Hot water for outbuilding next to house - under ground from UVC or electric?

[Navron]

We have a house and garden office build currently in progress. The garden office is 8m x 3.5m inside and is only a couple of metres from the corner of the kitchen. It is multipurpose so is both for use as an office and will also have a sofa bed and small bathroom with shower for occasional guests. The plumbers are saying it would be too complicated to get a hot water feed from the UVC to the garden office for the shower we have planned for that room. They said the run is too long, the pipe may freeze in the winter, the hot water may not be hot enough and we would have to wait too long for hot water. They have no concerns with putting a cold water feed in a trench from the house to the garden office. The position of the buildings is such that the hot water feed into the kitchen is about 13m in a direct line to the furthest corner of the garden office, then another 3m inside the garden office to the shower location.

 

The bathroom will not be used very often for showers - maybe two weeks per year at the most.

 

We are having trenches dug soon in any case for the waste pipe, electric, ethernet, and cold water supply for the garden office.

 

My uneducated opinion is that maybe a pre-insulated hot water pipe could work fine in this scenario, given we can accept quite a long wait before the hot water starts flowing, but the plumbers are suggesting either an electric shower or an under-sink water heater, either of which would require us to upgrade the electrics quite a bit (being 30m from the consumer unit).

 

Has anyone faced a similar challenge and/or what are your recommendations or comments?

[Dan F]

It won't be long if the garden office is just behind the kitchen.  I'm not sure what stage of the build you are at, but if you were to use a pre-insulated pipe (e.g. Rehau) underground between the two buildings then heat loss would be very minimal and the risk of freezing would be nill.  If you need to run piping externally with standard insulation then you could make it work, but may be less ideal I'd agree.   You will need to wait for hot water, but it wouldn't be that long and this wouldn't be a major concern IMO if it wasn't used very often. If you did use it more often you could easily use recirculation to avoid this, but you'd have increased losses unless you can program recirculation to only come on when the garden office battery door is opened for example.

 

We used an underground pre-insulated pipe for hot water in a garden office around 30m from our UVC.   The pre-insulated pipe wasn't cheap, but we're happier using water heated with ASHP, rather than direct electricity given there is no reason not to.  We used 2x25mm, to allow for recirculation if required, but I think there are some products with a smaller return pipe.   Based on the distance, design shower flow rate and temperature you plan to keep UVC tank you may be able to use a 20mm pipe which would reduce wait time for hot water.

 

I don't know what you are doing for heating/cooling in the garden office, but we have used a second pre-insulated pipe run for an additional UFH circuit which is hooked up as an independent zone on our heating.

 

 

 

[Iceverge]

For the DHW I would just run some 15mm pipe in the duct. So long as it's below the frost depth and there's not a gale of cold air blowing through the duct it should be fine uninsulated. 

 

The wait for hot water would be very small even at 16m. 

 

Say a 10l/min shower pulling  5l/min hot water would use about 1m of the water in the pipe per second so 16m would be about 16 seconds plus any dead volume in the shower body. 

 

 

 

 

 

 

[Navron]

Thanks for all your help people.

We ran some insulated pipes below ground for hot and cold water, taking a feed off the kitchen. There will be a bit of a wait for hot water, but that's far preferable to a separate water heating system in the garden office.

Sadly it was too late to get the UFH extended out into the garden office as that had all been completed, but had I known in advance I certainly would have done that too!