Secondary DHW loop on a combi?

[stonehouse]

The only thing I don't like about combi's is latency. It takes about a minute for the hot water to arrive and another half-a-minute for the temperature to stabilize. This is the reason why I've been thinking about other types like conventional open-vented or thermal stores with PHE but from reading the following thread (see link below) it seems so complicated.

https://www.diynot.com/diy/threads/convert-existing-vented-cylinder-to-thermal-store-and-phe.360877/

So, instead I'm now wondering whether it's possible to install a secondary DHW loop on a combi to fix the latency problem. Can it be done and if so, how?

 

[echoes]

Well all things are possible, but without a suitable buffer, this won't work.
If I understand the question, you are wondering about keeping a pumped loop of mains pressure water constantly heated by the boiler so that hot water is pretty much instantly available at the taps?
One problem (among others) with this is that as soon as hot water is drawn off, it needs to be replenished quickly with more hot water, and as you say, with combis (or any boiler) there will be a delay.
The good news is that combi boiler manufacturerers have solved this problem by keeping a smallish quantity of DHW pre-heated.
If latency is your only concern, and you're happy with the other pros and cons of combis, maybe a storage combi might be worth considering?

You might also consider having a combi supply some taps/showers with DHW and have other taps supplied from a cylinder.

Merry Christmas!

 

[JohnD]

Do you currently have a combi? Or a cylinder?

 

[stonehouse]

......

 

[stonehouse]

If I understand the question, you are wondering about keeping a pumped loop of mains pressure water constantly heated by the boiler so that hot water is pretty much instantly available at the taps?

Yes, I would like hot water pretty much instantly available at the taps. I was thinking a schematic something like in the attached diagram below. Can the circulation run in convection?

The good news is that combi boiler manufacturerers have solved this problem by keeping a smallish quantity of DHW pre-heated.

Great! Which brands have that?

Merry Christmas.

 

[stonehouse]

Do you currently have a combi? Or a cylinder?

None actually. My house (recent purchase) is currently undergoing a major refurbishment.

 

[echoes]

Great! Which brands have that

Most of them I believe. Google is your friend.

If you're undergoing a major refurbishment, is there any reason why an unvented cylinder would not fit the bill?

 

[echoes]

...a schematic something like in the attached diagram...

That's not going to work unfortunately. Well, it will supply hot water, but the loop does nothing; there is no means of keeping the loop hot. And the taps at the 'end' of the loop will wait even longer for hot water.

 

[hertsboilers]

Most of them I believe. Google is your friend.

If you're undergoing a major refurbishment, is there any reason why an unvented cylinder would not fit the bill?

Agree! Go for a system boiler with a unvented cylinder.

 

[dilalio]

And a secondary return/recirc.

 

[hertsboilers]

And a secondary return/recirc.

Why reinvent a wheel?

 

[dilalio]

Why reinvent a wheel?

Exactly. Does all depend on OP’s incoming main though

 

[stonehouse]

Thanks guys for all the replies. No, I can't think of a reason why an unvented cylinder would not fit the bill. It's just that I don't have any experience with these.

I like the look of the following diagram (from the Building Regs), which is sort of an intermediate system (unvented DHW and vented primary)

 

[Dan Robinson]

What makes you think it is "intermediate"?

 

[jeff the gasman]

It won't work on a combi.