Why are combi boiler/ hot water units installed in homes larger than 1500 sq feet?

jeanc

I had a combi unit installed as part of an oil to gas conversion. 4 out of the 6 companies who gave me quotes, steered me to the combi. With various rebates, the price was the same as a standard boiler. I was shocked to see that I lost my hot water capacity. No one explained this to me. They just touted this as the latest and greatest. Sure, there's endless water but only 1 shower runs at a time!! My family is going nuts. I called the boiler manufacturer and they confirmed these units do 3 gallons per min. Why in the world would any reputable company install this in any home larger than 1500 sq feet?? The only advantage I see is space. However, maybe at best a foot of space is saved. I can see this being used in a condo but not in a house.

Am I missing something? I had a well established company recommend this to me.

I can't stand this! Can I keep the boiler but get an old fashioned water tank? There should be some code against using these units for anything but a small home. I can't understand why any company would recommend this.

bmwpowere36m3

The installer "should" have asked as to your hot water needs/habits… For some 3 gpm would be enough. Maybe only 1 person takes a shower at a time. Maybe they have low flow head installed(1.5 gpm). Maybe they don't fill tubs… etc.

With that said, why do they make them? I assume their's a good cost savings to not having a hot water tank, circulator, associated piping and controls.

Obviously it's not for everyone.

jeanc

Thanks for the comment. So when getting a hot water tank installed, it would have its own circulator whereas with the combi the same circulator is used for heat and hot water?

Tom_133

Depending on the manufacturer they sometimes offer a domestic tank that can be installed right along with the boiler for more hot water capacity. I have also seen some people put in an electric water heater that is fed from the domestic hot outlet on the combi so that you would have 40 gallons of hot water ready and typically the electric elements are used only for topping off tank if at all.

kcopp

Many towns here in NH will not allow you to install a combi boiler in a home that has more than 1 bathroom. Combi's have their place but hot water output is not their strong point. Two things I have never heard anyone complain about.... "too much closet space and too much hot water."
If you want to boost your hot water production you could add a storage tank w a circulator. Often referred to as a booster tank.

http://www.htproducts.com/glasslinedtankdocuments.html

I think that as w/ a lot of things Combi boilers are misunderstood in their use. Many think that they are a one size fits all tool....not so.

jeanc

Fascinating. The plumbers just asked me who lived in the house . None of them explained the limitations. Heck it's not like I asked for this in particular. I could have cared less about space. This is in a basement.

Is adding a storage tank the way to go? Or is it best to just go back to a regular boiler and old fashioned tank? I 'm also reading that these combi units are expensive to service and repair.

Eric_32

What brand and model number are we discussing here?

icesailor

@jeanc:

"" I had a combi unit installed as part of an oil to gas conversion. 4 out of the 6 companies who gave me quotes, steered me to the combi. With various rebates, the price was the same as a standard boiler. I was shocked to see that I lost my hot water capacity. No one explained this to me. ""

It would be interesting to me to know what the 2 out of the 6 said. if anything.

The dirty little un discussed secret of Combi boilers. Lack of adequate hot water.

Some Americans are critical of people in other countries as being unclean. Because they don't bathe as often as most Americans.

Years ago, Marshall Dodge (Burt & I) told a story about a local (State of Maine) town character that never bathed. No one could remember the last time he had cleaned up. Everyone offered him a wide berth. One day, he was brought before the judge for being a town health hazard and a public nuisance. The judge asked him how often he bathed. "'Bout once a yea'a". he replied. How 'bout you your honor? Well, every day. Sometimes twice a day in the Summa' if it's hot and filled with Humdiddy. To which the defendant replied, "Yer Honor, how can you call me dirty when you take 365 bath's to my one?

To which the judge sentenced him to a bath.

With a Combi. someone who calls "Dibs" on the first shower gets to take the first shower.

hot_rod

It's really more about the DHW needs and wants of the family than then home size. Tankless is about all you get in Europe, even in the mansions and nice lodges:)

But not everyone wants to adjust their lifestyle to the limitations of the 3 gpm. They do make much larger capacity models, or adding a tank turns a tankless back into the tank style.

I'm not sure many homeowners understand the 3 gpm limit. Gpm and delta T doesn't mean a lot to homeowners. Unless the installer shows them or offer an analogy.
You could get Icesailor to come up with an unique analogy

icesailor

Straight gas instantaneous water heaters modulate to over 100,000 BTU's per hour. People are trying to install boilers far below that. Combi's split the load. It takes NUTS to heat water. There's no nuts in a improperly installed/selected Combi unit.

