Any boiler can easily become a combi

hot_rod

I will say when a combi boiler fails to make DHW, or flow drops off, it can be a job to repair them. The boiler needs to be down to disassemble the DHW "module", so no heat either.
They are mounted in a tough space to work on. Inside a sharp sheetmetal boiler jacket Proprietary OEM repair parts are usually required to repair.

To build your own external DHW "combi" module you need a circulator, heat exchanger and flow switch with a single switching relay box.
Shop wisely and you could acquire all the parts for well under 500 bucks. E-bay is one thought.

When a faucet opens on the hot side, the flow switch in the cold supply turns on the P:2 and the boiler. Many boilers have a DHW priority connection, so heat loads drop while in DHW mode. Or the double throw relay in most relay boxes could also drop off the heat call.

This allows you to custom size the heat exchanger, add flush valves, and use off the shelf components not specific to the boiler brand. This HX could be removed in minutes taken to a radiator shop for an acid cook if ever needed.

For most homes you would want at least 100,000 BTU boiler. This is my shop and I have it connected to a 55,000 Lochinvar Knight. It covers a hand sink and hose to wash the truck.

Alan (California Radiant) Forbes

When you have the room, it's certainly an option. Thanks for reminding us that we can be creative and have all kinds of options in out toolbox.

I like the angle unions on the HX. Where can I get some?

tim smith

Hotrod, I have always thought it would be nice to have an appx 4- 8 gals or so shell & helix coil heat exchanger for making your own combi. The minimal gals heat x would give you the buffer time for ramp up of boiler firing. With a helix coil you could get a fair amount packed in there for recovery. Make it out of 8 or 10" stainless pipe and stainless coil. Would be a good way to go. Boilers could even be make with side taps that you could attach it to with unions. Maybe pipe dream but think it would work well. Done quite a few with braze plates, they work well too but lack the little bit of storage.

hot_rod

Correctly sized tube and shell tend to be $$. Find plate HX on ebay for 100- $150.

Build one would be easy if you TIG stainless? You need to dome the ends for strength. Probably find stainless tank "heads" if you google them. Then find or wrap a tube to match.

If you strip regular CSST tube, heat it with a torch that will anneal it, soften it.
I was able to wrap 1/2" CSST tight enough to fit into a 2" nipple!

You get turbulent flow with the ridges and more external surface area. Be aware of pressure drop however.

I had a pressure drop chart somewhere for the 1/2, 3/4 and 1" SolarFlex that we sold.

WMno57

hot_rod said:

Build one would be easy if you TIG stainless? You need to dome the ends for strength. Probably find stainless tank "heads" if you google them. Then find or wrap a tube to match.

Could repurpose a Cornelius keg.
https://en.wikipedia.org/wiki/Cornelius_keg
"A Cornelius keg is a stainless steel cylinder that can hold a maximum of 130 PSI. There are three openings in the keg: a large central hole for cleaning and filling, and two ports for pressurizing and dispensing.
The central opening is used for filling and cleaning the keg and is large enough to permit reaching inside for hand scrubbing."
The coil could be sized to fit through this central opening. Then no cutting or welding needed.
A 9 Litre Corny keg is 8" diameter and 15" high.

EdTheHeaterMan

hot_rod said:

I will say when a combi boiler fails to make DHW, or flow drops off, it can be a job to repair them. The boiler needs to be down to disassemble the DHW "module", so no heat either...

...To build your own external DHW "combi" module you need a circulator, heat exchanger and flow switch with a single switching relay box.

For most homes you would want at least 100,000 BTU boiler. This is my shop and I have it connected to a 55,000 Lochinvar Knight. It covers a hand sink and hose to wash the truck.

I love that you even have a Caleffi Boiler and Caleffi pegboard
I would have gone with the standard pegboard from Home Depot

hot_rod

I've been buying tri clamp stainless stuff off Amazon, download the Dernord catalog for building serviceable vessels. A 2" end cap for 8 bucks!

Years ago a fellow called about thoughts for a homemade Hydro sep. He was a welder in the dairy piping industry, here is what he built with the tri clamp in the middle.