180 with a condensing boiler. Why not?

Brad White_191

Personally, I size coils for 140 or less when using a condensing boiler and even so, a 40 degree delta-T and counter-flow.

Have the engineer give me a shout via e-mail and maybe I can convince him or her.

My basic approach is to have all emitters use the same water temperature. Sometimes that is not possible of course so the next best thing is to keep the high end emitters (usually air coils) to as low a temperature as possible.

Do price out 4-row coils (they tend to work 90% of the time with 140F water or less; I reserve 6-row coils for 120F or higher delta-T's). In the big scheme of things, this cost will be minimal and the benefits will outlast the house.

Big Will

I am bidding a job that has some hydro air.

The mechanical is all specified equipment. They call for two Knight 399s linked to in slab and cast iron base board. But the hydro air coils are my concern. They show specifically that the water delivered to the coils needs to be 180. So I know that that is outside of condensing temps. I know that you don't get the efficiency you pay for with the Knight at those temps. Is there anything that has been written by Dan or others that really lays out the reasons for not doing this. I really need to have some third party in the letter I am going to include in my bid specs.

Glen

mainly -

because of the euro standards of limiting output to 167 F or even lower.

Big Will

I think the knight will produce slightly over 180.

Andrew Hagen_2

Specs

There is no reason not to run them at 180°F other than their efficiency is lower at higher temperatures. Typically if a condensing boiler is specified in a new heating system, the coils will be "oversized" so that they can run at lower temperatures where the boiler is more efficient. I would contact the specifier and see if they really wanted 180°F or if that was inadvertently carried over from an old spec. If they do want 180°F, they should be able to explain why. "I know the Knight is more efficient at lower temperatures, but I see the specs call for 180 degrees at the coils. Is this what was intended?" You will either get an addendum or an explanation of their logic.

Brad White_192

180 degree thinking...

The notion that hydro-air coils have to be 180 F should be challenged especially if the coils have not been ordered yet.

There is nothing that says you must use 180 F. water, that is just a conventional rating point (and who wants to be conventional??)

Try this:

Figure your heat loss and the airflow you have for heating. (May match cooling, could be less if a multi-speed system.)

Divide that BTUH load by (CFM x 1.085). That will give you the temperature rise you need. Select the coil for that temperature rise using say 140F entering water. This may be a 3-row or 4-row coil. I have even use 6-row heating coils on very low water temperature systems.

Say you have a 3-ton, 1200 CFM system with a heat loss of 37,500. If you divide 37,500 by (1,200 x 1.085) you will see a 28.8 degree temperature rise is needed. If return air is 70 F., that is a 98.8 degree discharge temperature, about as low as I would go unless distribution is excellent, not all in one place.

(Try to keep the air discharge temperature at least 100 to 110 degrees to avoid any chilling effect. That will also be about your limit with low water temperatures.)

As a second pass example, the same system using 900 CFM over the same heat loss would get you a more ideal discharge air temperature of 108.4 degrees, affording less risk of 'wind chill'.

Point being, overall, why defeat your higher efficiency based on a couple of 1-row or 2-row coils? Why have the tail wag the dog? Buy deeper coils-once. Save high temperatures for domestic water production and save on heating.

My $0.02

Brad

Big Will

I must say the engineer did some homework for me here.

The heat loads were laid out and cfm per fan coil calculated based on the 180 deg water. about half of the fan coils are a custom fabricated unit with a Dayton blower. This in it self is a problem in my mind. I don't like standing behind Dayton. I can just put a disclaimer in the bid. I intend to add the necessary costs to use coils that will allow 150 deg water or lower. I just need to justify that cost. I was hoping there was something in print that explained this. I find third party documents really help when questioning a engineer.

Wayco Wayne_2

I'm not as familiar

with the Knight, but with other mod cons I have heard of using the domestic hot water side of things for higher temperature needs. A lot of mod cons boost the water temp up for domestic hot water so you can put zone valves seperating the domestic and the hydro air and get your high temps. When your not using the hydro air the water temp can do it's condensing thing. WW

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Mike T., Swampeast MO

That's a tough mix efficiency wise.

Can one hydro air load be met by one boiler? If so, can it be piped/controlled as the higher of a two temp system (as seen by the single boiler) with the lower temp contibuting to the remaining load if necessary? Is the hydro-air perhaps boost, basement or backup?

Kal Row

I have done lot of hydro-air with knights...

you will discover that you don’t really need 180, me and moses did a huge house that is sitting on hold because of the nyc building dpt and the owner don’t exactly see eye to eye on space usage, needless to say we had to heat the shell through the winter with no insulation, and huge air leaks all over the place, and the place was quite toasty on a howling 7 degree night, and I had limited the knights to 170 –


Sure at those temp mod cons are not condensing much – but they are modulating the fire to the load, and take cool water back so that you can design for much more efficient larger delta T’s, and even at 180 a modcon is venting with pvc because it has put almost all of the heat into the water instead of up the flue, look and any cast iron natural draft boiler input and net – there is usually a 25% difference – but on a mod con its about 10-12%

Yes, if you need to go higher then you need another type, I got a pair of laars pennants running at 220 and 60psi, in a plating process plant, because the coils were meant for steam, it’s ok according to laars, as long as you are not cold shocking the boiler with sudden cold loads, when they called me in to the job, the pennants where there, but not doing the job, even though they were big enough for the calculated btu load, and when I looked the heating coil’s doc I had an “ah:ha” moment realizing they ain’t gona do squat with 175 degree water

PS the house above has a pool on the top floor, why there, and not in the basement?, because my dear friends, god gave men only enough blood to run their brains or their “you-know-what’s”, but not both at the same time – there is such a thing as too much money

Uni R_2

Buffer Tank

Maybe just run the 180 to the air handlers and the then return them to a buffer tank for the low temp circuits. The boiler returns from the buffer. What's the maximum ΔT for that Knight? It would need to be 50° or at least pretty high to get to the condensing range for return temps.