 140F, you'll be able to take reasonably long showers, and still heat the house with warm air coming out of the registers.

I'm currently heating my house with a tankless buffered by a "reverse-indirect", with one large zone heated with an hydro-air (First Company air handler with both a hydronic coil for heating and a 5 ton air conditioning coil, a legacy part of the prior setup). I keep the buffer at 130F (max), and have the aquastat set to cut out the air handler at a water temp of 110F, and never run out of hot water. The tankless never exceeds 60,000BTU/hr output even during extended showers, in part due to a drainwater heat exchanger pre-heating the incoming water.

I've yet to observe the 110F cut-out point on the air handler trip, but when set it to 120F it will, during extended winter showers. My heat load is likely higher than yours, and incoming water temps much lower than you would get in Oakland- you should be able to get the performance out of the HTP unit even without the drainwater heat exchanger once you have the 76K burner rather than the wimpy burner on your existing heater.

Answered by Dana Dorsett
Posted Mon, 01/13/2014 - 18:15

Helpful? 0

Hey Dana,
Thank you so much for that explanation! It is a significant help!! and makes me feel like we can get this to work just fine once the HW tank is switched out. This whole field is pretty fascinating and I'm glad I've switched technologies just so I can learn about this type of system...

I'm so grateful for everyones perspective and input!!

Answered by Dana Whitlock
Posted Tue, 01/14/2014 - 13:47

Helpful? 0

Dorsett. Your last post is spot on. I was about to post about having the potable water set up as priority which is standard procedure in the HVAC trades.

Answered by aj builder, Upstate NY Zone 6a
Posted Tue, 01/14/2014 - 16:34

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