WB Greenstar vs Valliant EcoTec

[divorcingjack]

I need to choose a large gas system boiler to supply UFH and lots of DHW for our MBC passive-ish house. Thermal store (400/500l), possible PV at a later stage - I was persuaded on an earlier thread that it's the way to go, but budgetary reasons make it a non-starter for a while. 2 adults, 2 young kids, very fond of baths and long showers. 

 

So, I'm thinking 35kW? Is this ridiculously oversized? I've seen WB and Valliant both get good feedback on here - does it just come down to price between them? Warranty? Any other top recommendations? 

 

Thanks in advance for any advice, you ever-helpful bunch. 

[pdf27]

Passivhaus has a heating load 10W (0.01 kW) per square metre at the design heating condition, so if you're a bit off then you probably need 0.02-0.03 kW per square metre for heating.

As for hot water, you get about 20 litres of hot water for a kWh of heat (a bit less in winter, a bit more in summer - depends on the starting temperature) so a 35kW boiler should be able to give you about 700 litres per hour of hot water. As a cross-check, a 35kW combi is rated at about 15 litres per minute (870 litres per hour).

 

So unless you actually want to get rid of the thermal store and run the place off a combi, 35kW is absurdly oversized - you want the smallest boiler you can get your hands on (probably 12kW) as otherwise you'll end up with short-cycling nightmares.

[nod]

7 hours ago, divorcingjack said:

I need to choose a large gas system boiler to supply UFH and lots of DHW for our MBC passive-ish house. Thermal store (400/500l), possible PV at a later stage - I was persuaded on an earlier thread that it's the way to go, but budgetary reasons make it a non-starter for a while. 2 adults, 2 young kids, very fond of baths and long showers. 

 

So, I'm thinking 35kW? Is this ridiculously oversized? I've seen WB and Valliant both get good feedback on here - does it just come down to price between them? Warranty? Any other top recommendations? 

 

Thanks in advance for any advice, you ever-helpful bunch. 

I’ve just bought a Greenstar Cdi and 250 ltr cylinder yesterday 

That’s to run 155 mtrs of UFH

and radiators for five beds three baths 

[divorcingjack]

Hi both, 

 

Thanks for your replies. @pdf27, I know it seems ridiculous, but it is an idea explained by Nick in another thread of mine in the heating section. We use a lot of DHW and have high flow showers, so the idea is to use a thermal store both as a buffer for the UFH (avoiding short cycling) and basically as a high flow combi, hopefully meaning we never run out of hot water. 

 

This is is why I’m having a problem sizing it because the radiator/heat load formula just doesn’t work. Very useful to know that 1kw = 20 litres though, thank you. The reason we decided against a high flow combi is that our bathrooms are quite far from the boiler and we wanted to implement a hot water return system. 

 

@nod what size is your boiler? We have 3 bathrooms to run as well but no rads. 

[pdf27]

Not really a problem - essentially you just need to size the boiler for the hot water load plus probably the standing losses of the cylinder, and your heating losses will probably come out in the wash. I'm assuming the thermal store is sized such that everyone can have a shower/bath at the same time before it's recharged? 500 litres translates to about 25 kWh to heat it up, so your maximum hot water heating power required is more or less 25 divided by the minimum number of hours between everyone using up all the hot water (8 hours for a 3kW load?).

 

DHW is only the dominant heat consumer in a Passiv or near-Passiv house because the heating demand is so low - in absolute terms it isn't actually all that big. If you go for a big storage tank plus a large boiler, you're essentially ensuring that you can have unlimited hot water twice - a big tank and small boiler or small tank and big boiler would both be absolutely fine to do what you want.

No reason that the heat load formulae wouldn't work for sizing the heating fraction of the total demand - you still need the same amount of heat to keep the place warm, it's just being delivered by a different route at a lower temperature.

[CC45]

You might find the coil in the tank has a rating - ours was 18kw max so there isnt much point massively exceeding that.

