Boiler / HW Cylinder choices

[nickd01]

Morning all,

We're in the middle of a whole house refurb, and the builder is going to put in a heat-only system boiler (They prefer Valiant or Worcester Bosch) and a 300L Megaflo. This is to work with wet UFH on the ground floor, radiators through the rest of the house, and 3 bathrooms. I'm just doing some research so we don't get their standard install as it 'works' as I'd like to put a bit more thought in to it.

I've got a few questions, as if I go against their recommendation (Which they've said they're happy to do) then I want to ensure I'm doing the right thing.

- I like the Vitodens 100W heat only, as it goes down to 3.2kW min output; I understand this is important to reduce 'cycling' of the boiler. The Valiant only goes down to 5.2kW so I assume the Vitodens is more flexible and should be cheaper to run?
- Is weather compensation is important, all boilers seem to support this now?
- Is OpenTherm support necessary? If not, is Valiant / Vitodens or WB a better platform to buy in to?
- I've seen that "Hot Water Priority" is also important, is this boiler specific? I can't see if the boilers support it

Is there an independent person / company I can call and pay for an hour of advice?

[nickd01]

Thanks both

I understand the thinking behind being able to properly control and manage the boiler is likely more important than chasing efficiency numbers on a chart. I've used Tado before, but this area confuses me...

Some things I've read are to literally leave the weather compensation to do the hard work and don't turn off radiators or add room thermostats
Yet, some systems have connected TRV's on each radiator feeding back to the central 'hub' and everything gets controlled individually.
Others stick temperature probes on to pipes and the platform uses that to keep it efficient along with diverter valves and all sorts of magic

We're likely to use the Heatmiser controls for the UFH but what do we do upstairs? I've seen the Valiant platform configured on UrbanPlumbers website and it seems to make sense. Maybe I go with that platform.

Sorry, there seems to be lots of options and I'm just trying to get my head around them all.

[nickd01]

Thanks.

The rad's are slightly over-sized (A lot of this depended on available space on walls etc) - but possibly not as much as they should be.

I'm hoping the concrete slab on the ground floor and UFH will keep the house warm without having to boost the bedrooms too much with radiators. Despite being an old detached house, the renovation has added loads of insulation on both the existing house and the extensions so I'm hoping it'll help overall reduce the heating requirements.

[nickd01]

We looked at a thermal store (I think it was through Octopus) - but it was only suitable for smaller homes - I think 1500sq ft or less.

I'm looking at Oso as an alternative to Megaflo - they appear to be more compact and have what appears to be a high quality of insulation around them.

I think we're going to stick with the System boiler / Hot water tank setup for now; but it looks like there's a lot that can be done to optimise the install.

I put a call to the Heating Hub, so I'll see if they can guide.

[nickd01]

HarryW said:

When you say heat only system boiler, do you mean open vented heat only or just system boiler.
The reason I ask is the Vaillant 400 series open vent goes down to 5kw ish but the new 600 series system ones go down to 2.9kw.

First thing is to ensure you do a proper heat loss calculation on your boiler sizing, no point getting 35kw one when a 25kw or 15kw will do.

Re PDHW this is how Vaillant are designed, they allow for different power levels between DHW and CH, however you will if having a vaillant boiler need to use their controls to do that though. My 418 boiler actually has 18kw of heating but in PDHW mode can produce 24.5kw for DHW.

What size of property do you have and how many baths/showers? as 350ltr unvented is pretty big and perhaps a waste of energy. Agai. I have 1 bath 2 showers and most times my 210ltr tank is pretty big enough to heat once a day. It’s easy to boost it if you start to run out, mine only takes about 20 minutes to heat…

It's approx 2500 sq ft, 3 showers, 2 baths and 6 people.
From what I've read (Always dangerous!) - a property similar to ours needs as little as 12kW to heat but it's the DHW that requires more 'power' to warm up. So I'm looking at around an 18-24 kW boiler but this is based on a hunch rather than a calculation.
It looks like the heat-only boilers are less common, and the system boilers with the expansion vessel in seem to be more available as well as more advanced. I can't see why I'd chose heat-only, as the one with the expansion vessel in is the same size so surely it'd save me some space in the cellar where the hot water tank goes?

[nickd01]

HarryW said:

As previously stated the Vaillant 600 series system boilers were new to market last year and therefore state of the art, modulating to below 3kw.
Get a vaillant engineer to design and quote for your system, you also get up to 12years warranty with them doing it that way too.
Links to your local one will be via this.
https://www.vaillant-advance.co.uk/findaninstaller

What ever you do though is get what you need, not what’s easiest for your installer to fit as it’s something you should only be doing every 20 years or so.

I have the vaillant controls and can’t fault it myself, they are a bit Germanic in their thinking but it’s worth understanding it. I have the sensoComfort controls with outdoor temperature sensor….

Thanks, our on-site chap is Valiant certified so that's his preferred boiler. I'll take a look at these.