sizing boiler with tankless coil

Constantin

... now that all depends on your intended use, doesn't it?

Let's take this an example: All you aim to do is take a shower with a 2.5GPM shower head. In the depths of winter, your tankless coil will have to raise that water about 70 degrees or thereabouts. That's 2.5 GPM x 60 min/Hr x 70 deg F rise x 8.3 lb/gal = 87kBTU/hr DOE. I suppose one should de-rate the tankless coil since its thermal transfer from inside the boiler is not 100% perfect, pushing the DOE net requirements ever further up. If you want to take two showers at the same time, your DOE output would have to be doubled, etc.

Now, let's say you end up buying a 100kBTU/hr boiler to cover the hot water heating load with the tankless coil. The boiler will be 100% oversized even on a design day, and without the addition of a buffer tank may short cycle unless you buy a boiler with a modulating burner. Such burners currently only exist on the gas side of the US residential heating business.

What I am trying to tell you is that using a tankless coil is probably not an optimal solution for you. You're better off buying an indirect water heater and piping it into your heating system as if it was a separate (primary) zone. Barring a requirement for endless hot water, etc. it is my understanding that you won't have to add any capacity to the boiler and still enjoy the benefits of a heating plant that can heat your home and your hot water.

STEVEN MARKS

sizing boiler with tankless coil

small colonial - 1,000 sq ft. heat loss calc shows 41,000 btu/hr. i have been told to add 50,000 but for tankless coil. Would i add 50,000 to input, doe rating or net ibr rating. would determine if i went with .75 or 1.00 input for oil.

thank you for your responses steven

Constantin

I'm but a mere homeowner...

... but I presently do not follow the logic of adding a tankless coil to a small boiler such as yours.

A properly-sized indirect water heater on a primary loop will allow you to size the boiler to the heat loss, not the water heating load. Doing the former will improve the seasonal efficiency of the boiler and probably result in superior performance to boot. Are you looking for endless hot water or are there specific dump loads like jacuzzis, monsoon shower-heads, etc?

STEVEN MARKS

Sorry Constatine but you didn't any my question. I am installing a boiler with a tankless coil for proper performance how much do i add to heat loss and do I add to input, doe rating or net ibr rating.

Joe Grosso

Coil

Steve, it depends on the boiler you would use.Personally, I would use a boiler with a 5 gpm coil at the lowest setting.For example- I use EFM boilers. I would go with the PK 400 at .75 gph which would be a 79,000 output.Choosing a small boiler that has a large 5 gpm coil is the important thing.Another small one with a large coil is the Thermo-Dynamics CWL. I don't think you will get much smaller than 75,000 output with any manufactuer.

Joe_63

Constantin do not waste your breath here.He does not want to hear logic.

Unknown

Or ........

It could be that he doesn't have the space to add an indirect or storage tank ? Like about 75% of the homes we work in on L.I. ?

Going strictly by homeowner feedback , a boiler with a coil works well enough for most homes with up to 2 baths . We take out 55+ year old GE boilers where the customer never had a problem with hot water .

Having said that , I recently installed a 30 gal. indirect under my stairs and capped off the tankless . In the shower , I can run a steady temp continuously now , whereas with the coil I needed to turn the lever a bit every few minutes if my shower was 15 minutes or better .

STEVEN MARKS

Constatine and Joe I know all about the benefits of indirect water heating but that is not what I was asking. Constatine you are not totally correct anyway. If my heat loss is only 40,000 btu/hr I would still need to at least double my boiler size in order to achieve the output ratings for a 40 gallon indirect.
Do most contractors add to the boiler input, DOE rating or net IBR rating for a tankless coil and how much?

Firedragon_4

This is the math

to do it right, every time.

gpm x 8.3 x rise x 60 = btu/hr required.

gpm = gpm flow required

8.3 = the weight of one gallon of water

rise = the desired rise in water temperature at design condition

60 = number of minutes in an hour

Science always rules the day!

As coils in tanklesses and indirects lime and calcify this proves to be the great equalizer. BTW, this is in our book on DHW and is a universal formula.

BTW2, I have an external tankless in my home and it has outperformed and outlasted three different types of indirects

If you do have a contractor install a new boiler use a rear section mount regardless of whether it's steam or hot water, there's just more free coil exposed to the water. The more coil exposed, the better the heat transfer and the lower the friction drop across the coil, that's why externals have also worked so well since the 1920's.

Constantin

Hi Ron,

You're absolutely right, there are some very good reasons to have a tankless coil, and many people have been very happy with them. My guess is that older boilers could take advantage of several factors: their thermal mass and being sized for uninsulated homes, if they were sized properly at all. Dan has a nice description of Levittown homes having early flowers around their foundations due to the lack of slab insulation, for example.

In a Levittown home with a slab as a heat emitter, an oversized plant isn't going to be much of a problem because the slab acts as a huge thermal battery. Even if the slab coils have been disconnected, the mass of the slab is still there, soaking up and dispensing BTUs... just not as much as if it was part of a RFH system.

On the other hand, we recently heard from a homeowner in CT with a 4x-oversized boiler and baseboard that was short-cycling like crazy... and his boiler wasn't even that "big", something like 120kBTU/hr DoE, IIRC. It's a good possibility that the heating plant there had other issues as well, but isn't a low-mass emitter in a low-mass home combined with a high-input boiler inherently asking for trouble?

As fuel prices climb higher, more and more homeowners are right-sizing their heating plants via conscientious contractors. Between insulation, weatherization, etc. some very small heating loads (and boilers) can result. Yet, as my (perhaps erroneous?) math example showed, a 2.5GPM shower will require at least an output of 87kBTU in a tankless setup in the depths of winter. As a result, an indirect tank makes more and more sense.

But the question regarding the performance of tankless coils is a good one... how big are the boilers you install these days vs. the ones you remove? And, perhaps more importantly, when do we get to see your next episode of a boiler install? :-)

Constantin

Steven,

I laid out the formula for you above. So has Firedragon below. So, go ahead and calculate your DoE capacity requirements and be done with it. Yet I disagree with the notion that you have to upsize your boiler to meet the demand of an indirect - that all depends on your water and flow needs.

In most homes of the NE where you seem to live, the heating needs for hot water were traditionally a pretty small fraction of the total heating needs. As homes are getting better insulated and people are gravitating towards jacuzzis, waterfall showers, etc. that balance is starting to shift. Yet, based on the data I have seen, the heating needs in your neck of the woods usually still dominate.

The use of an indirect allows you greater flexibility with respect to the input rating of the boiler attached to it. In other words, you shouldn't go by the BTU input rating that the manufacturers publish to attain the maximum first-hour capacity, rather you should go the other way.

First, figure out what your flow requirements are. How many gallons over how much time. Then look into the indirect literature to find a tank whose first-hour rating can meet your needs when fired by your ideal boiler. Now you have an indirect that meets your needs while maximizing the seasonal efficiency of your heating plant.

Lest we forget, there are millions of gas water heaters in this country that have very happy customers despite the fact that a standard 50 gallon gas-fired unit transfers less than 40kBTU/hr into the tank when it's firing. The secret in the sauce is the buffer. Electric water heaters rely on the buffer even more due to their poor recovery rates.

Obviously, you may have hot water needs that are so constant that they far outstrip your heating needs by far. Under those circumstances, a heating system optimized for the latter makes more sense. All the best with your project.