Combi in small house

[Deleted User]

I have a small house with in- floor radiant heat the load is 25,000btu and their is two small bathrooms one of them has a small soaking tub. I would like to use a Triangle Tube challenger combi boiler cc105. My question is, is this a good match for this system? If not could someone please make a suggestion.

Tom_133

I don't think it's a good match

Here are a couple reasons why,



1 Short Cycling- your boiler will modulate that is true, but when the desired water temp is reached in the slab and the zone is still calling then the boiler can't go low enough to just simmer along and not turn off and on.



2 DHW- depending on what size the soaking tub is and how much flow you have going thru the valve that feeds it, there is a good possibility you run out of hot water, especially if you are filling that and other things in the house are using hot water.



My thinking is a small boiler like a Cadet CDN40 and a rinnai on demand for water heating.

jonny88

buffer tank?

what if you used a 30 gallon storage tank to prevent short cycling,we have a customer with a similar situation who insists on a combi Navien ch 180.to prevent short cycling we are installing a storage tank.Will let you know the results

Tom_133

Buffer

Buffer tank is perfect if it is the correct scenario, in this particular situation the boiler cant produce enough hot water so this combi is out. A combi that can produce the GPM for DHW and has a buffer would be fine, except the price of a buffer is a bit high. So I guess you want to do the math, and make sure a small boiler and on demand or a different water heating source doesn't make more sense before you buy.

kcopp

A small house....

prob also means that space is a problem. The perfect system/ set up does not always happen. Sometimes the the perfect system is not always affordable either...(again small house) Combis are not perfect. The Challenger combi is not a great boiler if you want decent hot water. You may want to get a heat only Solo version and do an electric water heater or go for the Prestige Excellence w/ the Trimax control. Other options.. Bosch Greenstar combi or the Smith GC160.

SWEI

CC105

is four times larger than your design day heating load.  Its minimum firing rate is roughly equal to your design day load.  It will short-cycle constantly.  Size your heating boiler as close to your design day load as possible.  No mod/con is currently available that small.  The closest I have seen so far is the CDN040 http://www.cadetboiler.com/InstallersModelsSpecs which should work relatively well.  You would also need either a large indirect or a separate DHW heater.  The payback on a mod/con may be quite long with a load so small.



How much sunshine does this house receive?  Does it have any passive solar gain?

[Deleted User]

small house combi

Thanks for your replies. The small house,little heat load, big domestic is the problem. Would be nice if their was a combi to fit the bill. So I am think of using the attic space putting in a Triangle Solo 60, which turns down to 16,000 btu with a 30 gal indirect for dhw. That way the indirect temp can be raised and tempered down for more output.

SWEI

I'm a big fan of Triangle Tube

But you should also consider a Lochinvar WHN-055, which is based on the original TT heat exchanger and downfires to 11k.



Just be sure that 30 gallon indirect will meet your needs.  The SMART 40 costs about 10% more than the SMART 30 but has 27% more capacity.  Even that may be too small for a big soaker tub.

[Deleted User]

combi in small house

Thanks Swei, Think I,ll stick with TT know then pretty good but your right about the tank think the 40 will do it thou.

Tom_133

My 2 cents...

After you buy the indirect and pipe it you are at the same price as an on demand. The on demand allows you to get the smallest boiler possible and you won't be storing hot water for no reason when not in use. The math tells me that for this scenario the smallest boiler you can get and an on demand would be most efficient and (if there is enough space) the most logical way to go. No short cycling, or at least very, very minimal. I also see the buffer tank and combi as a good solution