Vaillant 831 vs Worcester-Bosch 30Si vs ?

Our 25-yo Netaheat suffered a fault last night. It turns out it's the fan and we decided to have it fixed for one last time.

My very friendly installer only does the Vaillant 831 and the Worcester-Bosch 30Si, and as I would prefer to use his services, I thought I would investigate. If I read their specs well (I could not find much on the Vaillant website) they are pretty marginal for my property (3 bed detached, 2 baths, Scotland; the W-B site recommended the 42CDi or the Highflow 440CDi), but let's not dwell on that.

When the time comes, I would like to get something that will last, is proven but reasonably state of the art and is good value for money. In terms of cars, I would buy Honda post face-lift, not Rolls, nor Lada and I buy used.

Back to boilers, in terms of features, I (think) I would like weather compensation, for example. Also, they both appear to modulate, but who can tell how low they modulate? I also (think I) would like stainless steel exchangers; have these got them?

Importantly, how reliable are these? I understand they are pretty popular brands, so spares are unlikely to be a problem. And that's a big thing; I had a look at Broag (hail Dr Drivel) for example and they don't appear to have installers north of Hadrian's Wall.

Thanks in advance,

Reply to
Kostas Kavoussanakis
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Try here, for full installation instructions:

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here for the controls:

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property (3 bed detached, 2 baths, Scotland; the W-B site recommended

Marginal for hot water or for heating?

The vaillant's have SS HEs, the WB's use silicon coated ali. Modulation range is something like 3:1 IIRC on both (so a 24Kw unit will go down to

8kW). I think you probably need the WB CDi range to support the posher controls like weather compensation - although that may only apply for split temperature operation with a cylinder rather than a combi)

Difficult to sort opinion from data here, but they seem to be up the better end of the market. The Vaillants also support quite detailed internal diagnostics (accessed via a USB port and PC based software).

Reply to
John Rumm

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> and here for the controls:

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thanks John!

Well, I changed the settings of the search from 2 bathrooms to 1 plus shower room, and it returned the 30 and 37 CDi and the 30 Si. But the search is pretty basic:

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> Back to boilers, in terms of features, I (think) I would like weather

The page you linked above suggests the following central heating heat output range for the ecoTEC plus 831: 8.7 - 24.0kW. Does this mean that the lowest it will modulate is to 8.7, or is this a different measurement?

I read and saved your post last week re added parts for W-B.

Does modulation happen anyway as a result of demand variation, or does one need suitable room or external sensors?

Re Vaillant, is the only option for 2 zones the 358-quid Vrc 430 + Vr

61 2 Heating Zone Kit, or can I just add another zone using any stat and a valve?

So many options, such high costs...

Cheers,

Reply to
Kostas Kavoussanakis

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No, that's the minimum output...

No, modulation will happen anyway. Without a weather compensator, the boiler will maintain a set flow temperature, and also possibly attempt to maximise condensing efficiency by keeping the return temperature lower if it can. So as the return temp creeps up it modulates the power down.

The VR430 is usually about £100, and the VR61 about £70. There are quite number of ways the above beasties can be configured, for multiple zones

- some offering temperature mixing for things like UFH. You could (I expect - not read through in detail) do a bog standard S+ plan system with ordinary stats and zone valves for a fixed flow temperature all over. Or you could go for the weather compensated version and have two zones running with compensation - there is another add on control to go with the VRC430 if you want fully temperature sensing stat on the second zone.

Costs are not so bad if you shop about a bit.

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Reply to
John Rumm

Both types are probably pretty reliable. I've been fitting W-Bs for a while and only know of one, predecessor of the current ranges, that's given problems with the boiler itself (if you Google back a few weeks there's also an issue with Y-plan controls which AIUI may not be confined to W-B). I've had 2 or 3 Vaillants (only one of which I fitted: the only one I've fitted) that have had problems ranging from howling noises when running at full bore to leaking heat exchanger seals.

W-B's tech support is not always 100% on the ball with the more obscure problems (such as the Y-plan issue) but at least they answer the phone. Vaillant's are, IME, a complete waste of time. They actually suggested it was better to email them and they'd call me back (which to be fair they did, but that's not much help when it's next day and the customer's still without heating in the middle of winter).

Reply to
YAPH
Reply to
Kostas Kavoussanakis

This is all extremely useful, esp. as, if I can afford it, I would also like to move the boiler in the (unfloored) loft.

I should say that Broag (Remeha) emailed me back within a day with the details of a local installer that does their range.