[nod]

1 hour ago, divorcingjack said:

Hi both, 

 

Thanks for your replies. @pdf27, I know it seems ridiculous, but it is an idea explained by Nick in another thread of mine in the heating section. We use a lot of DHW and have high flow showers, so the idea is to use a thermal store both as a buffer for the UFH (avoiding short cycling) and basically as a high flow combi, hopefully meaning we never run out of hot water. 

 

This is is why I’m having a problem sizing it because the radiator/heat load formula just doesn’t work. Very useful to know that 1kw = 20 litres though, thank you. The reason we decided against a high flow combi is that our bathrooms are quite far from the boiler and we wanted to implement a hot water return system. 

 

@nod what size is your boiler? We have 3 bathrooms to run as well but no rads. 

35 kw

[Nickfromwales]

A fast flowing bath and one shower running simultaneously should be possible, as long as your cold mains flow will support that of course. 

20+ kW to the bath and maybe 10kW should be your worst case scenario TBH, so for that reason I wouldn't go less than 35kW for the boiler. 

Remembering also that space heating may be drawn at the same time too, so in order to match demand ( so no drop in DHW performance, which nobody wants after forking out for a new system ) your better off safe than sorry. 

An 18kW boiler would just about support the bath filling, or even fail at that if it's a big bath and good flow rates at the taps.  

A 35kW boiler will modulate down to less than 10kW, even lower with better makes, so you absolutely will not be wasting gas if that's a worry. 

As for makes, I prefer a Vaillant TBH, but WB have a guarantee, not a warranty, for 10 years. You have to adhere to their installation criteria ( their filter their controls and an accredited WB fitter ) so check out the small print ( speak to them directly prob best ) and see which has the most appeal. WB have got a good reputation, Vaillant similarly so, but for an excellent daily driver I would easily recommend the cheaper but afaic fit and forget Baxi Megaflow. 

First two are the Mercedes, the Baxi is your VW golf. . Never ever had issues or comebacks with the Baxis, an excellent boiler. 

[PeterW]

Just on the Baxi Megaflo- they seem to have a much better QC process than a few of the others as we recently had a main board fail and couldn’t get one for 2 weeks as Baxi had rejected the previous batches and binned the supplier due to poor quality. Basically every supplier in the UK then went to back order..!!

 

They were failing at the 3-4 year point and Baxi said that was unacceptable. I know of other brands with the same issue (and same board....) and they are still failing ... The difference is that Baxi have a real set of people to talk to, and understand customer service. 

[divorcingjack]

Thanks all, as ever, your advice is much appreciated. I'll speak to my plumber and see if he is WB accredited, but I'll definitely look at the Baxi too. 

 

Just while we're here - are there any disadvantages/problems about oversizing the store to 500l? Trevor said that the price difference between 4 and 500l is negligible, and we have the space... 

[Nickfromwales]

Go for 500 as you can run a bigger TS slightly cooler. Also, more volume to store PV down the line. 

[divorcingjack]

Your wish is my command @Nickfromwales  

 

So, my shopping list... 

 

500l Thermal store from Trevor at Telford

- 22mm DHW coil

-  6 KW immersion coil low down,

- 3 KW immersion coil higher up for possible future PV. 

- Tapping for UFH (at the bottom of the tank?)

- Tapping for DHW (top of tank presumably)

 

If my plumber is a WB guy, then 35kw Greenstar Cdi, if not then the Baxi mentioned above - although it only goes to 32Kw, or the Vaillant.

 

Sound OK? Valves, thermostats etc should all come from my plumber, unless there are specific ones I should have? 

[Alexphd1]

Maybe of a interest. I have one from Trevor and this one going into this build. I got the eBay one on a cheeky offer Inc vat and del. Can't comment on quality both are still wrapped up. 

[Alexphd1]

Remember if sealing the system you need allow 10% of total volume for expansion tanks. 

