Navitron Renewable Energy and Sustainability Forum

HEAT PUMPS & Geothermal Energy => Heat Pumps => Topic started by: Alastaid on June 01, 2006, 06:33:12 AM

Title: Boiler Replacement
Post by: Alastaid on June 01, 2006, 06:33:12 AM

We currently have a an oil fired boiler that is rated at 32-44KW,  will the 9KW GSHP you market cope?    The house has warm water underfloor heating throughout,  so I am hoping this is going to be an easy swap?  I don't have the U value calculation of the house,  but it is heavily insulated, and pretty modern.

For the hot water tank,  do I make the assumption,  that the GSHP will get it up to say 30-40 degrees and then I use the immersion heater to get it to say 60 degrees?

Also as a rough guide how much of a trench,  how long,  how wide and how deep will I need to dig for the ground collection pipe?

Thanks in advance.  And are there any other questions I should be asking  ;)


Title: Re: Boiler Replacement
Post by: NickW on June 01, 2006, 09:22:47 AM
Whilst I wouldn't want to undermine Ivans business - you might want to look at Veissmanns range of heat pumps - their 350 model will raise DHW to 58 degrees. I believe that with trenching systems you need about 1 metre for every 100 watts of heat drawn. So if you want an efficent output of 10kw about 100 metres of trench is required. I believe 3-4 feet is perfectly adequate. An ideal situation is if you have to dig trenches for land drains or septic tank tails - two uses for the price of one!

I very much doubt that a 2.5kw wind turbine would give you sufficent and reliable power on a stand alone basis to run a heat pump drawing 2.5 - 3kw of electric



Title: Re: Boiler Replacement
Post by: Alastaid on June 02, 2006, 07:04:20 AM

Thanks for the reply,  I will look around re heat pumps,  I was interested in the Navitron product as well.  With regards to the turbine, I realise that I will never be able to run solely on this for power.  I was hoping that over a year period that I would be pretty much even, the question I really wanted to know, was am I better (ie is it easier and cheaper to) simply push everything I generate back to the grid and sell it, or use what I generate, sell any excess, and consume from the grid when I am short.

I suspect it is easier to just push everything back onto the grid from a setup and installation view,  and hopefully when I sell my generated electricity it will pay for most of my electricity bill   :)



Title: Re: Boiler Replacement
Post by: Ivan on June 03, 2006, 01:39:16 AM
You can use our heatpump to heat water to around 50C. However, heating domestic hot water is inefficient for heatpumps (although probably still more efficient than a straightforward immersion heater). The reason for this is that heatpumps work best at low temperature difference. A typical heat pump COP would be around 4 - 4.75. However, if you can utilise heated water at say 35C and take it from a source that is not too cold, then you can achieve something in the region of 6 or even 6.5. If you are trying to heat water to 58C and drawing water from a cold ground, you can reduce the COP to below 1.5 - 2 or even below this level.

We work on the basis of 30W/m, or 40W/m in milder areas. It does depend to a certain extent on local conditions eg whether the pipe is in rock, clay, sand etc. The better thermal contact there is with the pipe the better the heat exchange. Also, there is no damage caused by having a little too much pipe. On the other hand, too little pipe will cause your heat pump to work harder.

Wind turbines are sometimes used for direct heating ie just to drive heating elements. It would make a lot of sense to be able to use this heat via a heatpump, and therefore multiplying the heat  by 4 or 5.  However, heatpumps like to run for long periods without interuption. Wind turbines by their nature, are stop/start devices, so the only way to utilise would be to grid-link  - you can sell the electricity you produce, and buy it back to run the heatpump


Title: Re: Boiler Replacement
Post by: NickW on June 05, 2006, 12:03:23 PM
Even if the COP does drop to 1.5 - 2.0 its still substantially better than the COP of 1 that an immersion heater has and would be required to lift the water temperature to 55-60 deg/c. Note - legionella risk with water held below 50 deg/c.

Title: Re: Boiler Replacement
Post by: Ivan on June 06, 2006, 02:37:35 AM
Yes, a COP of 1.5 is still better than an immersion heater, but not if it means running the entire central heating system at the same COP!

Even if you do decide to use the heatpump to heat the domestic hot water cylinder, we would recommend topping up the heat to 60C with the immersion heater.


Title: Re: Boiler Replacement
Post by: NickW on June 06, 2006, 12:55:06 PM
Hi Ivan

I looked at the viessmann website and they quote the particular model I mention as having an annual work factor of 3 - I assume they mean coefficient of performance - even at circulating temperatures of 60 degrees. This model would appear particular suitable for direct replacement of an existing oil boiler as the circulating temp would be compatible with the existing radiator circuit. I have no idea who much viessmann charge for one of these units - I have a feeling someone once said 2950??

Some friends of mine in a largish rural house are looking at replacing their aged oil boiler and a unit like this would appear to be a good long term investment.