Combi sizing

icy78

My buddy wants a combi boiler to replace his natural draft 80k input boiler. He's thinking a 155 because he can get 4gpm domestic out of it as opposed to an 80 which gives him 2 gpm.
They both have 10/1 turndown.

Let's pretend that the present boiler is sized right at 58k output.
How can I convince him that the 155 is too big? Price is almost identical.
What are the cons other than possible short cycling on the most border line days?

NY_Rob

Measure the fin-tube in his shortest zone, calculate it's BTU output using 100F AWT... that should do it!

KC_Jones

Present and analogy, ask him if he wants to use an 18 wheeler as a commuter vehicle?

gschallert

icy78 said:

What are the cons other than possible short cycling on the most border line days?

Going with the premise that heatloss is 58k as you state, unless he's micro-zoned there probably aren't any cons. He should go with the larger BTU for DHW, which he's apparently already figured out. ;-)

icy78

> @gschallert said:
> What are the cons other than possible short cycling on the most border line days?
>
> Going with the premise that heatloss is 58k as you state, unless he's micro-zoned there probably aren't any cons. He should go with the larger BTU for DHW, which he's apparently already figured out. ;-)

There are three zones. I'd forgotten about that. 58k is most likely oversized, at least everything else is in this area.
But yeah, I'll get him to add up the base board for each zone.

j a_2

What is his factor value for hot water? If he has a large draw fixture he is not a candidate for a combi...On the other side short cycling can be an issue, me I can’t stand the short cycleing
boilers...Don’t chase rebates, there is a very good reason for rebates....As a former plumbing and heating small business owner rebates sucked, and caused issues because customers heard at work so and so got a pissa rebate, I gotta get me one of them...jmo