New Combi Boiler questions (possibly Vaillant Ecotec)

[Locoblade]

Hi All

Im new here so go easy on me please, and apologies for the lengthy initial post

We've currently got an old (very old!) Baxi back boiler with gravity fed hot water tank which is all well past its sell by date, so when we extend the house this autumn we intend on getting a new combi system installed in the extension, plumbing onto the existing rads.

Firstly can anyone advise on how best to go about accurately sizing a combi? Our house is a marginally bigger than average 3 bed semi in Hampshire, built in the 1950s. We have concrete floor, (old-ish) UPVC double glazing, good loft insulation and reasonable cavity insulation - we couldn't get the entire house cavity insulated because the first floor front and back walls are tiled so don't allow access to drill/fill, but most of it is done.

Using a Whole House type boiler size calculator, it says we only need around 10-11Kw of heating output but I've seen it mentioned that this calculation doesn't work for Combis? Im guessing though that I can use that figure to spec a combi that modulates below ~10Kw on CH? If so, does that mean the size of the boiler doesn't make a lot of difference, at least in terms of it heating the house? Is there an efficiency issue still though, or will a 30kw boiler running at 30% capacity to supply 10Kw be as efficient as a 20kw boiler running at 50%?

Regarding hot water requirement, we currently have two adults living in the house, 1 bathroom with electric shower which may become a Mira type shower run from the combi when we re-do the bathroom next year. We rarely if ever run a bath at the mo but if kids come along in the future that may change. Downstairs there's obviously the kitchen sink to supply, plus another two sinks in the extension (utility room and washbasin in the loo), although I don't see them being used more than one at a time very often at all.

We've had a plumber friend-of-a-friend around to have an initial look and he said he'd be recommendeding something like a 30Kw Gloworm CXi for our requirements, but having become an "internet expert" by reading various sites like this (8)), the boiler that seems most recommended is the Vaillant Ecotec Plus series, which I think would point us at an 831 (31kw DHW?) if wanting one roughly equivalent to the Glow Worm CXi that's been recommended?

Im happy to spend the extra cash on a good boiler as the Glow Worms seem to have mixed reviews, and being a techie type I'd like to go with the all singing/dancing control options with weather compensation etc. The question is really though, is a 30Kw boiler roughly what we'd need, and is main sizing consideration more for the increased hot water flow rate of the bigger boilers rather than CH capacity?

Regarding the choice of the Vaillant, the concern we have going from a hot water tank to combi is losing the reliable/quick supply of hot water we have now, and also how often the combi system loses pressure and shuts off. We previously lived in a house with a cheap combi and that put us off a little, as it took a while to run hot water and also lost pressure quite regularly and shut itself down without notice until I re-pressurised it again.

The hot water speed issue I know can be helped with the "Aqua Comfort" option on the Vaillant pre-heating the water heat exchanger every so often, but I can't find anywhere that can quantify how much additional gas this would use? I believe though this can be controlled via the control box (VRC430) so it's only running at times when hot water is most likely going to be required, but Ive also seen comments saying its not really worth having on anyway?

Regarding the pressure issue of combis, I know this is a bit of an unknown and will need doing on all combis some time or other, but how often/quickly should you expect the pressure to drop, because our old one seemed to leave us with a cold house about once a month because it had lost pressure and shut down? Can the higher spec boilers like the Vaillant monitor this and give a warning beforehand via the control unit or even re-pressurise themselves, or do they all just shut down without any warning too and rely on the user to keep them primed?

Finally regarding the VRC430 weather compensation controller, Ive seen it mentioned a couple of times on here and elsewhere that you need a north facing wall to mount the external temperature sensor? Our house is on the diagonal if you like, so the three exterior walls face NE, NW and SW, with the boiler external wall facing NW, is this likely to cause a problem?

I know some will say that my plumber/heating engineer should be advising on all this but although CORGI registered and very experienced, he's a bit "old school" and Im not sure he's totally clued up on the latest higher spec boilers and may just be recommending based on what he always fits, so I'd like to form my own opinions as well. That probably suggestd that I should change my engineer for someone with more expertise/knowledge on the boilers Im interested in, but I always like to give work to friends / personal recommendations where at all possible so would prefer to continue with him if I can.

thanks in advance

 

[Locoblade]

Apologies to those that read this far, didn't realise it was quite that long until I re-read it after posting

 

[Andy165]

Hot water cylinder with cold water storage tank {50 gallons) usually supplies about 20 litres a minute hot water. The vaillant ecotec plus 837 condensing combi can supply 15 litres a minute and you can turn off the preheat function by the programmer. Would recommend getting system cleaned/ powerflushed first and then adding a spirovent dirt buster to the ch return pipe if getting new boiler. Alternatively you could get a vaillant system boiler and keep the hw cylinder supply and have your ch supply on the sealed system. If your ch system and radiators are in good condition you should only have to top up the system pressure about once every two months and sometimes even longer. The programme display will give you a fault code when the pressure is too low. You can also press the bottom left hand minus button on the control panel and it will tell you the current pressure. Don,t know about weather compensators yet, never had a customer ask for one and the extra costs are often offputting and can lose you a job on price quote.

