Combi or not?

josephny

I need a hot water heater (LP) for a house with 3 full bathrooms, 2 half bath, kitchen sink, dishwasher, laundry machine.

I would like a single unit that can also provide 30kbtu heating for hydronic radiant.

Should I use a combi? Any suggestions?

Thank you!

mattmia2

It depends on your incoming water temp and what your usage looks like, are you using all the loads at once or sequentially, what sort of shower heads do you have in the baths, are you filling a tub, and a few other things. Most likely a combi isn't the best solution for this situation, it would have to be much larger than your heating load, even at its lowest firing rate, the unit that could provide that much hot water would be significantly oversized for your heating load. Most likely a very large indirect that could slowly recover with a small boiler but could hold enough hot water for all of your loads would be a good solution. One or more conventional powered vent LPG water heaters and a boiler is another solution.

josephny

My readings confirm exactly what you wrote, and that an indirect would make a lot of sense.

But then I read more and learn more of the details like larger tank and smaller boiler vs. smaller tank and larger boiler, among a million other details.

I have no idea how often all loads will run at the same time, or how often 90/80/70/60/50/40% of the loads will run.

There are 2 tubs, 1 shower, 5 bathroom sinks, a kitchen sink, a dishwasher and a laundry machine. I think that's all the hot water loads. If I had to guess, I'd say that it wouldn't be too uncommon for 2 showers and either a dishwasher or laundry machine to be running at the same time. Less common but certainly possible: 1 tub, 1 shower, dishwasher and laundry at the same time. I believe that last scenario would demand about 8gpm.

Am I making any sense?

Thank you!

EzzyT

I wouldn’t go with a combi unit. Go with a boiler and a 40 indirect tank with a thermostatic mixing valve. 

mattmia2

I think you would really have to be careful about a 40 gallon indirect being large enough for that load. It is possible under some conditions but it also would be woefully inadequate under other conditions, especially if you size the boiler for the heating load. No one ever complained that their tank was too big and they never ran out of hot water.

Jamie Hall

Well, @josephny , you are making sense in your analysis. Now let's consider just what that means. Suppose you are indeed demanding 8 gpm of hot water -- which I regard as quite likely. Let us further suppose that the incoming water is at 45 F, and you like nice hot showers -- say 105 f water. That will require about 4,000 BTU per minute, or if you are using an "instantaneous" water heater or combi, a power input to the unit of about 250,000 BTUh.

Units that large do exist, but the heating side of them will have a minimum firing rate more or less equal to your heating load on a design day -- so most of the year the thing will be cycling like mad, which is hard on the hardware, not to mention the efficiency.

Now also consider. Suppose, instead, that we supply that hot water from a nice indirect It's unlikely that that peak demand will last more than 10 minutes. 80 gallons of hot water, even if the indirect is held at 105 (which it wouldn't be, if properly installed -- more like 140 and mixed down). Also 40,000 BTU of energy. Taking the full output of a mod/con boiler sized for your heat load would take an hour and 15 minutes for the tank to recover -- quite manageable once or twice a day, and the boiler would be condensing while doing it, so good efficiency.

Perhaps at least as much to the point, with the indirect if the peak load goes over that 8 gpm for a couple of minutes -- someone decides to wash their hands, for instance -- the parties in the shower don't get treated to a blast of cold water when the combi is overloaded, which is not likely to make them happy.

To me, it's a pretty easy decision... though I would also look at a direct fired gas water heater as well. But I wouldn't even consider a combi.

Robert_25

Small heating load + big domestic hot water load is not a great match for a combination unit. Go with a properly sized indirect or a separate water heater.  

Also - keep in mind that combi units do not tolerate hard water very well.

josephny

I love this forum -- you guys are so wise, experienced and knowledgable -- and generous with all of that! Thank you!

Combi is out.

Tankless looks like it would be a challenge for times when demand is at or above 8gpm, right?

That leaves boiler+indirect vs. gas-fired-water-heater.

I'm not sure of the down side of a gas fired water heater.

But, if I understand correctly, a boiler sized for heating load combined with a large indirect tank would be both cost-effective and highly functional solution.

So, something like an 80-100 gallon tank connected to a modulating boiler (maybe an IBC SL series 20-160kbtu) would be a good solution?