It will be interesting when the time comes for me. I think the challenge is to find a recommended fitter who supports the improved controls and features (and even more basic things, like combi vs system), while offering a personal service. In the meanwhile I will keep collecting information.

Many thanks!

-- Kostas

Reply to
Kostas Kavoussanakis

You mean for having weather compensation? My guestimate is it might reduce the gas bill by about 10% - some gain from extracting a few % more condensing efficiency from the boiler, and the remainder from maintaining better temperature control with fewer overshoots in temperature. So you can translate that into money and payback time based on your bills. In my case its worth having, since it will payback fairly quickly.

To an extent its a simple or as complex as you want to make it. You could go for complexity not much dissimilar from a normal S or Y plan system without any need for blended temperatures. Let the weather compensator set a flow temperature for the whole house based on conditions and then have conventional zones that all run at that temperature.

With a "normal" install you set the boiler flow temperature with a knob on the front. It then generates water with that as a target flow temperature. (apart from when recharging a hot water cylinder where it may run hotter). If you set the temperature high then you reduce the boiler efficiency, if you set it low then you may not get the house warm enough on cold days. This would lead to a certain amount of manual intervention to get best performance from the system. The weather compensator automates this, by dropping the temperature to the minimum necessary to heat the place, it also feeds control to the boiler based on actual temperature data rather than just "on" or "off" requests.

No, that falls into the scope of what a prog stat (and by extension the weather controller does anyway).

Where is gets more complicated is when you want say a flow of water at

60 ish for a rad zone, plus say 50 for an UFH one at the same time. The boiler can only produce water at one temperature (even if the weather comp has final say as to what temperature that actually is). Here you need a blended circuit, with a variable mixer and sensors that can run the cooler zone using some primary flow water mixed with some cooler return water to keep that zone working at a lower temp. The VR61 will also let you do that (in combination with the right valves) if you want.

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You need "more" in that you also need the zone valves. I don't think you need any extra electronic kit (the external sensor for the 430 comes with it, and a cylinder sensor comes with the VR61)

Reply to
John Rumm

How much of this can one get with a (programmable), decent, digital room-stat, like the CM907? I mean, do you get better temp control and fewer overshoots with the modulating room stats?

Is this correct even in modulating boilers (w/o external or modulating temp sensors)?

Two things to say here:

- Thanks!

- Wow!!!

(probably out of my scope this, but way cool).

Ah, thanks for this. Yes, the extra valve is unavoidable for the zone (and I guess one can use any valve, rather than a Vaillant own thing?).

This has all been very enlightening, many thanks.

Reply to
Kostas Kavoussanakis

In general the digital stats are usually a little better than the mechanical ones, however they are all feedback based systems ultimately. Some of them "learn" the characteristics of the house (so called optimising stats) and attempt to anticipate when to turn on an off based on assumptions derived from past data.

A weather compensator is fundamentally different in that it is partly a feed forward system, and also offers proportional control. The feed forward aspect is that it takes account of the actual outside temperature as an input parameter. It then also has an addition user selected input which is the response curve (you choose this match the type of construction of the house - steeper curves are used for properties with poorer insulation). In operation, it also is not limited to "run" / "don't run" control, but instead can demand "run at 53 degrees flow" or 60 degrees flow etc. The demanded temperature can change dynamically to match the heat loss curve and also the outside temperature. So during the space of a day you would expect it to sweep the flow temperature over a range of values to maintain a comfortable environment in the house. Hopefully this variation of demand should track the heat losses such that the temperature does not overshoot, and you save some energy that would normally be lost heating the place to above the required temperature. Because the compensator is using the "right" flow temperature at all times, then the boiler should be running at the best condensing efficiency it can in the circumstances.

Yup generally. The modulation will vary the input power to prevent the set point temperature being exceeded - some may also optimise the return temperature for better efficiency. Most people will set the flow temperature to be "enough" to cope with most days, then perhaps tweak it up on cold ones.

(with older non condensing boilers, conventional wisdom was to leave the boiler state set to a high value to ensure the return temperature was above the dew point as much as possible. Thus preventing condensing as much as possible - since this would corrode the HE rapidly)

Don't take all of this a gospel on the internal operation of the Vailliant - I don't have visibility of their algorithms, and can only base this stuff on standard control theory and what they publish. The general thrust ought to be about right - the actual detail of how control is achieved can vary.

Yup off the shelf valves etc should be fine.

Reply to
John Rumm

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