Combi boiler replacement - upgrade to 35kw or stick with 30kw?

noitsnotme

Our old Halstead Ace HE 30 has come to the end of it's life after a long service.  I'm looking to replace it with a Viessmann Vitodens 050-W or a 100-W, which are both available in 30kw or 35kw.

Our water flow was measured at 18lpm so the fitter has advised upgrading to a 35kw boiler.  A possible downside to this is the existing gas supply might be undersized and if that needs upgrading will require some long external and potentially ugly pipe runs to navigate a particularly awkward route around a single storey extension to reach the boiler which is situated upstairs in the bathroom.  Apparently its 50/50 on whether the supply will need upgrading or not but because the existing supply pipes are hidden within walls and floors they can't calculate it before hand.

The current Halstead boiler is a 30kw and we've never had a problem with the shower or bath, both always being more than hot enough.  We're adding one new radiator which will bring it to 11 in total.  It's a large-ish 3 bedroom, 1 bathroom, semi.

My thoughts are if the existing 30kw Halstead has provided enough hot water, then a 30kw Viessmann will be fine.  Or should I be upgrading to the 35kw and accepting the potentially required but not ideal external pipe runs?  Is the 35kw more than I need anyway considering the relatively small number of radiators we have?

ThisIsWeird

noitsnotme said:

Our old Halstead Ace HE 30 has come to the end of it's life after a long service.  I'm looking to replace it with a Viessmann Vitodens 050-W or a 100-W, which are both available in 30kw or 35kw.

Our water flow was measured at 18lpm so the fitter has advised upgrading to a 35kw boiler.  A possible downside to this is the existing gas supply might be undersized and if that needs upgrading will require some long external and potentially ugly pipe runs to navigate a particularly awkward route around a single storey extension to reach the boiler which is situated upstairs in the bathroom.  Apparently its 50/50 on whether the supply will need upgrading or not but because the existing supply pipes are hidden within walls and floors they can't calculate it before hand.

The current Halstead boiler is a 30kw and we've never had a problem with the shower or bath, both always being more than hot enough.  We're adding one new radiator which will bring it to 11 in total.  It's a large-ish 3 bedroom, 1 bathroom, semi.

My thoughts are if the existing 30kw Halstead has provided enough hot water, then a 30kw Viessmann will be fine.  Or should I be upgrading to the 35kw and accepting the potentially required but not ideal external pipe runs?  Is the 35kw more than I need anyway considering the relatively small number of radiators we have?

I suggest your answer is in bold above. The additional radiator is will have next-to-zero effect on demand, assuming your house isn't huge and already on the edge of the boiler's CH performance. Extremely unlikely.

With combis, the kW figure relates to the DHW performance, ie how many litres per minute of hot water it is capable of delivering. In terms of what the radiators need, both the 30 and 35kW figures are WAY in excess of what they will require - which is typically 10 to 15 kW for most properties. So, only go '35' for extra hot water flow from your taps.

If starting from scratch - and I was in this boat a good decade+ ago - I would probably suggest the 35kW version, just for that wee extra showering and bathing oomph. But, if the small additional performance from 30 to 35 required a whole new gas pipe run, and this was anything other than completely straight forward, I'd personally stick with 30kW.

You are happy with 30kW, and so am I. Cracking showers, and ouch-hot dishwashing.

FreeBear

Both the 050 and 100 have very similar ratings for DHW flow rates.

30KW - 12.2 l/min @ 35°C temperature rise

35KW - 14.3 l/min @ 35°C

If you are prepared to drop to a 30°C rise, the flow rates are 15.6 l/min and 17 l/min respectively. If you want hotter water, just turn the taps down to limit the flow rate.

For comparison, the 111 (32KW) will provide 18 l/min @ 35°C, but it incorporates a 46l storage tank which will cost you money to keep heated. I discounted that particular model for just that reason when i was shopping for a new boiler earlier in the year (ended up going for an 050 30KW).

Viessmann claim a 98% efficiency for the 100, and a slightly lower 92% for the 050 - Unless you can run with low flow temperatures, you are unlikely to hit 92/98% with either boiler. low flow temperatures need bigger radiators if you want to maintain a comfortable room temp. How much bigger really depends on the insulation levels and number of cold draughts.

At the moment, some of the online installers (Boxt, Heatable, etc) are offering 10 or even 12 year warranties on Viessmann 050 30KW boilers and prices for a straight swap from around £2200.

Early days, but I'm liking my Viessmann aside from a few technical quibbles. Just waiting for the weather to turn cold so that I can tune the system and see how it performs.

noitsnotme

Thanks both, that’s really useful.

@FreeBear - the local fitter we have the quote from is a trained Viessmann installer so we’d qualify for the 10 yr warranty on both models (the 100 can be increased to 12 years for an additional £200).  And the total price they have quoted, which includes adding the one extra radiator and replacing 2 others, is very reasonable.

I’m not sure the 100 is worth the additional £300 over the 050.

Eldi_Dos

Would adding a weather compensation sensor to new installation make a difference to boiler output you choose.

And could you put a weather compensation sensor in the loft space above the insulation rather than outside wall.

noitsnotme

Eldi_Dos said:

Would adding a weather compensation sensor to new installation make a difference to boiler output you choose.

And could you put a weather compensation sensor in the loft space above the insulation rather than outside wall.

I am planning to add the weather compensator.  Apparently that works best on a North facing wall and our boiler is on the inside of a North facing wall so it should be easy to fit.  I have no idea if that influences the output I should be considering.

ThisIsWeird

noitsnotme said:

Eldi_Dos said:

Would adding a weather compensation sensor to new installation make a difference to boiler output you choose.

And could you put a weather compensation sensor in the loft space above the insulation rather than outside wall.

I am planning to add the weather compensator.  Apparently that works best on a North facing wall and our boiler is on the inside of a North facing wall so it should be easy to fit.  I have no idea if that influences the output I should be considering.

It shouldn't.

Both combis will have waaaay more than enough output for your CH system (assuming you don't live in a castle).

The max boiler rating only applies to the DHW heating capacity. If you listen to a combi in operation, it's only when you open a hot tap that it powers up like a jet fighter.

The weather compensator should automatically turn down the CH flow temp rate to as cool as possible, that still gets the job done. The cooler the flow, the cooler the return, and the cooler the return, the more heat iy can extract from the flue gases = more efficiency.

FreeBear

ThisIsWeird said: The weather compensator should automatically turn down the CH flow temp rate to as cool as possible, that still gets the job done. The cooler the flow, the cooler the return, and the cooler the return, the more heat iy can extract from the flue gases = more efficiency.

But to get the maximum benefit out of a lower flow temperature, the radiators need to be sized appropriately for all rooms. This could mean that some/all the radiators need to be up to 50% larger. And in some cases, the CH pipework needs to be bigger too.