Underfloor heating (wet) - Vaillant or Worcester-Bosch?

sergiup

Distinguished Member
This isn't actually for me - we're not putting underfloor heating anywhere except the bathroom (and possibly the living room in future, but only as a secondary heat source), so we're going electric for that

However.. I did ponder wet (ie: water based) underfloor heating for a while and a friend of mine is also considering it; he's been told that the control systems in Worcester-Bosch boilers tend to work better with it - does anyone know if there's any truth / reasoning behind that?

Personally I actually went with a Vaillant 837 because they seem to have stainless steel heat exchangers as opposed to aluminium, and I didn't really care about the underfloor heating.

Thank you!
 

SteveCritten

Distinguished Member
Has he considered doing away with the boiler altogether and going for ground source or air source heating?
 

sergiup

Distinguished Member
Hi Steve - I have, however we both live in built up areas which poses a problem with finding a suitable place for the outdoor pump, plus neither one of us can really afford the increased initial outlay. I realise that it may well pay for itself in a few years, but we can't really afford to put either one in while also doing all the other works; it also gives us something to upgrade in future I guess :D
 

SteveCritten

Distinguished Member
I spoke to a friend who fitted his own underfloor system and he just kept the boiler he had in, it was only 84% efficient but he says it's done a great job.
 

blue max

Distinguished Member
I'm a little confused because I put my own system in and they have independent mixing manifolds and pumps. Because underfloor heating runs at approx 45 degrees, the typical radiator temperature is significantly above that. That is why the system mixes returning water with the hot supply to lower the temperature to the working requirement.
The boiler has no part to play apart from delivering hot water. Unless it is solely used for the underfloor heating in which case the only advantage may be that they can run at lower temperatures, but I'd imagine any boiler could.
 

DJT75

Distinguished Member
We've put wet underfloor heating through our approx. 60sqm groundfloor extension, the rest of the hous is still on rad. I was given the choice of Worcester-Bosch, Vaillant or some other cheaper brand. I went with Worcester-Bosch because the bloke that installed it was associated with them which meant I got a 5 year warranty. I'd also never heard of the other 2 but he did say Vaillant are the only other one he'd recommend. I'm petty sure he was just talking about the boiler though, nothing else
 

SteveCritten

Distinguished Member
Yes they have a mixer valve so the old boiler heats up to 65º and goes to both the taps and the mixer for the underfloor heating where it mixes with the return to bring it to 45-50º. It was quite a simple job and although he would have preferred a more efficient boiler he just couldn't afford it at the time.
 

sergiup

Distinguished Member
Thank you all! So it sounds like there may be no good reason to pick one over the other as far as wet underfloor heating goes.
The only thing I could think of (though I haven't verified) is the minimum power level that each one could produce - the lower one may be more suited if only a small temperature difference existed I guess, but that's hardly the case most of the time.
 

blue max

Distinguished Member
Thank you all! So it sounds like there may be no good reason to pick one over the other as far as wet underfloor heating goes.
The only thing I could think of (though I haven't verified) is the minimum power level that each one could produce - the lower one may be more suited if only a small temperature difference existed I guess, but that's hardly the case most of the time.

That would be my view too. I'd say that a correctly specc'd boiler is more important than the brand. IE not too large as it may struggle to produce less output without 'cycling' or going on and off continuously. But you may find that the hot water output is worse with a smaller one, so bear that in mind.

Unless you are on a tight budget, WB or Vaillant are always recommended.

That's if you use a combi of course. We are adding another shower room, so have decided to go for an unvented system with a system boiler. The boiler heats up a cylinder of hot water for instant delivery. And apparently it is easier to wire it all up, but we're not ready to cross that bridge yet, so have no idea why.
 

SteveCritten

Distinguished Member
I wouldn't recommend a combi for an underfloor system. The most efficient system is a pressurised unvented system mine is 96% efficient and you have a lot more control over your water and no need to buy extra bits for showers as the pressure is plenty for a very powerful shower without extra pumps.
 

blue max

Distinguished Member
I wouldn't recommend a combi for an underfloor system. The most efficient system is a pressurised unvented system mine is 96% efficient and you have a lot more control over your water and no need to buy extra bits for showers as the pressure is plenty for a very powerful shower without extra pumps.

Nothing wrong with using a combi though. That's how ours is currently. Our shower is fine, just don't think it would cope with two at the same time as there is obviously a limit to how much hot water can be delivered. Adding a pump is not going to help either as it can't heat the water any quicker. As for 96% efficiency, the boiler may be, but you are storing hot water. That is never efficient.

But for the reasons in my previous post, a system boiler and unvented system is a better system. But only if you need more hot water. The underfloor heating doesn't really make a difference apart from being slightly easier to wire-up (apparently).
 

SteveCritten

Distinguished Member
Yes agree mate a combi can be used, I have to have stored hot water due to the way my house is configured. My inlaws live in an annex which is self contained apart from the heating/water system so 2 baths could be filling at the same time as well as both washers being on. Having said that I still believe that a combi struggles with anything bigger than the average 3 bed terraced house all you need is the underfloor heating pumping round plus the washing machine on and it will take an hour to fill the bath.......unless they have improved ten fold in the last 5 years of course as that's the last time I took one out and replaced it (in one of my rentals, if only I had waited a few years I would have got it for free :()
 

sergiup

Distinguished Member
Agreed, there are different situations which will require a different solution; even the WB 42CDi - I don't think they make a larger condensing combi boiler - can only do 17.2l/min at 35C dT, so even if your water supply could keep up (a lot probably don't) you'd still be looking at one concurrent main use of hot water, maybe plus that little bit on top. You'd be completely up a creek without a paddle if you didn't have a storage system.

The other issue with most combis of course is that you can't have both the heating and the hot water running at the same time; but they're much more convenient, so provided your usage fits within its "limitations" then they're the right solution.

That would be my view too. I'd say that a correctly specc'd boiler is more important than the brand. IE not too large as it may struggle to produce less output without 'cycling' or going on and off continuously. But you may find that the hot water output is worse with a smaller one, so bear that in mind.

Unless you are on a tight budget, WB or Vaillant are always recommended.

That's if you use a combi of course. We are adding another shower room, so have decided to go for an unvented system with a system boiler. The boiler heats up a cylinder of hot water for instant delivery. And apparently it is easier to wire it all up, but we're not ready to cross that bridge yet, so have no idea why.

The minimum output vs hot water heating capacity was the main reason I went for the largest Vaillant in the range - the 831 would've probably been plenty for heating, but the 837 has the added hot water capacity which I wanted to have. Mind you, our current rental has a WB 42CDi and I still can't do more than shower and have someone wash their hands, but I strongly suspect that's because of the mains supply flow and not the actual boiler. In our new house, the plumber ran as much as reasonably possible on 22mm pipe but the main feed in is still 15mm so it may not make a huge difference. I've yet to do a flow test on it.
 
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