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System boiler and UVC sizing


divorcingjack

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  • 2 weeks later...

OK lads and ladies - being trapped in the house under feet of snow has meant that we finally got a chance to discuss the heating and PV again and we have come up with more stupid questions for you. 

 

As regards the PV - we're still tending towards not having it. We live by the sea and have a lot of seagulls. We are putting in special measures to avoid them landing/nesting on the flat roof - we think they will perch on the panels and shit all over them, so we'd have to factor in fairly regular cleaning, presumably at a cost. This would pretty much wipe out the earnings from FIT. However, it's not a complete no - depends on where the energy prices go over the next 5-10 years. 

 

So, we are now tending towards @Nickfromwales suggestion of a high flow combo combined with buffer tank for the UFH. Having lived with a (probably undersized) combi in a previous house, I'm a bit wary of baths taking a millennium to fill or underpowered, lukewarm showers :( Some of our showers for the house have very high flow rates of 24l/m and I notice that the combi Nick recommended does "up to" 20l/m hot - I appreciate with blending this comes down, but presumably it wouldn't manage with multiple outlets at once. Would anything though? 

 

My questions: 

 

- Will the Valiant 938 cope with very high flow showers? Should we go for something higher powered?  

- What size buffer would we need for the UFH and how would this be linked up with the combi? 

- Is there a way to retrospectively connect any PV to a combi? Perhaps another buffer with a PV coil?  Or is that adding another layer of unnecessary complexity? 

 

Thanks again for all your help - in answer to a previous question, we haven't been connected to the water yet (long story....) but the local pressure at the point we are hoping to connect was approx 10 bar, I think. Don't know about flow rate, but nothing was mentioned about it being problematic. We are next to two blocks of flats, so hopefully the supply is reasonable. 

 

Thanks all - off to pick up the wee one, so will catch up later. 

 

Cheers, 

dj 

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Two showers at max wallop in a regular dwelling would prob use 9-10Lpm max, so forget anything that needs to generate 48Lpm as that's a fire hydrant. 

You will get two ADEQUATE simultaneous showers at around 8Lpm from the 938, and one I fitted with my mate a while back did so, 8 & 8 respectively from a max 16Lpm cold supply ( which is good btw ). Don't expect to be in there for 20 mins each though. You have to be realistic. 

If you have 10bar available and you really want high flow showers to run together then it would have to be a 250-300L TS with a 28mm 46kw DHW instant coil inside it. You will need a pressure reducing valve to bring the supply under control, say set to 4 bar, but that should still give sufficient cold water volume to run two 10Lpm showers if, as I expect with such high pressure, you'll also have in excess of 20Lpm coming into the TS. 

Gas system boiler & TS may be best, and the TS is your buffer AND hot water tank so a '2 box' solution. Any pv you generate would go into it via 2 immersions ( which you could / would use if the boiler ever failed too ). 

Time to decide if one blasting shower is good enough, as the 938 ( or even the 838 regular combi / other large regular combi ) will do that, or if you definitely need to be able to run two hot bathing outlets at the same time. That's the decider. 

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  • 4 weeks later...

Hi all, 

 

Time to bump up this thread again as we have to make these purchases before new baby arrives (due yesterday!). After moving house into another rental with inadequate HW setup, we're more keen than ever to make sure we get this right.  So - more questions: 

 

- Am I right in my assumption that you cannot have a HW return circuit with a combi? So, you could potentially be waiting for HW at far away taps? 

- If we oversize the thermal store now (perhaps to 400l), we could run it at a lower temp and add in PV via an immersion at a later date if we decide to install it? 

- If we are ordering a thermal store, we need one with a 28mm 46kw DHW instant coil and 2 immersions to future proof for later PV? 

 

I'm going to read through the thread again and probably come back with more questions :)

 

Cheers, 

dj 

 

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4 hours ago, divorcingjack said:

Am I right in my assumption that you cannot have a HW return circuit with a combi? So, you could potentially be waiting for HW at far away taps?

Yes your right. Read a screw fix forum the other day where they were doing this, but it would be massively detrimental to the boiler, plus it would instantly void any warranty. 

 

4 hours ago, divorcingjack said:

If we oversize the thermal store now (perhaps to 400l), we could run it at a lower temp and add in PV via an immersion at a later date if we decide to install it?

If serviced by a gas boiler then yes. As theres no potable water stored ( its a nigh on instant water heater ) there is also virtually no risk of legionella.

 

4 hours ago, divorcingjack said:

If we are ordering a thermal store, we need one with a 28mm 46kw DHW instant coil and 2 immersions to future proof for later PV? 

No. You can get the smaller 22mm coil fitted and still run two good showers. I only suggest that a 28mm coil is used if theres 3 or more showers. Always 2 immersions on a tank this size, id probably say fit 1 x 3kw and 1 x 6kw so you have the 6kw for emergency back up ( failsafe ) and the 3kw for PV integration. 6kw lowest and the 3kw above it. That way if the boiler ever goes down you can dump 9kw of electricity into it and have at least some heat / hot water.  

Sparky will need to know about this asap to give you the correct supplies. 

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  • 1 month later...

Bumping this thread again for specs as I've had a chat with Trevor at Cylinders2Go. We're pretty much decided on the TS + big boiler suggestion from Nick with a HW return for the further away bathrooms. We looked again at the showers we have and they are a lower flow rate than I thought, so we should be OK. 

 

So, my shopping list: 

 

Thermal Store (500l?) 22mm DHW coil, 6 KW immersion coil low down, 3 KW immersion coil higher up for possible future PV. We are not too short for space, and Trevor said that the price difference between 400 and 500l is negligible. Are there any potential problems in oversizing the store to this extent? As mentioned by @Nickfromwales earlier, we are planning to run it at a lower temperature and put it inside an insulated box to reduce heat losses. Realistically, any PV addition will probably not be for 2-3 years. Do I need any other bits and pieces (valves etc) to go with this? Controls? 

 

Now - boilers... Do I need to worry about modulation to UFH temps on the boiler, or will the TS take care of that? What size should we be thinking about? Valiant or WB come up regularly - are these all pretty reliable, or does it vary by model? 

 

I'm off to read some more threads.... 

Edited by divorcingjack
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  • 2 weeks later...
On 06/06/2018 at 20:50, divorcingjack said:

Apologies for bumping this up, but I need to order this kit ASAFP and am worried about missing something crucial ... 

 

Any advice anyone? There's a beer and a pie in it for you when visiting scotland ;)

How did I miss this ? Beer and a pie ! 

Is this sorted now?

  • Thanks 1
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World championship winning pie! I asked this question on my other thread, which you very helpfully answered, so pies all round when you're in scotland :) and the brewery is just next door .... 

 

Trevor is going to send me a drawing to double/triple check before we go ahead, but hopefully we should have delivery this week. Found out my plumber is not WB accredited, but knows a guy who is, so he will sign it off for the guarantee. Just comes down to price between the Vaillant and the WB. Next set of questions about linear drains ... 

 

Thanks! 

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54 minutes ago, divorcingjack said:

Macaroni pie! or the classic Scottish double carb... pie on a roll :) 

 

Or potato scone in a roll. I never get the double carb thing really lol. Cannae beat a bit of meat! :D

 

 

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