Oversized boiler

[Swwils]

My house has a ridicously oversized boiler - it short cycles like crazy.

Would it be sensible to install a buffer tank and run the CH from this?

30kW ideal logic system boiler - I think the minimum modulation is 6.1kW.
250L DHW tank
2 Zones
apparently 11kW demand at -3, although i've never needed to use the upstairs rad zone at all.

I can do plumbing to a reasonable standard, just looking to bounce ideas around.

[Stinsy]

A buffer tank can indeed reduce short-cycling. However the solution to your problem is fitting an appropriately-sized boiler (or better yet a HP).

[Oldgreybeard]

This seems to be a common problem. Our old house had a system boiler that was around double the capacity needed. When we opted to replace it (as it was ~30 years old and inefficient) most of the people we had around to quote wanted to fit an equally oversized one. I think it's just in the mindset of a lot of heating engineers to fit boilers that are way bigger than needed, that way they don't get customers complaining about not getting enough heat . . .

A buffer tank will partially fix the short cycling, but will need to be pretty big to have a decent effect. It will also increase the overall system losses, so may not be the best solution.

[Swwils]

The system was provided with the house so it's only 1 and a half years old, which is my hesitancy to just replace the boiler.

My thinking was that getting a tank would work well with an eventual heat pump and I also have a decent amount of PV so I could also eventually divert there too.

I could also probably pickup a 500L tank for £300 with enough waiting.

I would assume this model of boiler was chosen because it offered the most cashback at the wholesaler lol.

[Moxi]

Swwils

As with everything there's the optimum solution and then there's what you can afford or what you have space for - I tend to procrastinate and lose valuable time designing a "perfect" solution - what I have taken from the forum most recently is to stop overthinking and get on with doing something.

In your case it sounds very much like the tank is a reasonable and sensible modification to what you presently have and as you yourself note its a modification that will be suitable for incorporation into other plans in the future so I would say go for it!

Moxi

[Swwils]

Yeah it made sense in my mind just wondered if there were any major gotchas.

Space/access isn't an issue!

Eyes peeled for someone's tank now.

EDIT:

Just bought 2 x 300L tanks for £90! 20kw coils in them too.

[Moxi]

Don't forget we love photos when you start the project!

Nice score on the tanks too

Moxi

[spread-tee]

I am fairly sure you can change the burner assembly on the Logic as most of the other components are standard. Also are you sure the boiler is modulating down to 6kW on the minimum setting, it shouldn't be cycling too much at that output unless you only have a couple of rads turned on.
Ideal have a pretty good tech helpline, or did last time I contacted them.

Might be worth a try.

Desp

[nowty]

And cycling on a gas system is nothing like as bad as cycling a HP system.

[Swwils]

It's definitely cycling.

Would be very interested if I could switch it to the heat 15. But I don't think the lowest modulation is that much better.

Basically the house is over 3 floors, the lower floor just has a hall, playroom, storeroom and utility. But it's on the same zone as the middle floor.

So the middle floor TRV are almost never open, so yeah just 4 rads needing heat.