Correct pipe sizing from unvented cylinder (heat pump driven) for efficiency

[AndyRP]

I'm installing a new plumbing system in a retrofit that I'm doing to a small Victorian terrace.

 

Heating will be direct electric panel heaters downstairs (peak heat demand should be under 2KW).

 

Hot water will be supplied via a Vaillant AroSTOR 200, which is an unvented hot water cylinder with a heat pump on the top of it. The reason for me choosing this is that the property has little outdoor space but a decent basement. This heat pump water cylinder allows me to position the unit in the basement, thus easily drawing on external air and not interfering with the MVHR system in operation within the building's thermal envelope.

 

I welcome any thoughts on the setup in general but my specific question is about correct pipe sizing for supplying hot water through the property. I have an eye on the challenges of speedy hot water delivery, energy efficiency and adequate pressure.

 

The supply runs are:

- GF kitchen sink, diswasher; 3m from tank

- 1st floor bath/shower, toilet, sink, washing machine; 10m from tank

- 2nd floor shower, toilet, sink; 13m from tank

 

Mains pressure is 'good' (I guess I should probably measure it somehow but haven't). Baths are rare so quickly running a full bath is not a high priority.

 

I realise convention would be to run 22mm to the bathrooms but this seems to overly favour speed of water delivery over efficiency. Can I get away with doing everything in 15mm plastic pipe?

 

At the risk of muddling the thread, any thoughts on the merits of supplying dishwasher and washing machine with hot water from the cylinder also welcome.

 

 

Edited January 7, 2023 by AndyRP

[PeterW]

1 hour ago, AndyRP said:

The supply runs are:

- GF kitchen sink, diswasher; 3m from tank

Use 10mm here

 

1 hour ago, AndyRP said:

1st floor bath/shower, toilet, sink, washing machine; 10m from tank

- 2nd floor shower, toilet, sink; 13m from tank

Use 15mm here

 

1 hour ago, AndyRP said:

This heat pump water cylinder allows me to position the unit in the basement, thus easily drawing on external air and not interfering with the MVHR system in operation within the building's thermal envelope.

How are you making the basement airtight from the rest of the house ..? And those units are designed to use waste heat from the building so in a basement that may be very inefficient. Also consider condensation due to the cold air it will create. You will also need a G3 blow off to a drain - don’t forget this.

 

1 hour ago, AndyRP said:

any thoughts on the merits of supplying dishwasher and washing machine with hot water from the cylinder also welcome.

There are none - most work on cold feed only now and you’ll just be pulling water out of the tank and it is unlikely to fully fill the DW/WM anyway due to deadlegs.

[AndyRP]

Thanks Peter. Hadn't thought about 10mm but makes sense for such a short run.

 

8 hours ago, PeterW said:

How are you making the basement airtight from the rest of the house ..?

The basement is separated by well taped chipboard and vapour control membrane.

 

8 hours ago, PeterW said:

And those units are designed to use waste heat from the building so in a basement that may be very inefficient. Also consider condensation due to the cold air it will create

I saw a lot of similar units that take waste heat from the house and extract it. These look neat but I'd already committed to a full MVHR. As I understand it the AroSTOR is different and designed for garages etc. It is often ducted to bring air in from outside and kick cold air out somewhere else. These ducts are in theory optional but I do see the risk in blowing cold air up at my timber joists so I'll probably duct at least the cold air outside.

[Nickfromwales]

10 hours ago, AndyRP said:

I'm installing a new plumbing system in a retrofit that I'm doing to a small Victorian terrace.

 

Heating will be direct electric panel heaters downstairs (peak heat demand should be under 2KW).

Hi.

Are you now at EnerPHit standards then? ( I just quickly scan-read through your posts from '21 onwards ). Bloody good results if you're defo at this level of space heating requirement!

I know it's a bit of a digression, but are you referring to IR panels or convectors? I would not be going to IR panels for space heating throughout the GF, as they are very crude giving masses of heat out locally, and only "heat what they see". Surfaces get stupidly hot too, to the point of becoming a hazard.

 

10 hours ago, AndyRP said:

I welcome any thoughts on the setup in general but my specific question is about correct pipe sizing for supplying hot water through the property. I have an eye on the challenges of speedy hot water delivery, energy efficiency and adequate pressure.

 

The supply runs are:

- GF kitchen sink, dishwasher; 3m from tank

- 1st floor bath/shower, toilet, sink, washing machine; 10m from tank

- 2nd floor shower, toilet, sink; 13m from tank

Dish washer and wash machine should be modern energy efficient appliances and fed with cold water.

Kitchen sink; I'd go for 15mm to get the flow as 10mm will hold that back a little, and it's only 3m. I only recommend 10mm for a kitchen if the run is over 9m or so, and there isn't a desire / budget for a hot return system.

Run a separate 10mm pipe to each of the upper floor basins.

15mm to everything else.

 

10 hours ago, AndyRP said:

speedy hot water delivery

Not going to happen without a hot return system. How often will the 2nd floor bathroom be used? If frequented, consider treating yourself to a hot return?

 

10 hours ago, AndyRP said:

Mains pressure is 'good' (I guess I should probably measure it somehow but haven't). Baths are rare so quickly running a full bath is not a high priority.

OK, fine. As the UVC is in the basement, you could also consider a cold mains accumulator ( as you have multiple showers ) which would allow you to use both showers at once plus draw off water elsewhere ( appliances filling etc ). Being creatures of habit could negate this eg if you know the limitations, and you can stagger showering.

[HughF]

I’d certainly consider running a hot feed to the dishwasher. Better to heat that water with a cop of 3 from the heat pump than a cop of 1 using the appliance heater, surely?

 

Although a typical dishwasher run is 3kWh or so, not a massive amount I guess.

 

The washing machine will usually be quite happy on a 30 degree wash, so that can have a cold feed unless you’re dealing with soiled bedding or grease covered overalls when only a boil wash will do 🤣

 

Edited January 8, 2023 by HughF

[Iceverge]

19 hours ago, AndyRP said:

Heating will be direct electric panel heaters downstairs (peak heat demand should be under 2KW).

 

Do the sums on an A2A split unit for the lions share of the heating. Maybe topped up locally by a few electric rads. You sound like you have a similar heat load to us and I think it's the way forward. 

 

Have you considered switchable ducting on the ESHP ? It could be useful to cool the house during heatwaves if you extract and flow to inside the thermal envelope. Noise may need to be considered however.

 

At 10mm and 3bar we see 5-6 l/min at the hot tap. A 15mm cold pipe to the same tap is 10 l/min.  It's a 13m run and time to hot is about 9 seconds to hot. It works for us as the reduction in wasted hot water balances out the lower flow. I did attach the hot manifold vertically to the top of the tank which preheats the water before it gets in the pipe. 

 

I recently used 10mm for replumbing my parents house for everything except the bath and showers and cold taps in the kitchen. Its very easy to cable through tight spaces, almost like a wire, and does the toilets fine in my experience. Similar trick with the manifold from the thermal store. It isn't as fast as a hot return for the one distant basin but for the sake of a 5-10 second wait it isn't worth the hassle I think.