And piping something like this:

dopey27177

Look at AO Smith Gas fired AT10-910n or AT10-910An instantaneous water heater.
380,000 Btu input. 15 year warranty
Space Saver and continuous Hot Water up to 8 gpm

Jake

Hot_water_fan

@josephny not much downside to a separate tank heater, lifespan is about the only concern but that will be variable. It also provides redundancy.

maybe an IBC SL series 20-160kbtu

Way too big for a 30k Btu heating load. With an indirect heated to a high temperature, the 85k Btu boiler will be more than enough. An option worth considering if you have access to the drain pipe would be drain water heat recovery - it saves energy and adds capacity by increasing the incoming water temp of both the cold side of the shower and the water coming into the heater itself.

josephny

dopey27177 said:

Look at AO Smith Gas fired AT10-910n or AT10-910An instantaneous water heater.
380,000 Btu input. 15 year warranty
Space Saver and continuous Hot Water up to 8 gpm

Jake

Looks nice indeed. Not cheap.

josephny

Hot_water_fan said:

@josephny not much downside to a separate tank heater, lifespan is about the only concern but that will be variable. It also provides redundancy.

maybe an IBC SL series 20-160kbtu

Way too big for a 30k Btu heating load. With an indirect heated to a high temperature, the 85k Btu boiler will be more than enough. An option worth considering if you have access to the drain pipe would be drain water heat recovery - it saves energy and adds capacity by increasing the incoming water temp of both the cold side of the shower and the water coming into the heater itself.

My thinking is that when heating is called for, it will modulate down to as low as 20kbtu, but when the tank calls for recovery, it can produce 160kbtu. Is that wrong or inappropriate?

mattmia2

josephny said:

My thinking is that when heating is called for, it will modulate down to as low as 20kbtu, but when the tank calls for recovery, it can produce 160kbtu. Is that wrong or inappropriate?

It is an approach. A likely more efficient approach is to use a smaller boiler and larger tank so that the boiler can modulate down to the heating load during warmer/average periods but the tank stores enough hot water that it does not need to recover quickly.

Hot_water_fan

@mattmia2 is exactly right - if you only need 30k of heating load maximum, you're mostly using much less than half of that, which is lower than the minimum modulation leading to short cycling. A larger indirect tank or reducing usage deals with the DHW draws.

josephny

Okay, I'm getting it (slowly).

IBC has an SL model boiler that modulates from 10 to 85kbtu and another from 14 to 115.

I'm just not clear if my understanding is correct that the full 85 or 115kbtu will be available for DHW recovery?

A back of envelope calculation (because that's the best I can do) would show that with a 100 gallon tank, at 8gpm draw, I'd have ~11 minutes of stored water usage and at 115kbtu firing, recovering at about 2gpm. With a max usage of ~15gpm, there will be the occasional running out of hot water.

Reducing usage is just not practical -- even a complete lay person like myself gets eye rolls of disinterest when trying to discuss these (super interesting) topics with the typical hot water user.

I can see how a larger tank makes a large impact but so does the recovery rate (which is a function of the boiler capacity, so I'm reluctant to reduce that).

mattmia2

I'm not quite sure in the previous calculations where the 8 gpm comes from. Only the dishwasher and washing machine will use that water at the temp stored in the tank. In a tub or shower it will get mixed down to something like 100 degrees. A conventional washing machine uses a lot of water, high efficiency washing machines and dishwashers only use a couple gallons at a time. Since you are mixing the water in the tank with cold water for most of your big uses, you aren't using all of it out of the tank. Stagnation is also a factor that prevents you from using the entire volume of the tank.

The boiler values are inputs. If you can get the return temps low enough that it is condensing, you may be putting 90%-95% of that in the water. If you are running higher tank temps you may only be putting about 85% of that in to the water.

Perhaps @Larry Weingarten can point you at a more scientific way to guess at what your usage will be.

Larry Weingarten

Hi, I don’t know just how scientific it is, but the data from a number of studies is that more than 99% of the time, hot water usage doesn’t exceed three gpm. The tubs are a big variable. If you gave them their own heaters, the rest of the system would be much simpler to size.

Yours, Larry

josephny

Larry Weingarten said:

Hi, I don’t know just how scientific it is, but the data from a number of studies is that more than 99% of the time, hot water usage doesn’t exceed three gpm. The tubs are a big variable. If you gave them their own heaters, the rest of the system would be much simpler to size.

Yours, Larry

That is very interesting. I'm assuming the data (99%/3gpm) is from "typical" single family homes. That's encouraging.

It seems to me that it wouldn't be too rare for have 2 showers, a kitchen sink and either a dishwasher or laundry washer to be running simultaneously. I think that means about 8gpm. Maybe the size of the house and number of bedrooms explains the great difference between 3 and 8.

But, if we go with 8 in that configuration of showers, etc., then what would be the wisest water heating system (that can also feed a radiant load of 30kbtu)?

