What a lovely project to have! You are right it will mean a heat exchanger since you can’t use the water directly it would be too risky on water acidity etc, I have little experience of heat exchanger selection but I recall some discussion a while ago which might have been @cathoderay. Any chance you guys can recall? Heat exchange as a search term doesn’t work it’s used too widely here.

its a fairly small reservoir you have there and once you take heat away it will cool. Is the flow steady? Does it vary with air pressure rainfall or general external temperature?

This is a super interesting question. I’m recording more podcasts tomorrow and will raise this with one of the panels. It’ll be really interesting what they propose as a solution.

Posted by: @allenc

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I have access to a hot spring, temperature of the water is 45 - 55 deg C and the flow rate is 4 litres per minute.  

So if the water from the spring were cooled to say 30"C running through a heat exchanger, 4l per minute is 20×4.2/60 x 4 kW, about 5kW.  That's a useful amount of heat.  If you had ufh, or largish rads, you could definitely use this.  In principle you just need a heat exchanger and a water pump.

Obviously if your house loss is > 5kW  you need another source of heat also, either operating independently or in tandem.

In Iceland there are quite a number of houses heated by hot springs and they sonetines pipe the water to the hot taps as well, which gives it a very sulphurous smell.

Posted by: @judith

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Any chance you guys can recall?

The discussion was initially specific to my installation, a standard retrofit ASHP, and then broadened out a bit, with a trend towards a general view that the main purpose of a PHE (plate heat exchanger) was to protect the suppliers rear end, at the cost of a performance hit for the customer, plus ça change. That said, I still haven't removed my PHE, chiefly because I like the idea of separating the glycol treated primary circuit from the rather ancient but still OK (it survived a rather severe pressure test) secondary radiator circuit.

It seems to me the idea of extracting heat from a hot spring is on paper at least very do-able, Buildings and their heat loss/demands are heat source agnostic, they don't know or care where the heat comes from. As far as they are concerned, a kWh is a kWh, wherever it came from. In theory, you could even use a compost heap as the heat source, but you might find the neighbours start to complain after a while.   

@allenc

According to Wiki geothermal power heats 89% of the houses in Iceland, and over 54% of the primary energy used in Iceland comes from geothermal sources. 

4-5kW of free, green, energy is not to be sniffed at; even at gas prices its worth around £2000 per annum.    I don't see that you necessarily need a heat pump BTW, so its no good talking to heat pump vendors, you probably just need some plumbing.  A lot of the heating systems in Iceland are district systems rather than individual, but I bet there are plumbers there who know how to do it.  It might (seriously) be worth finding an internet plumbers forum in Iceland and asking the question.  Or try plumbers in our spa towns, there must be one or two that have been involved with the spa itself.

On another forum and in a different context someone (@johnmo who is also on this forum) has just posted this link https://beetbg.com/products/dhw-plate-loading-20xplate-kit

Its designed for DHW (which presumably you also need) but the same principle, and likely the same components, would work for space heating.  You may need to find out a bit about the chemistry of the spring water in case its likely to be corrosive.

Thank you everyone for you comments and advice!  I am sincerely grateful!

I was thinking about making a heat exchanger from 15mm copper tubes - a series of rectangular loops to fit inside the tank which would give a significant surface area for the hot spring water to be in contact with the tubing carrying the central heating water. The problem with using a plate heat exchanger is providing a pump to circulate the hot spring water round the primary section of the heat exchanger.

The spring water has a constant flow and temperature throughout the the year , and is clearly not very acidic /alkaline as it is used for the 'hot tub' (Japanese 'onsen') inside the house - and its source feeds many public 'onsens' throughout the location.

Any specific help and advice is very welcome - thank you everyone!

Allen

Posted by: @allenc

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Thank you everyone for you comments and advice!  I am sincerely grateful!

I was thinking about making a heat exchanger from 15mm copper tubes - a series of rectangular loops to fit inside the tank which would give a significant surface area for the hot spring water to be in contact with the tubing carrying the central heating water. The problem with using a plate heat exchanger is providing a pump to circulate the hot spring water round the primary section of the heat exchanger.

The spring water has a constant flow and temperature throughout the the year , and is clearly not very acidic /alkaline as it is used for the 'hot tub' (Japanese 'onsen') inside the house - and its source feeds many public 'onsens' throughout the location.

Any specific help and advice is very welcome - thank you everyone!

Allen

Modern DHW cylinders designed for heat pumps have quite sophisticated finned coils to maximise the surface area and thus heat transfer.  Ideally you probably want that. 

Is there any way that you could replace the existing tank with an off the shelf cylinder fitted with such a coil.  Alternatively it might be worth contacting a couple of the DHW tank manufacturers (there are several in the UK) and seeing if you can buy a coil off of them.  The fundamental problem is that the spring water isn't enormously hot so you will likely need quite a large area to extract most of the energy available.

There again you could just start with something home made and, if its good enough then its done, but you do need a fairly substantial contact area (3m^2 is common for heat pump cylinders).