Air source heat pump with log burner

[Nikki Locke]

I am having an ASHP installed in my home next month. I have had an EPC survey done, and everything is OK for getting RHI. 

 

In view of the fact that our house was without electricity for 5 days after the storm last week, I would like to have an alternative source of heating for such emergencies. I was thinking of adding a log burner with back boiler, feeding into the same heat store as the ASHP.

 

However, I read somewhere on the Internet that this would not be allowed under the RHI. 

 

Is that right? 

 

Also, I am concerned about using a heat store with an ASHP. If the hot water runs off a mains pressure coil in the tank, and the tank is only at ASHP temperatures (55 deg or less), will the hot water actually be hot enough? I like my hot baths! 

 

Nikki 

[joth]

Are you replacing a gas boiler with ASHP? f so, you already have a dependency on both gas + electricity both being available simultaneously for heating, so moving to ASHP actually slightly reduces the probability of an outage.

Likewise, a log burning feeding into the heat store will be useless in the case of a power cut, as you need electricity to pump the water from the log burning to the store, and from the store to the heat emitters (and to run the control systems).

 

Aside from the RHI complications, plumbing a log burner to an ASHP system massively complicates the system and will increase install and maintenance costs. If you're worried, put the log burning in as a stand alone heat source and huddle around it when there's a power outage.

 

 

28 minutes ago, Nikki Locke said:

Also, I am concerned about using a heat store with an ASHP. If the hot water runs off a mains pressure coil in the tank, and the tank is only at ASHP temperatures (55 deg or less), will the hot water actually be hot enough? I like my hot baths! 

 

Technically domestic hot water shouldn't be delivered above 48ºC, so 55 is plenty. The only downside of storing it at this temperature is you need a larger volume of store to hold the same amount of energy. i.e. the lower the temp you store it at, the bigger the cylinder. This is why ASHP are typically installed with a 200-300L cylinder.

btw most people find an unvented cylinder is better than a thermal store, as it performs better when being run down to lower temperature. (Stratification means you get more useful HW from it)

 

Edited December 12, 2021 by joth

[ReedRichards]

The RHI rules do allow you to have a secondary heat source but it makes everything more complicated.  However a log burner without a back boiler would not count as a secondary heat source so would not have any effect on your RHI.  That would be far simpler to implement.  It's what kept us warm during the recent power outage.

 

At 55 C you get second degree buns after 17 seconds of exposure.    

Edited December 12, 2021 by ReedRichards
Added info.

[J1mbo]

As said, do get log burner(s) but don't connect them to the ASHP. Bear in mind that over a certain output (I think 7kW) they have to drill a massive hole in the house for air supply. So you might conclude that a smaller log burner without the 5" hole is better, depending obviously on the overall heat demand.

 

As an aside, a log burner can also be a useful way to achieve a really warm lounge on a cold evening without having to play with the heat pump too much. i.e. let the ASHP keep the fabric broadly steady pretty much 24x7 and use the log burner when you fancy a toasty warm night in as well.

[SteamyTea]

1 hour ago, Nikki Locke said:

I like my hot baths

Have you ever measured the temperature to see what it actually is.  Probably nowhere near as hot as you imagine.

1 hour ago, Nikki Locke said:

without electricity for 5 days after the storm last week

It this a common occurrence?

There may be better ways than a polluting log burner.

[Nikki Locke]

Thanks, everyone,for the replies. I will take your advice, and not try to integ the log burner with the ASHP. 

 

Although having our electricity cut off for days at a time has not happened before, with climate change we can expect more extreme weather, and our overhead supply has a line of crack willow and ash trees alongside it. 

 

[Redoctober]

24 minutes ago, Nikki Locke said:

Thanks, everyone,for the replies. I will take your advice, and not try to integ the log burner with the ASHP. 

 

Although having our electricity cut off for days at a time has not happened before, with climate change we can expect more extreme weather, and our overhead supply has a line of crack willow and ash trees alongside it. 

 

 

Yes, just to underline what has been mentioned already as I have picked up on this thread somewhat late in the day - we too have an ASHP and a stand alone Log burner. Our water is set at 55 degrees and trust me, that is more than hot enough for baths etc. We also suffered a loss of total power for 6 days recently and it was the log burner that kept us cosy and warm. The other thing worth mentioning as part of any forward planning - we have now invested - cheap as chips really - in a camping stove and kettle. Our neighbours lent us theirs and it was a real morale booster and well worth its weight in gold.

[bassanclan]

You could have a log burner with boiler feeding into a buffer tank.

 

Log burners with boilers work more efficiently when they have a electric pump but MUST work via thermo-syphoning (i.e. hot water rises) in the even of a powercut, otherwise they could cause the water in the boiler to boil like a kettle

[George]

On 19/12/2021 at 15:06, Redoctober said:

 

Yes, just to underline what has been mentioned already as I have picked up on this thread somewhat late in the day - we too have an ASHP and a stand alone Log burner. Our water is set at 55 degrees and trust me, that is more than hot enough for baths etc. We also suffered a loss of total power for 6 days recently and it was the log burner that kept us cosy and warm. The other thing worth mentioning as part of any forward planning - we have now invested - cheap as chips really - in a camping stove and kettle. Our neighbours lent us theirs and it was a real morale booster and well worth its weight in gold.

 

This is my set up and agree with not even trying to integrate ASHP and biomass. I briefly investigated it but found all the issues mentioned here - I did have a back boiler pre-ASHP but got it decommissioned (probably should have filled it with sand but want to replace it with an external air fed, room sealed fire sometime in the next year or so)

 

So yes, I have an ASHP with two log burners as space heaters (both were already in the house and used to be our only reliable heat source, brrrr). Gas cylinder, camping hob and kettle in the shed. 

 

Since finishing the renovation I've only experienced one 4hr+ power cut and I dug out the territorial army hexi-stove to make a cup of tea. 

 

I do light one log burner fairly often as it's in my office and means I can switch off heating everywhere else if no-one else is around. Although I've never really needed them as the ASHP can heat the house in all conditions we've experienced so far. 

 

Log burners with flat tops can be used as ovens in a pinch, or to toast marshmellows.