Thank you!

Jamie Hall

The old National Plumbing Code (whatever did I do with my copy? Somewhere around here...) had a set of very very useful tables for both supply and drainage "fixture units" for various arrangements and combinations of plumbing, which took into account the probability of more than one fixture in a bathroom group being in use, or several bathroom groups and other fixtures and so on -- all developed from decades of experience. Useful table. Do the newer codes have that?

In any event, though I don't recall the exact numbers, the general assumption as I recall it was that in a residence with multiple bathroom groups that all but one of them would be in use simultaneously -- so 2 out of the 3 in this case -- and that at least half of the other uses would also be in use.

Today the issue is somewhat complicated by the advocacy (and in some areas requirement) for low flow fixtures -- and the inevitability that someone, someday, will substitute a 4 gpm rain shower for that original 1.5 gpm pipsqueak (or just take the flow restrictor out of the pipsqueak...). And will then complain bitterly that there isn't enough hot water.

Bottom line: unless you are in a situation where space is really truly tight, or where there are mandatory water restrictions and the building inspectors actually come around and do flow checks, it does not pay to undersize either for piping or particularly for the hot water heater (in the case of a well, the well pump and pressure tank, either).

josephny

Plenty of space available and no water restrictions.

Given that, and my general approach to work hard to avoid undersizing anything, should I be looking at a 100 gallon indirect?

I'm still not sure my understanding is correct that a modulating boiler (20-115kbtu for example) will heat water needed for recovery at 115kbtu?

Thank you.

GW

anti combi guy here, I’m a tank guy 

Jamie Hall

josephny said:

Plenty of space available and no water restrictions.

Given that, and my general approach to work hard to avoid undersizing anything, should I be looking at a 100 gallon indirect?

I'm still not sure my understanding is correct that a modulating boiler (20-115kbtu for example) will heat water needed for recovery at 115kbtu?

Thank you.

The 100 gallon.

As to the boiler, size it for your heating load, not your hot water load. You want it sized so that it's output at full song is slightly greater than the heating load of the house -- and no more, if you can find on that small. Or the smallest one. The idea is that it will modulate down to meet the heating load at other times.

When the indirect calls to recover, the boiler will come up to its maximum output -- but you are not trying to recover that hot water indirect as fast as you use from it (you are not running those peak flows all the time!) so you don't need anything close to that power to do the job. The tank supplies the volume of hot water needed to handle the flow for a short period of time.

Robert_25

How many bedrooms are in the house? The number of simultaneous showers or back to back baths is what will demand the most DHW. Modern washing machines and dishwashers use very little water - and occasional hand-washing in the half baths doesn't amount to much either.

We are a family of 4 living in a house with 3 full baths. 99% of the time our DHW needs are met just fine with a 50 gallon electric water heater (1st hour rating of 57 gallons). The only time we ever challenge the water heater is when the kids take back to back baths. Two showers at the same time with the dishwasher or washing machine running is not a problem...but we are also using 1.8 gpm showerheads.

Unless the house has a large soaking tub or a shower with high-flow showheads, I think a 40/50 gallon indirect with a boiler sized for the heat load will do fine. e.g. A 40 gallon boiler match fired with 40k btu has a first hour rating of 89 gallons. Set the indirect at 145 degrees and mix the water down to 120.

Rich_49

Here's your Combi . https://www.htproducts.com/versahydro.html

josephny

Jamie Hall said:

josephny said:

Plenty of space available and no water restrictions.

Given that, and my general approach to work hard to avoid undersizing anything, should I be looking at a 100 gallon indirect?

I'm still not sure my understanding is correct that a modulating boiler (20-115kbtu for example) will heat water needed for recovery at 115kbtu?

Thank you.

The 100 gallon.

As to the boiler, size it for your heating load, not your hot water load. You want it sized so that it's output at full song is slightly greater than the heating load of the house -- and no more, if you can find on that small. Or the smallest one. The idea is that it will modulate down to meet the heating load at other times.

When the indirect calls to recover, the boiler will come up to its maximum output -- but you are not trying to recover that hot water indirect as fast as you use from it (you are not running those peak flows all the time!) so you don't need anything close to that power to do the job. The tank supplies the volume of hot water needed to handle the flow for a short period of time.

Could you please explain why a modulating boiler that goes down to well below my heating load but also has the capacity at full throttle create a quicker hot water recovery wouldn't be better?

Thanks!

josephny

Rich_49 said:

Here's your Combi . https://www.htproducts.com/versahydro.html

Very nice looking device. Pretty pricey there.