You have to throttle or flow restrict the cold through the coil and the pressure drops. Then, when it goes through a required pressure balance shower valve, the balancing spool compensates to the hot,, and slows the cold into the valve. Then, if the water isn't hot enough, the cold overcomes the hot, and you get complaints.

There's probably a way to install a smaller water heater tank with a circulator, pumping through the potable water side and circulated through the Combi. The initiated flow from the pump will trigger the boiler to run and store the water.

If something makes potable hot water, you can store it. There's always a way.

SWEI

I'm still curious what combi this is and what shower/bath configuration led to the unhappiness. 3 GPM at a 77°F (nameplate) rise is somewhere around 115k BTU's.

icesailor

Then, there's always the issue with single lever pressure balance shower valves with sticking balancing spools and not seating in the proper place to allow enough hot water through the valve. If the water is hotter and flows well in any other sink, but the shower won't supply enough hot water, the spool can be sticking.

Depending on how cold the cold water inlet is, it doesn't take that much hot water to make a shower safe and hot.

And don't put a positive shut off fitting on the shower head. It will wreck the balancer

What kind of shower valve is it? If you don't know, post a photo of it.

bob eck

Is there a cross connection in the house cooling the hot water down?

Rich_49

The answers to your question are as follows .
1 . Builders are cheap and want every square foot of living space available .
2. Contractors are in the majority are stupid and have no idea how what they install works and will believe any salesman who gives them a free lunch and lies to them .
3. Because gas and electric utilities always pay rebates for stuff that will hurt their bottom line (SARCASM) . That's why real good stuff does not really qualify for any real incentive or rebate in large metropolitan areas .
4. Homeowners believe what their licensed contractor tells them .
5. It just plain cost too much to install quality systems so components get installed with no regard to a system .
6. It must be more efficient to require 199,000 BTus to deliver less hot water than a unit using 76,000 .
7. I could go on but won't .

icesailor

3 GPM at what rise? 3 GPM is equal to 180 gallons per hour. At what rise? 30 to 50 gallon gas water heaters recover 35 GPH.

Combi spit the loads. But still, what is the rise? Does anyone know? Is the DHW hot enough? Something is wrong. Is the water hot at the kitchen sink?

You HAVE to know what the recovery rate is or what the steady heat rate is. 3 GPM means nothing without the recovery rate. If the recovery is from 55 degrees incoming cold water temperature, and you want to heat it for use to 120 degrees, , that's a 65 degree rise. Is the unit with a combi capable of delivering 3 GPM at a 65 degree rise?

Single lever mixing shower valves won't work well with less than 120 degree incoming hot water at the valve. There may not be enough hot water available to overcome the cold water temperature.

No one has said what breed this Combi unit is so ratings can be checked.

Leo_G

So we need the unit make and model. The Navien 240 can do 5 GPM at around 60* rise. So entering water temp is crucial.

Also, agree with above about a cross connection possibility.

What is the outflow temp set for?

In my area, all new fixtures, except tub fillers are now limited to 1.5 GPM, so these units are quite capable in keeping up with demand in newer construction, renos.

Also agree with the electric HWT as a buffer. Use to do this in situations where a gas tank was the primary and customer wanted more capacaity, but we had no way of running a new vent (before condensing units). The electric may come on once or twice a night, and not for very long.

I have installed combi units in 3500 to 4200 square foot houses with good results. One has a 4 body spray, 3 shower head shower and so far no complaints.

Snowmelt

You have a bad contractor.......we can always add more water heaters, I have these tankless doing large laundromats. But you have to put 3 or 4 of them in.

mars_6

I do not recommend these units.!!!! they have there place but the niche is very small. I have seen 2ea 200,000 btuh units paired up to satisfy a 4,000 square foot home and it baffles me where the energy efficiency may come to play. Just my 2 cents. Matt

hot_rod

Remember too, they will need periodic cleaning especially with large quantities of water consumption. This is true for both the tube or flat plate HX styles.

The performance numbers are new, clean, out of the box, after a few months of use and those numbers may drop considerably with hard water conditions. Pipe them with bypass valves to allow easy de-liming procedures.

There are plenty of great applications for tankless or instantaneous, but installers should present all the facts, including the cleaning requirements.

Rich_49

Bob , it might help if the installers knew those facts also may I point out .