 

[Deleted member 105431]

Go for the Vaillant ecotec 837 or 831. Most reliable and efficent combi on the market and the heating output is downrateable to 5 kw. Make sure you get the system cleaned out first and would recommend a magnaclean is fitted on the c/h return. I have fitted loads of these with no problems at all.

 

[Andy165]

Forgot to say, the condensing combis require a higher output to give a better hot water flow rate per minute . e.g. 24kw boiler 9.6litres a min. at 35 deg. c rise, 28kw 10.6 litres, the vaillant 837 15 litres. The central heating of your house would probably be about 18kw or thereabouts and the boiler will modulate according to the heating requirements. A system boiler would have to sized according to the heating requirements of the ch and hw combined .

 

[simond]

The VRC weather sensor can be mounted in an area that gives the best representation of the ambient outside temperature. Therefore it should not be on a warm wall or in direct sunlight.

You can use the VRC430 and VR65 with a conventional cylinder because the Vaillant electronics have no dea what cylinder they are connected to.

You may think you are doing your plumber a good favour with this system, but if he is 'old school' it may not be a job he wants!

Weather Comp is nothing new and if a heating installer doesn't know what it is he must be totally disinterested in his trade.

 

[Locoblade]

Many thanks for the replies chaps, and thanks for sticking with it reading the long post, I just like to get all the info in there as Ive lost count of the number of threads on here and in forums in general where the first reply is "need more info"

Andy/Dangermouse - would the 837 go as low I thoretically require, as the Vaillant website seems to suggest it only modulates to around 12kw? I strongly suspect we'd need more than that anyway as our double glazing isn't great and we have quite big windows, plus I obviously couldnt account for only 75% of the house being cavity filled when using the whole house calculators.

Simon - I may well be doing "my" plumber a disservice by saying he doesn't know about them, but I believe the work he does (for local authority mainly I think) is more often than not general plumbing with the odd new boiler, so I suspect he'll be more clued up on cheap 'n cheerful brands with no frills, hence my reservations. Because of that I'm certainly not against the idea of bringing in an unknown, and having looked on the Vaillant website for a list of their accredited engineers, there's one based within 100 yards of my house so I may give them a call too to compare prices and expertise.

With regards to having a new cylinder and not going combi, we could do this but the airing cupboard is in our bedroom which makes it extremely hot in summer, and makes the room's radiator redundant in the winter, so ideally I'd like to lose that "feature".

 

[gas4you]

Most reliable and efficent combi on the market and the heating output is downrateable to 5 kw

Lowest setting is 9Kw for heating.

 

[Locoblade]

Considering the non combi option for a moment where we'd keept the tanks in their current location, are there any statistics on how uneconomical that is in "normal use" compared to a combi, if at all? I obviously understand the theory of storing hot water and it wasting energy going cold if its not used, but how much heat would a well insulated tank actually lose? Also, if its a well insulated brand new tank, presumably our airing cupboard wouldn't get nearly as hot, so we wouldn't have such an issue with that either?

 

[simond]

Different manufacturers have differing levels of insulation. In a bedroom no sort of heat is desirable in the summer.

Some insulation types deteriorate after manufacturing; it is also important to lag the pipes from the unit or this compromises the insulation of the cylinder by water circulation.

With respect to the combi versus cylinder argument, this is best had with an experienced savvy installer on site who can measure your mains flow rate and ask all the right questions. It is a mistake to design your system on a forum, there is just so much we can't see.

A Vaillant uniSTOR is an ideal partner for the boiler, if you don't go down the combi route.

 

[Locoblade]

Yep I completely understand Simon, I'll certainly be getting my plumber around for a proper quotation to see what he comes up with, and will also get the local Vaillant listed installer to quote so if nothing else I can compare their expertise and see how they reach their recommended install.

I must admit though, this process has made me appreciate that its far more complicated than simply gutting the old system and putting a reasonable combi on instead and I can certainly now see how good engineers earn their money, so unless the recommended friend can convince me he knows his stuff and isn't just a plumber who's done a few heating systems, I'll be looking elsewhere.