[Nickfromwales]

37 minutes ago, Alexphd1 said:

Remember if sealing the system you need allow 10% of total volume for expansion tanks. 

The installer should know about that ( hopefully lol ). /

1 hour ago, divorcingjack said:

Your wish is my command @Nickfromwales  

 

So, my shopping list... 

 

500l Thermal store from Trevor at Telford

- 22mm DHW coil

-  6 KW immersion coil low down,

- 3 KW immersion coil higher up for possible future PV. 

- Tapping for UFH (at the bottom of the tank?)

- Tapping for DHW (top of tank presumably)

 

If my plumber is a WB guy, then 35kw Greenstar Cdi, if not then the Baxi mentioned above - although it only goes to 32Kw, or the Vaillant.

 

Sound OK? Valves, thermostats etc should all come from my plumber, unless there are specific ones I should have? 

The DHW ( tappings ) are on the DHW coil so cold in / hot out are sorted . 

A pair for boiler 

A pair for space heating ( all )

Make sure you ask for two stat pockets, one upper and one lower. I'll explain why later down the line when your plumbers on board. 

Expansion vessels / valves etc will be down to the design your plumber adopts.  

[divorcingjack]

OK, got it  Thanks! 

 

@Alexphd1, thanks, I'll have a look - the price is not that far off what Trevor quoted me, but always good to have options. 

 

Cheers all - no doubt back with questions later on! 

 

 

[PeterW]

Don’t forget that a Telford cylinder comes with everything inc valves and expansion vessels where needed 

 

That on a 500 litre UVC can be worth £150+ in bits

[Alexphd1]

My telford thermal store cylinder didn't come with anything, have I been had?? UVC usually comes with all the extras. They accepted 900 for the eBay tank. 

[PeterW]

Thermal store doesn’t need an expansion vessel or zone valve - only needed on a UVC.  

[Alexphd1]

 

35 minutes ago, PeterW said:

Thermal store doesn’t need an expansion vessel or zone valve - only needed on a UVC.  

Depends on how you plumb it (sealed v vented another topic)

 

 I have seen 28mm compression blow off on dhw coil linked to a solid fuel boiler. Tank was up at 90 degree, installation was nothing to do with me! 

[PeterW]

A sealed TS Is an odd one as it will need a  fair number of controls I agree but they are pretty much sold as a tin can with holes in. Any sealed circuit would need its own T&E valves etc and expansion vessels. 

 

But yes, I can see how you would need them although the majority seem to be sold as open pipe. 

[Nickfromwales]

TS's don't typically come with expansion vessels for the primary water system as you normally decide what additional volume there is beyond the TS and then fit a vessel or vessels to suit the entire system volume. 

The TS's that have a DHW coil always come with a small EV ( aka shock arrestor ) so @Alexphd1 can you confirm that you have one fitted? The Telford TS's usually arrive with the TMV fitted and all Tectite fittings in place with the upper cold feed going in on a tee. In that open end of the tee goes that small EV.

Nowt else to go with a TS, that's the beauty of them. 10 bar DHW coil so cold mains straight in, if the mains is sub 5 bar at your property, and oodles of high flow DHW. 

[divorcingjack]

OK, one more question from trevor before I can order :

 

Do I want open vented or unvented - about £200 difference in cost. Need mains pressure for the showers, ideally .. 

 

Thanks all - delivery next week! Exciting. 

[Alexphd1]

@Nickfromwales box still sealed and stored just now, will check and report back later.

[PeterW]

13 minutes ago, divorcingjack said:

OK, one more question from trevor before I can order :

 

Do I want open vented or unvented - about £200 difference in cost. Need mains pressure for the showers, ideally .. 

 

Thanks all - delivery next week! Exciting. 

 

The mains coil provides the hot water at mains pressure. What you’re being asked is if you want a sealed system TS (which needs G3 sign off and expansion vessels) or an open vent TS that you can install yourself but needs a feed and expansion or header tank in the attic. 

 

What has been designed ..?