Can you help me reconfigure my hot water supply layout please?

[Adsibob]

This is partly an MVHR post but mainly a HWC/secondary loop/boiler post, so hopefully I've picked correctly in posting it under hot water cylinders and boilers.

 

The house we are refurbing will have its two principal bathrooms on the first floor, with the utility room centred below them at ground floor. On the second floor there will be a further shower room, which is directly above one of those bathrooms. I had originally planned to site the hot water cylinder also on the second floor in the eaves space directly above the other bathroom. As the three bathrooms, the kitchen and the utility are all on the same side of the house, this would have resulted in nice and short pipe runs from the cylinder to each bathroom: approximately 0.5m, 1.5m and 2.25m and maybe 5m to the utility, 8m to the guest WC and 10m to the kitchen sink. For the kitchen sink and guest WC we were going to have a secondary loop to give instant hot water.

 

Our MVHR designer has worked out that the eaves storage can just about fit our Flair 400 unit, he has suggested it will look as shown in this drawing:

 

This drawing actually doesn't make it look like it will fit, but it's because it shows an older version of our eaves storage area - we are actually extending it by 25cm or possibly a fraction more, so instead of the cross section being a right angled isoceles triangle which is only 168cm on each side, it will be a right angled isoceles triangle which is 193cm on each side. The width of the space is 170cm. Because of the 45 degree angle of the roof space, as one gets x cm away from the highest point in the room, one loses x cm of head height, so whilst 193cm sounds nice and high 56cm away from that, which is the approximate diameter of a 300 litre cylinder one only has 137cm of height. It is looking unlikely that I will be able to fit a 300 litre hot water cylinder here as well. The Telford Tempest unvented one I was looking at is 56cm in diameter and 1650 in length as a horizontal. Whether I do it as a horizontal one or a vertical one, i can't see how I can make it work without blocking the access to the eaves storage (which is shown by the orange arrow in this diagram:

- we are doing away with that bathtub in the adjacent bathroom and making it a shower room instead, with a waterproof door/hatch that will lead from the shower area into the eaves storage area).

 

So I think my only option is to site the HWC elsewhere. There is an eaves storage area on the other side of the stairwell (the stairwell is in the middle of the house). If i were to site it there, I would add at least 4.25m to all of my pipe runs to the outlets, so pipe run lengths for hot water would be:

4.75m bathroom 1

5.75m bathroom 2

8m shower room

9.25 to the utility room

6m to the guest WC and

15m to the kitchen sink

 

Water pressure from the street is approximately 2.7 to 3.1 bar depending on the time of day, but probably safest to assume 2.7 bar. My flow rate is about 15l a minute, but this might increase slightly when we widen the connection to the water supply. Two questions please:

 

1. Will a secondary loop with a timer that runs the water for 1 minute every 20 minutes during the day (and switches off whilst we're asleep) still give me instant hot water in all my bathrooms, guest WC and kitchen, or are these distances problematic?
2. Where would you site the gas boiler that heats the HWC? Originally this was going to go in the utility room, some 5.5m directly underneath the HWC. But if the HWC moves to the other side of the stairwell, we will have about 10m between them and so maybe it makes sense to move the boiler up into the eaves storage space right by the HWC, with the flue going straight up and out the roof?

 

 

Edited April 22, 2021 by Adsibob

[Dan F]

27 minutes ago, Adsibob said:

Will a secondary loop with a timer that runs the water for 1 minute every 20 minutes during the day (and switches off whilst we're asleep) still give me instant hot water in all my bathrooms, guest WC and kitchen, or are these distances problematic?

 

If this would works running 1min every 20min, depends on pipe size, amount of insulation, DHW temperature, and what you consider hot as a % of UVC temperature.  The length impacts total heat loss of the system and how long pump needs to run for, but not the feasibility per se.  The speed of heat loss will be the same, whatever the length, just there is more water to flush through when the pump runs.

 

I thought there was an online calculator somewhere, but I can't find it now..

 

Edited April 22, 2021 by Dan F

[Adsibob]

39 minutes ago, Dan F said:

depends on pipe size, amount of insulation, DHW temperature, and what you consider hot as a % of UVC temperature.

No idea about pipe size, what would you recommend? 
i will make sure all pipes are insulated with whatever is recommended here - any suggestion?

DHW would probably be set to 63C or 64C. I guess I could set it to 65C on the basis that there will be some heat loss that will mitigate scalding risk, but rather not. 
I like my baths hot; I’ve never checked the temp, but I’d be surprised if it was hotter than 45C, so let’s call that 70% of the 64C HWC temperature.

 

[PeterW]

6 hours ago, Adsibob said:

No idea about pipe size, what would you recommend? 
i will make sure all pipes are insulated with whatever is recommended here - any suggestion?

DHW would probably be set to 63C or 64C. I guess I could set it to 65C on the basis that there will be some heat loss that will mitigate scalding risk, but rather not. 
I like my baths hot; I’ve never checked the temp, but I’d be surprised if it was hotter than 45C, so let’s call that 70% of the 64C HWC temperature.

 

Hot water has to go through a blending or TMV - you store at 65°C but to outlets especially baths is limited in building regulations to 43°C from memory. You would not want to put 65°C water direct to a tap - it is scalding temperature.  
 

For your HRC to the kitchen sink you could go with 15mm and a 10mm return, all insulated with 25mm of standard polyethylene pipe insulation. Using Hep2O means no joints hidden and you could have a manifold for this purpose. Showers can be fed direct off the UVC as long as they go through a thermostatic mixer valve. 

[Adsibob]

Thanks @PeterW that’s helpful. All showers and baths will have thermostatic valves.  At the end of the day, a lot of this is going to depend on £ costs vs savings. If the boiler is at the other end of the house from the hot water tank, say 16 to 18m away in total length, with about half of that being vertical, but I insulated the pipes properly, what £ will I lose per year? The rest of the house is pretty well insulated, not passive haus, not even close, but still 100mm of Celotex over the cement slab, 40mm of insulation on the exterior of all the external walls, plenty of insulation in the loft, all windows and rooflights at least double glazed with thermally broken frames, MVHR, etc. With all of that, if I take this one short cut and put the hot water tank 16m to 18m from the UHWC, what £ will I suffer, but I’m heat loss and pump running costs? How do I work this out?

[PeterW]

You’ll lose naff all as the heat is lost into the house anyway. Pump timers are the best idea and 1 min in 30 is more than enough if you are insulating the flow and the return loops. 

[Adsibob]

4 hours ago, PeterW said:

You’ll lose naff all as the heat is lost into the house anyway. Pump timers are the best idea and 1 min in 30 is more than enough if you are insulating the flow and the return loops. 

Thanks @PeterW. Which pump do you recommend? I need something very quiet.