What does a combi do for very short DWH calls, like hand washing

In_New_England

We have an indirect currently for DHW. I don't even bother to use hot water for hand washing because of the long run of pipe from the tank to the faucet. Some family members do (they still just get mostly cold water).

If there is such a short call for hot water, will the combi turn on? Or is there a delay built in? I read somewhere that there are flow lower limits too, so a slow trickle won't trigger the boiler. Some of them, like the Noble, have a setting to keep the HX warm. Do such short calls cause heavy stress on the boiler, or are they designed for this?

Zman

They short cycle. This makes for less efficient performance and more wear and tear on the combo.

Tom_133

The combi boiler has its place in the world. Its not going to last like an old cast iron unit, but the price point and relative reliability do make it a good fit in some cases. I have always hated the short cycle. Some manufacturers put a tank inside the unit to eliminate the the short hand wash scenario. those units have a similar life span but are HEAVY when you gotta swap them. Triangle did quite a bit right in the beginning but sort of went the way of the world and got cheaper. The NTI Vmax is a good design but they really are limited in hot water production.

Jamie Hall

If the combi were placed where the hot water tank is now, it won't change the delay to getting hot water for hand washing anyway. That's in the pipes.

pedmec

use a point of use recirc pump and you will have instant warm water.

lochinvar noble combi uses a fire tube heat exchanger. so depending on size of unit it has 1.2 gallon to 2.7 gallon capacity of boiler water waiting to run thru the brazed plate heat exchanger which limits some of the short cycling.

hot_rod

Same delay with tankless style water heaters, they need to respond and fire up also.

In_New_England

I'm sorry my question was confusing for some. The delay in getting hot water was a distraction - I was asking specifically about the cycling.

I suppose though, that this is all built into the design. It is expected that there will x number of short calls for DHW. The Lochinvar Noble as a stay warm option, on by default, which I suspect makes periodic brief cycles, and they design for that.

If we do kitchen and bathroom remodels I think I will add point of use, tankless under sink heaters. Wait, more things to go wrong and have to replace ... but this stuff I can do myself.

I do overthink things.

hot_rod

I had a combi at my mother in laws. She lived alone and used a lot of short hot water demands, maybe a dozen times a day for hand and dish washing. I replaced the unit after 15 years of those short cycling, never replaced a single part in those years. It had developed a few leaks on some of the o-rings inside and we were selling the home, so I replaced it. A Laars mascot, which at the time was a Baxi product.
Yes, those short cycles drove me crazy when we would have a family gathering, but it is the way they work unless you add a buffer.
I suspect manufacturers design all the components with that in mind, a few hundred thousand cycle or more for design criteria.