mod con with high supply temps

nmac86

Hello Everyone,

I’m considering a HTP EFTC-140F combi boiler for a small house with 62 feet of baseboards and a design day load calculation of 37K. The EFTC-140F has a minimum fire rate of 28K BTUs so high temp or more emitters would be needed to keep the boiler from short cycling. I’m less concerned about fuel cost and more concerned about shortening the life of the boiler or needing expensive repairs. Won't the efficiency of the boiler still be around 80-85% with high water temps?

Besides reduced efficiency is there another reason not to use a mod con boiler at high water temps (165 -175F?) If I need to keep the supply temp high in order keep the boiler from short cycling (baseboard emitters) am I going to reduce the lifespan of the boiler or require excessive maintenance?

My primary reason for looking at a mod con is to be able to side vent with PVC and also provide DHW in a relatively small amount of space.


Thanks for taking the time to read!

Gida

Watch the Viessmann Vitocrossal webinar. It is designed for 194-deg temperatures (although less efficient when running high return), it'll still be more efficient than a cast iron conventional boiler. https://www.youtube.com/watch?v=0_t_Y0FzsqU

nmac86

Thanks for suggestion Gida! I've liked everything I ever read about Viessmann boilers but they're a bit too expensive for this project.

Gordy

What are your hot water demands? Big tubs, car wash showers?

You need to calculate how much emitter is in place compared to the actual load. If the emitter is capable of more than the load than lower water temps can be attained. Especially when outdoor temps are above design day which is most of the season.

That's a big boiler for a 37k load. Remember a 40 gal conventional gas water heater is about 36 k input. If a 40 gal satisfied your needs I don't see a reason to have such a big boiler.

nmac86

Gordy,
Thanks for the input!

Hot water demand is not that high. Usually 2-3 short showers a day not back to back. 1 shower head using less than 2.5 gallons / minute. Even though the hot water usage isn’t that high I will be asking a lot out of the boiler since I live in a cold climate (Minnesota.) In the dead of winter my incoming cold water could be as low as 40 degrees which would require an 80 degree temp rise to get to 120.

I figure I have about 37,200 BTUs of emitters at 180 degree supply temp. However at 150 degrees I think this number goes down to 24,800. Since this would cause the boiler to short cycle I think I would need to keep temps no less than 165-170.

I agree that the EFTC-140F is oversized for my house but I love that it has a 15 gallon indirect in it. I feel like this negates some of the hot water problems people experience with combi boilers. In a perfect world a smaller boiler with an indirect tank would be ideal but the EFTC-140F is so much cheaper. I’m just curious if I’ll shorten the life of the boiler by using high water temps for the baseboards.

Thanks

Jamie Hall

If you are concerned -- and perhaps rightly -- about having a large number of short cycles because the boiler output is greater than the building heat demand, one of the better ways to handle it is to use a storage buffer tank, with the building heat circulating too and from the buffer tank, hopefully using a mixing valve and outdoor reset on that, and then using the boiler to reheat the buffer tank water when the temperature in that gets too low. There are a number of control strategies which can be used.

This will not get a mod/con boiler into the condensing range most of the time, however -- the buffer tank water will be too warm for that.

And you are basically correct, the efficiency of a mod/con when not condensing will be in that 80 to 85% range. Basic physics, and nothing to be done about it.

Gordy

So did you do an actual heat loss, or are you just going by the emitters capability?

@Jamie Hall has a short cycle bandaid solution, but it also takes space.

nmac86

Jamie,

Thanks for the suggestion. The buffer tank is a very creative solution to this problem. However if I’m going to spend the money on a buffer tank, outdoor reset and mixing valve I’d probable be better off just spending more upfront for an appropriate sized boiler/indirect. So much to consider…

Gordy

Interesting that the EFTC 140 has a 10:1 tdr, but low end modulation is 28k.

Have nothing against HTP products. Maybe give lochinvar a look. The Nobel line. Its a combi with 10:1 tdr. The 199 supplies 4.8 gpm with 77 degree rise, and low end modulation is 19.9k.

Or if you can get by with less gpm dhw there is smaller sizes with even lower low end modulation.

Jamie Hall

nmac86 said:

Jamie,

Thanks for the suggestion. The buffer tank is a very creative solution to this problem. However if I’m going to spend the money on a buffer tank, outdoor reset and mixing valve I’d probable be better off just spending more upfront for an appropriate sized boiler/indirect. So much to consider…

Quite true...

hot_rod

62' of fin tube at 550 btu/ ft is 34,100 BTU/hr, if it it typical 3/4 fin tube?
Really no need to have a boiler larger than what the heat emitters can move into the space? Seems a 110 or 120K Combi boiler would be a much better match, if you can live with 2- 2.5 gpm DHW..

If you run any mod con at high temperatures, consider a better venting material than PVC, that tends to turn brown at high operating temperatures, stainless or PP vent material would be wise.

How often at design condition? Here is what Boston looks like, hours of occurance, you may be able to dial in ODR to get down into better efficiencies and lower operating temperature.

EBEBRATT-Ed

I don't think you will be happy with that boiler. Way to large for your heat loss no efficiency will be gained. Put in the right boiler and an indirect. It will smooth out the operation.

Their are cast iron boilers that can be horizontally vented

NY_Rob

Is the 62' of emitter one zone or is it divided into two or more zones?

Why not go with a HTP UFT-80W with a 30 gal indirect for DHW?
It's a very simple setup with a low fire rate of 8K BTU's.

If you have one 62' zone, you could drop SWT to 105F and still not short cycle on the UFT-80W.

Plenty of hot water and a correct size mod-con that should last years.

SuperJ

Like the others are saying... install a bit of storage on your DHW so you can have a sane boiler size. Putting a boiler in that is 5 times your heat loss and (almost 1x at min fire) is not good design.

I have a 100k boiler, running about 12kbtu of heating (panel rads), not ideal, but it works for me since I have 34 gallons of buffer.

hot_rod

With combi boilers it sort of comes down to what you expect for DHW production. Incoming water temperature changes their output considerably I think they figure a 77 rise

We seem to get along fine with a 120 combi, but our well water stays around 50- 55 year round

I agree if you want DHW fast and for dump loads an indirect is a better fit and then size theboiler closer to design.

Gordy

The baffling part about the eftc 140 is it has a 15 gallon storage, and is 10:1 tdr, but only modulates down to 28k.............

flat_twin

The 140 is 5:1 turndown 28K min. The 199 is 10:1, 19k min. per HTP website info