25kW or 30kW combi boiler?

FataVerde

Hi, I live in a 1BR Victorian conversion flat, but I intend to expand into the loft to create another double bedroom with a small (shower) en-suite. Would a 25kW boiler cover me for the extension as well? Or should I go for the stronger 30kW? I only have 2 small single rads in the lounge (replaced a large double rad), one in the bedroom, a single one in the bathroom and an unused large one down by the groundfloor entrance. With the extension, I'd add 2, max 3 rads. 

Trying to decide between the Vaillant ecoTec pro 30 (which shows heat output of 25 and water output of 30; with LPM 12.3) and the Vaillant ecoTec pro 24 or 28, which have lower LPMs of 9.6 and 11.1. 

I'd go for the 30 but worry it would be oversized for me now with only 4 rads and might increase my bills.

markin

The valillants are the best at modulating down when used for heating

daveyjp

You need to know your cold water flow rate.

FaceHead

I don't see why an oversized boiler would increase your bills. Only the installation cost.

If you were to get a radiator sizing calculator out and calculate the radiators needed to heat your rooms you'd probably not get past 10kW for a 1BR flat. 

It's all about hot water. 

Adding a second bedroom (and person!) will mean it's more likley that someone might want to wash the dishes/their hands while another is in the shower - what happens currently if you wash the dishes with someone in the shower? 

If you want to improve this situation, you might upsize the boiler. Are you adding a second bathroom? If you aren't adding extra hot water appliances, I would stick with the size of boiler you know works - if it works satisfactorily currently. 

FataVerde

FaceHead said:

I don't see why an oversized boiler would increase your bills. Only the installation cost.

If you were to get a radiator sizing calculator out and calculate the radiators needed to heat your rooms you'd probably not get past 10kW for a 1BR flat. 

It's all about hot water. 

Adding a second bedroom (and person!) will mean it's more likley that someone might want to wash the dishes/their hands while another is in the shower - what happens currently if you wash the dishes with someone in the shower? 

If you want to improve this situation, you might upsize the boiler. Are you adding a second bathroom? If your has it. aren't adding extra hot water appliances, I would stick with the size of boiler you know works - if it works satisfactorily currently. 

Thank you for this! Yes, when I wash dishes while someone is using either the shower or the sink, I get less water (that's pressure though, isn't it?). I have just moved into the flat by myself and haven't done it often enough to notice impact on heat. But I'd like a higher l/m value and the 30 boiler has it.

FataVerde

daveyjp said:

You need to know your cold water flow rate.

I Do you mean my tap water flow? I listed the boiler l/m in my post.

daveyjp

Yes.  Flow of water per minute at a cold water tap, usually the kitchen.

For a 30kw boiler it needs to be higher than 12.3 LPM as the boiler can't heat more water than is flowing through it. 

FataVerde

daveyjp said:

Yes.  Flow of water per minute at a cold water tap, usually the kitchen.

For a 30kw boiler it needs to be higher than 12.3 LPM as the boiler can't heat more water than is flowing through it. 

That makes perfect sense. Boiler has to work with what it gets. Will check the flow. 

Bendy_House

Hi Fata.

As said above, you do need to check your incoming mains flow. For either of these combis, you really want it to be in excess of 20lpm, tho' you might 'get away' with lower. So check that first - pan under cold kitchen tap, turn on full for 10 seconds, times the litres produced by 6.

You need that extra flow because there's more going on here than 'hot water delivery'; you'll need to blend in cold water to get a good shower, and you'll want the flow to be as unaffected as possible by other taps being used. So, really, even a pants 25kW combi should have a solid ~20lpm driving it.

If your flow (and pressure - yes they are different, but kinda linked) is borderline, then you will have issues when folk turn on taps/flush loos when someone else is having a shower. That's pretty much unavoidable, but doesn't have to be an issue as the shower mixer will be thermostatic, so 'all' you'll experience is a temporary drop in flow.

Unless there's a good reason to go 25kW, I'd recommend you avoid it - really, the DHW flow is pretty poor. A 30kW will be fine - hot taps will give a full flow, and showers should be great (provided your incoming whatsit is youknowwhat).

The next issue is, will the 30kW be too powerful for a smallish flat? It shouldn't be, since they modulate right down - like turning down a gas hob flame - to suit demand. Check the min CH output rate of each boiler - see if there's one that goes down to ~4kW.

You've swapped some rads in your flat? I'm guessing you didn't choose (over)sizes that can run on lower water temps? The idea is to have rads running on coolish - ~45oC - water temps, so that the boiler is running at its max efficiency. You can only get enough heat out at these sorts of temps if the rads are larger.

So:

1) Check your incoming lpm

2) Check the min CH kW output of each boiler.

3) Don't consider a less-than 30kW unless you really have to (imo).

4) If it ain't too late, fit larger rads, especially in the main living areas that'll be used more constantly.

BUFF

Bendy_House said:

4) If it ain't too late, fit larger rads, especially in the main living areas that'll be used more constantly.

They don't have to be physically larger in terms of width & height but may be deeper to add an extra panel or fins - what you are after is higher output.

shiraz99

BUFF said:

Bendy_House said:

4) If it ain't too late, fit larger rads, especially in the main living areas that'll be used more constantly.

They don't have to be physically larger in terms of width & height but may be deeper to add an extra panel or fins - what you are after is higher output.

^This, the difference in heat output between a single type 11 (K1) radiator and a type 21 or 22 is quite a bit, especially if you want to turn the boiler down to run at a lower temperature.