Wood Burner and Hot Water

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33 minutes ago, Ballynoes said:


I wish we had gas, but live in the country, so don't even have sewage, or decent broadband, but that's the price you pay for piece and quiet surrounded by your own fields... not complaining though.¬†ūüėČ


I still find it difficult to comprehend that you cant "feed" a hot water cylinder with two different form of heating, after all I was under the impression that you had to occasionally heat the hot water tank to a certain temperature, to make sure certain bacteria don't thrive... or is that another myth.¬†ūüôĄ


To feed a hot tank from two sources is OK, but controlling them so that they can both work effectively is the challenge.¬† For example, I installed a big thermal store initially, and pre-heated this when it was cold from the ASHP.¬† This raised the temperature of the tank up to about 40¬įC, as long as it wasn't already at that high a temperature.¬† The immersion heater, run from excess PV generation, then provided the energy needed to get the thermal store up to around 65¬įC.¬† If the tank was above 40¬įC then the ASHP couldn't provide any useful heat to it.


The need to boost the temperature of stored hot water enough to kill off legionella is probably an over-blown risk, provided that you're using mains water and a sealed UVC.¬† Legionella cannot get into the mains water supply (the reason mains water is usually post-treated with a persistent disinfecting agent is to ensure this) so if it isn't in the supply water, and the tank is sealed, there's no way for any bacteria to multiply, as there won't be a way for them to get in in the first place.¬† Also, legionella stop multiplying at a temperature of about 45¬įC, and so holding the tank at 50 to 55¬įC may not kill them, but will cause them to go dormant (not that they can get inside a sealed system anyway).¬† Finally, regular hot water use will flush the contents of the tank through before any bacteria (which can't get in) have a chance to set up home.¬†


If you have a vented hot water system, then things are different, and an anti-legionella temperature cycle with an immersion heater, to heat the tank to above 60¬įC every week or two would be a good idea.

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I'm sure I saw some sort of heat exchanger unit, that could basically take various heat supplies and supply them to a store. For the life of me I can't find where I saw it :(

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Posted (edited)

If you got mains elec--then why not just run ASHP for underfloor 

sounds like a lot of complications 

I have run a 2 source heat system and  as has been said good control to get best   from both systems is not easy 

when i up graded heat source to ashp from previous system --nothing was changed on the exisiting UFH 

just the heat source 

I used a mitsubishi ashp with their pre-plumbed tank+ pumps etc  which does all the hot water as well 

simple solution 

the only complication Iadded was solar thermal panel ,as the tank was already plumbed for that as well

80% of hot water over the year comes from the solar thermal 

resulting energy savings over the lpg system+solar thermal  was impressive  

lpg bill was £1500 --that dissappeared and my elec bil only went up by £500--so £1000 saving 

worh a look i feel  for a simple integrated system 

then you get the RHI  payment for next 6 years as well


Edited by scottishjohn

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Out of interest, what size of hot water store should you have for a wood burner/boiler, is there a certain amount of litres per KW output?

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1 hour ago, Ballynoes said:

The trouble with a thermal store is you need to store water hotter than your required DHW temperature, otherwise it will only be able to deliver a tiny amount of water before the temperature drops.  A heat pump will struggle to get a thermal store hot enough  That is why an UVC is better for use with a heat pump as you heat it to the desired water temperature and it will then deliver pretty much that temperature water until it runs out.


Which brings us back to the issue that is is very complicated to link a stove to an UVC. Much less so to link a stove to a thermal store.


There does not seem to be an easy solution for stove and ASHP heating the same hot water system.

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5 hours ago, epsilonGreedy said:

Relative to absolute zero (-273) your maths looks plausible but surely what is significant to ASHP is the heat capacity available down to the minimum chilling operating range of an ASHP. If an ASHP can chill to -10, then the available heat capacity at -5 is half of that available at Zero.


Obviously there's a low temperature beyond which any particular refrigerant will not work but thinking about the amount of heat in the input air to a heat pump is confusing. If there's less heat in a m³ of air the cure is just to blow more air through. That'll mean a bit more electricity to drive the fan but that's all.


The limitation on the CoP of a heat pump is set by the basic laws of thermodynamics. Real heat pumps will get about half this limit but do tend to scale roughly in proportion.

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