Rich_49

josephny said:

Rich_49 said:

Here's your Combi . https://www.htproducts.com/versahydro.html

Very nice looking device. Pretty pricey there.

Ya think so ? Price the equipment others are recommending and all the associated stuff you'll need also and whatever your time or whomever you hire is worth and get back to me . Trust me , it's a bargain , and I do this for a living .

By the way , read my tag line and really think about it .

Larry Weingarten

Hello, To work towards an answer for @Jamie Hall , here is a link for a recent presentation on the IAPMO water demand calculator: https://www.youtube.com/watch?v=2CTDU5fNhSo . Also, attached is a presentation by Gary Klein. The eleventh slide is a mind blower!

Yours, Larry

Jamie Hall

I give up. But don't call me when the teenage daughter of your wife's sister runs out of hot water washing her hair.

mattmia2

I'll say it again. No one ever complained that their water heater was too big and they never ran out of hot water.

mattmia2

Oh, and i have an ssu-45 45 gallon indirect with a 100,000 btu/hr input modcon and i can run it out with one shower in winter when the incoming water temp is about 35 f if i'm not careful. i think i could fix that with a higher tank temp and mixing valve but that only gets you a little more if you are pretty close.

sunlight33

I have a modcon that fires when a 52-gallon indirect water temperatures drop to 118 degrees, when that happens it takes a bit of time before the modcon modulates to high fire, by then the tank temperature would often drop to 90 degrees, this is also with a 35 degree incoming water and just one shower.

mattmia2

sunlight33 said:

I have a modcon that fires when a 52-gallon indirect water temperatures drop to 118 degrees, when that happens it takes a bit of time before the modcon modulates to high fire, by then the tank temperature would often drop to 90 degrees, this is also with a 35 degree incoming water and just one shower.

That is essentially my problem as well. It takes a while for the thermocouple at about 1/3 up the tank to sense the draw then the boiler has to pre purge for a while before it fires. By the time it starts dumping heat in the water the draw has been occurring for several minutes. I'm not convinced there isn't a control solution to this, but I don't think there is anything off the shelf that would do it.

mattmia2

But @Jamie Hall did you get a disagree from @Rich_49 for it?

jad3675

No one is going to mention a reverse indirect, like a TurboMax?

mattmia2

jad3675 said:

No one is going to mention a reverse indirect, like a TurboMax?

This might be a good solution for @aroshtr who wants to microzone their system.

Jamie Hall

It's helpful to keep in mind with this sort of thing that there are two genuinely separate problems. First, you need a certain volume of hot water delivered at a certain rate. That is power -- but power can either be stored (the water kept hot in the tank), as energy delivered at a low rate (much less power) over time or it can be supplied at that rate (such as an instantaneous water heater); the same total amount of energy, but vastly different amounts of power involved.

An indirect -- or, for that matter, a direct fired or heated tank -- uses a relatively small amount of power, but continued for a long time, to store the energy needed to release a large amount of power in a short time.

Note that a properly sized hybrid water heater will take this principle to an extreme: it is a relatively low power device, but the whole idea if it is correctly sized is that the heat pump will operate continuously, all day and all night, but that there will be enough storage to provide the bursts of power needed for the shower, Jacuzzi, or what have you. Mess up the storage tank sizing and you either engage the resistance elements (much money) or you run out of hot water -- or both.

Rich_49

mattmia2 said:

But @Jamie Hall did you get a disagree from @Rich_49 for it?

Robert_25

Jamie Hall said:

I give up. But don't call me when the teenage daughter of your wife's sister runs out of hot water washing her hair.

I don't want to get called either, but I view sizing a water heater for a situation that happens <5% of the time the same way as sizing a boiler for a load that happens once every 5-10 years.

If endless shower(s) are a must have, that changes the math.

Alan (California Radiant) Forbes

If you were my customer, based on the information you provided, I'd suggest a 75 or 80 gallon indirect or stand-alone water heater.

I also like the new generation of Polaris water heaters. Their largest tank is 50 gallons, but they have a variety of inputs with impressive recovery rates.

http://www.americanwaterheater.com/media/26031/NCGSS00216.pdf

josephny

Rich_49 said:

josephny said:

Rich_49 said:

Here's your Combi . https://www.htproducts.com/versahydro.html

Very nice looking device. Pretty pricey there.

Ya think so ? Price the equipment others are recommending and all the associated stuff you'll need also and whatever your time or whomever you hire is worth and get back to me . Trust me , it's a bargain , and I do this for a living .

By the way , read my tag line and really think about it .

Thank you!

Leaving cost aside, do you think this is the better solution?