Heating Philosophy

Heating Philosophy

I thought I'd use this post to delve into two options I'm considering for space & hot water heating in my new home. One uses a Hot Water Tank and the other instant Hot Water with no tank.

Each has it's own attributes but some of the factors I'll be using to decide on which route to go are:

• limited space heating required, mostly wintertime
• Myself: limited hot water usage, 3 minute shower daily, some washing up at sink, shave once per week
• Other person: Unknown
• Occupancy max of 1 to 2 people

So, the reason this is more of a philosophical argument is that for years everyone automatically got a hot water tank. But my issue with that is for one person it's overkill. Even a small one. The standing looses and constant need to top it up means storing hot water at 55 degrees becomes a drain on a good air source heat pump. Why not just create it as you need it? It comes down to a simple principal:

Store & Loose OR Create & Use!

So, why not store it and go conventional - here are my list of pros / cons of a hot water tank system:

Pros:

• Plenty of knowledge around on how to do this, there are some interesting variations but it's every conventional, any plumbers can put one together for you
• Good store of ready to use hot water at all times of the day (hopefully!)
• Better for larger households
• Can use pressurised system to pump hot water around a circuit continually to ensure no dead legs
• Can supplement with Solar Thermal system

Cons:

• Recharge time, teenager 30 minute shower vs timing of your own can be frustrating! 
• Standing losses, both in the tank itself and in the hot water pipes, the water essentially goes cold in the pipes after an hour despite any insulation
• Heat from tank will leak into surrounding spaces, not nice in an air tight house in summer!
• Dead Legs between hot water tank and use points, wastes a lot of water each year....vs water charges!
• Space required for Hot Water Tank, position vs use points hard to optimize in most houses
• Legionella risk mitigated by storing at 55 degrees +, or must use immersion once a week, ASHP isn't optimised to heat hot water to this temperature, lowers COP. 

And as for creating it as you need on demand:

Pros:

• You create exactly the quantity you need, at the exact temperature you need, precisely where you need it (or close enough!)
• No risk of Legionella as no hot water is stored
• Stops waste of using kettle to boil imprecise amounts of water that's not even needed / used
• No Dead Legs if using electric, very short runs between showers possible if sited above / below each other
• Can supplement power requirements by using Solar PV & Sunamp PV to store daytime energy

Cons:

• OK for 1 or 2 people but do efficiencies drop after that for larger families? Or do you just scale it?
• If using Combi gas boiler - still has dead legs
• Electricity use for all hot water needs if no Combi
• Increases reliance on Electricity as sole energy point if not using Gas Combi, more sensitive to price fluctuations
• Little knowledge out there, relying on manufacturers / suppliers for diagrams, troubleshooting etc, riskier as few have done it in practice. 

My current goal is to use an instant water heater in the Kitchen under the sink. Then for the showers, I hope to connect Solar PV panels to a Sunamp PV (for preheat) and Stiebel Electric water heater:

http://www.ithouk.co.uk

http://sunamp.co.uk

https://www.stiebel-eltron.co.uk/en/products-solutions/dhw/instantaneous_waterheater/overview.html

I've found good info on some of these options on the excellent buildhub site:

http://forum.buildhub.org.uk

There are still some interesting options such as Eco Heating systems with a Twist:

https://originaltwist.com/2016/06/23/eco-heating-system-for-heat-pumps/

Tank in Tank systems also provide some skews to the traditional approaches:

http://www.greentherm.ie/solutions/thermal-stores-and-hot-water-cylinders-from-greentherm/

Greentherm do a great Solar Thermal drainback system which is safer than one which leaves fluid in the panel which when not being pumped turns to sludge. But these are options for another day!!

Now, the focus here was mainly on hot water where most of the house energy is concentrated but space heating is also required. An air source heat pump is a good option when paired with under floor heating but ideally you want to gain some synergy between the hot water and space heating. There's no reason why you can't add an ASHP into the Sunamp/Stiebel combination, it just adds to the cost though. Also some MHVR units combines an ASHP anyway. Lots to consider so it's not a choice to be made in isolation if cost and all those maintenance charges are a worry. 

I hope the information above gives you some insight into the possibilities available today that weren't choices even 10-15 years ago. I'm sure there will be more options in another 10-15 years! Best of luck investigating your own space/hot water heating solutions. Find a good oracle/tradesperson you trust and see what they suggest while leaving options open for tweaks in the future......

BTW - I'm reading an excellent paper called "Sustainable Energy - Without the Hot Air!" - you can download a free PDF here:
http://www.withouthotair.com
Chapter 21 really delves into the best way to heat your home and get hot water...!