Heating and hot water for granny annexe

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pasty_girl

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Hi - I'm really sorry if this has been posted before but these forums are extensive!!

We are building an annexe in our back garden, about 20m away from the main house. Two rooms internal dimension total about 32m2. It will be made from a SIPs construction so hopefully very well insulated.

We are trying to work out the most economical way to heat it. Options we are considering are:

1. Electric everything - how much should we expect to pay for 2 electric radiators with timer and thermostat functions?

2. Air con/heat pumps. Probably a split system, so one outdoor unit linked to two indoor. The thing we are struggling with this is how much do they cost to run in comparison to an electric radiator? Then electric shower and hot water for kitchen sink on separate systems.

3. Run an insulated pipe (Uponor style) from the main house down to the annexe and back which would contain hot water for the heating and separate pipe for the shower and sink.

Does anyone have any thoughts on this? We don't want to spend a huge amount upfront if it would take 20 years to make the money back on energy bills!

Many thanks for any insight, or any other ideas.
 
Run as much as you can on gas, it will be far cheaper. Plus, you won't need a huge electric supply to the annexe.
 
Gas is not an option sadly as cost prohibitive to run pipe down there. Many thanks for your reply.
 
I'm not a plumber - what's the difference between laying water and gas pipes?
 
pasty_girl said:
Electric everything - how much should we expect to pay for 2 electric radiators with timer and thermostat functions?
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Not a good idea, it will likely cost you a lot to run in the long term. Inferred heaters cost less to run than traditional electric heaters, but are still relatively expensive to run, and cast everything in a orange glow while they are on.

pasty_girl said:
3. Run an insulated pipe (Uponor style) from the main house down to the annexe and back which would contain hot water for the heating and separate pipe for the shower and sink.
Click to expand...

Also fought with problems, you will likely need to wrap the three pipes (HW and CH Flow and Return) in heating tape to stop them freezing up and possibly bursting in the winter time. You will also likely encounter a very long draw of time for the hot water unless you have a mains high pressure HW cylinder with a very fast draw of rate or do a pumped looped system to and from the HW tank in house - And that of course won't be very effective if you have a combi boiler.


pasty_girl said:
The thing we are struggling with this is how much do they cost to run in comparison to an electric radiator?
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You need to look at the kW rating for the electrical load or the amps rating times the rated voltage and not the rated heating/cooling output, that is the maximum it will cost to run.

I.e. A heat pump/air con system may have a 2.4Kw electrical load, but will produce 8.6kW worth of heating. Thus you treat the cost to run as if it was a conventional 2.4kW electric heater. Generally the efficient inverter type ones will cost 1/3 to 1/5 to run for the heat output they produce and in comparison with a electric radiator with the same rated heat output.


Also, how about portable gas heaters, or having a underground or above ground propane/LPG gas tank connected to the outbuilding that can be filled up every year or two when needed.
 
Thanks very much all this is great - food for thought :)

We think gas is out due to installation costs and having to buy a new boiler for the annexe, plus space is at a premium in the annexe.

Seems like the air con/heat pump idea might be worth exploring though. Does anyone know from experience whether the stated SCOP ratings are generally accurate? Do they really perform at the ratios advertised?

Ideally we would have a multi split system (so one outdoor unit powering two indoor units) - this one seems a lot cheaper than others - £640 - SCOP of 4.2 which would seem decent. It would take a long time of bills to seem making paying double that worthwhile (the price others seem to be)

http://www.appliancesdirect.co.uk/p...rPktfsdicOhkm2zpQglRCcqIelMz0paGQcaAuP28P8HAQ

Does anyone have any views? Do you get what you pay for?

You have all been so helpful, thank you.
 
pasty_girl said:
Then electric shower
Click to expand...
Before selecting that or anything else electrical, how is the electricity supply to this annexe being provided? A separately installed supply with meter, or connected to the existing supply for the house?

If it's the existing house supply, what large loads are already in use there such as electric shower(s), electric hob, electric heating, etc.
Will this annexe have cooking facilities, and if so what do you intend to put in there?
 
The fuse board is being upgraded as part of house refurb so our assumption was this would be done to whatever spec was needed. We are amateurs at this though!

All things in your list re the main house are gas apart from we are planning an induction hob in our new kitchen. Annexe will only have a microwave and fridge (plus probably an electric shower)....
 
Highly unlikely you can have an electric shower in there. You're talking 16mm cable to the place which will be cost prohibitive. You can get mini cylinders with a 3kw immersion which you can pump to provide a decent shower.

Have you considered lpg cylinders and boiler?
 
pasty_girl said:
The fuse board is being upgraded as part of house refurb so our assumption was this would be done to whatever spec was needed. We are amateurs at this though!

All things in your list re the main house are gas apart from we are planning an induction hob in our new kitchen. Annexe will only have a microwave and fridge (plus probably an electric shower)....
Click to expand...
And your air con system. A significant load.
 
Steve said:
And your air con system. A significant load.
Click to expand...

A modern efficient Twin multi split AC/heat pump system is unlikely to have a electrical load higher than 16A, the one the OP linked to is rated at 11.2A @ 240v as a maximum. See here for the spec sheet.
 
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