GSHP planned but not hot water??

[Selfbuildsarah]

We are planning for a GSHP and underfloor heating in our new build approx 290m2 but not sure of hot water. Just the two of us for most of the time but must be able to cope with 10 max on occasions and I like a warm house. Sips or timber frame/ good insulation. Will the GSHP heat our hot water sufficiently? I really don't like PV panels etc from an aesthetic point of view. Some kind of back up? Tank size? Any help/advice appreciated.  Thanks

Edited September 12, 2021 by Selfbuildsarah
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[nod]

Welcome 

We are also considering GSHP

170m2 ground floor It’s my understanding that the pump would supply hot water to the UF and 4 bathrooms 

 

We also hate PV 
Planners insisted we have it in our first build But took a.chance and left it off

BC really didn’t care either way 

[ProDave]

A GSHP will heat your hot water nicely in an unvented cylinder.

 

Perhaps for your occupancy you need 2 cylinders, one small one for when it is just you, and a larger one when you have guests and you just select that when you know they are coming.

 

It is a shame you don't like solar PV because it makes so much sense for an all electric house where it will reduce your electricity bills.

[SteamyTea]

Any reason you want a GSHP over and ASHP. There is a large price difference in the units and the installation costs.

Don't get temperature, the ⁰C or K, confused with the power, the kW, of a unit.

Power (kW) is also different from energy, the kWh unit is usually used in the domestic setting for energy, through the proper unit is the joule, you need 3,600,000 J to make a kWh.

 

What you need to do first us a decent thermal model of the house. This will set the maximum heat load and the most likely power needed satisfy it.

Going from an occupancy of 2 to 10 is a huge difference, you are probably financially better off with a secondary cylinder that just uses immersion heaters run directly off the mains electricity. To heat a cold, 200 litre cylinder to 60⁰ will probably take around 12 kWh of electricity. That will cost, even at 20p/kWh, £2.40 a day.

Less that a couple of metres of GSHP pipe.

Edited September 12, 2021 by SteamyTea

[markocosic]

Many (most?) GSHPs and ASHPs will have a direct electric backup.

 

The heat pump provides the base load. When it's too cold for the heat pump to provide the full capacity required; or the heat pump is broken; or you need to reheat stored water quickly; or you want to heat stored water to a higher temperature than the heat pump can operate at; you turn on the direct electric backup.

 

You might be surprised what you can do with this.

 

 

e.g. the little GHSP i'm using has a nominal output of 6 kW; plus three 1.5 kW direct electric elements (for 1.5/3.0/4.5 kW extra)

 

Reheat time for 180 litres from 10-55C is 95 minutes on the GHSP alone; or 55 minutes with both the GSHP and the direct electric elements running.

 

That's enough for pretty much back to back showers in one bathroom in a limit cas  "loads of guests" scenario:

 

Assume 10L/minute shower at 40C; from hot at 55C and cold at 10C. That's nominally 6.7L/min of hot and 3.3L/min of cold.

 

A 10 minute shower is 67 litres of hot water gone. Say it takes 10 minutes of brushing teeth / having a poo / getting naked before shower / getting dry and clothed after shower. In that 20 minutes that the guest is in the bathroom the 6 kW GSHP and 4.5 kW direct electric element can reheat (20/55) * 180 or about 65 litres of hot water. So provided that you set the controls for "On, Boost" you can back to back shower 10 people through a single bathroom over three hours without running a 180 litre tank at 55C empty.

 

Two bathrooms is trickier. That'll eat 135 litres of hot water whilst you only refill 65 litres. If your tank starts out full and the controls are set for "On, Boost" then after three goes (i.e. 6 people in one hour) you've drained the tank and need to wait an hour before the next 6 people.

 

 

 

Bump up to something like a 12 kW machine with a 3/6/9 kW direct electric backup and you're laughing. (300 litres from 10-55C in 45 minutes) That'll effectively run 2 bathrooms continuously even with a small tank. (assuming 10 minutes of brushing teeth / having a poo / getting naked before shower / getting dry and clothed after shower etc) With a 300 litre tank you might not even need the direct electric backup. 

 

Even if your space heat load is only 6 kW it'd be worth having the 12 kW capacity available for the DHW in your 10 guest scenario. Note the ground loops don't need to be twice the size if it is only for occasional use. (you're sizing on the average rate of heat pulled from the ground not the peak rate)

 

 

Useful tools for quick calcs:

 

https://myboiler.com/calculators/hot-water-recovery-time-calculator/

https://www.spiraxsarco.com/resources-and-design-tools/calculators/water-mixing/water-mixing

 

[Selfbuildsarah]

Thanks for your replies everyone, particularly markocosic. Will take a look at the links you suggested. Very helpful.