How to find modulation ratios?

[jayc89]

I have a Baxi Platinum+ 32kw. It's already on the large size for our house - approx 200m2 and want to double check that as I continue to insulate and make the house more airtight, it's not going to start cycling.

 

My expectation is that our heat demand will drop to approx 8kw once we're fully insulated (and extended). 

 

I can't seem to find the modulation ratio for this boiler, can anyone point me in the right direction? 

[JohnMo]

Obviously didn't try to hard, first hit on datasheet for the above.  10.7kW min while condensing.

 

https://www.baxi.co.uk/our-boilers/system-boilers/baxi-platinum-plus-system

 

[jayc89]

3 minutes ago, JohnMo said:

Obviously didn't try to hard, first hit on datasheet for the above.  10.7kW min while condensing.

 

https://www.baxi.co.uk/our-boilers/system-boilers/baxi-platinum-plus-system

 

 

Urgh. Thanks. I'd downloaded all the PDF docs I could find but didn't actually notice that drop down. Cheers. 

 

I assume a buffer tank will be in order once it begins cycling? 

[JohnMo]

It may be easier keeping most of you heating system open to the boiler, so it has plenty to work on. 

[jayc89]

5 minutes ago, JohnMo said:

It may be easier keeping most of you heating system open to the boiler, so it has plenty to work on. 

 

We currently have 2x zones. Downstairs UFH (approx 520m of pipe, or 62.5 litres of water) and upstairs radiators (7x rads 1x towel rail, approx 40 litres of water, based on the Stelrad data sheet).

 

Both work independently right now and my assumption is that the upstairs radiators will get little use once we're fully insulated and airtight. 

[jayc89]

Using the calculator I found on here suggests (assuming I reach my target heat demand of 8kw);

 

- Without a buffer the boiler would cycle after 9.5 minutes

- To achieve a 15 minute run, I'd need a 50 litre buffer tank

 

> 9 minutes is not the end of the world, so it's probably not something to worry about right now. 

 

[JohnMo]

You have calculated for the best case, which is the coldest day.  Most days will have a much lower heat demand down to about 1kW.  And you need to size for 62.5L, if the radiators are on a different zone and not always switched on.

[jayc89]

8 minutes ago, JohnMo said:

You have calculated for the best case, which is the coldest day.  Most days will have a much lower heat demand down to about 1kW.  And you need to size for 62.5L, if the radiators are on a different zone and not always switched on.

 

That would suggest it's massively oversized, requiring a 600l buffer  Unsurprising really, but a bit of a PITA. 

 

 

[Adsibob]

Are you sure you have done your heat loss calcs correctly? 200m2 of floor space is a fairly large house, must be at least a 4 bed, no?

What construction year and style is your house and how much insulation are you adding?

[jayc89]

11 hours ago, Adsibob said:

Are you sure you have done your heat loss calcs correctly? 200m2 of floor space is a fairly large house, must be at least a 4 bed, no?

What construction year and style is your house and how much insulation are you adding?

 

Assuming I've used Jeremy's heat loss calc spreadsheet correctly.

 

Walls - 0.35 (internal wall insulation so I don't want to go any lower)

Roof - 0.10

Floor - 0.15

ACH - 5 (I was deliberately conservative here as I'm sure there will be problem areas yet to be discovered, but I'm hoping to get significantly below this)