Converting from oil boiler to heatpumps (Air-air and air-water)

[crappy]

Hi folks,

I have:
* 25 year old oil boiler which is akin to Trigger's Broom.
* Live in a large converted barn, large open spaces and normal radiators which need changing.
* Excess solar generation (9KW)
* Large battery bank and the ability to recharge it at night at 7.5p/unit.


I want to get install a few aircons in the house which will use excess solar/battery to warm the house up.
I also pondering using a air-water heatpump to run the rads. The best I can find is one which goes up to 65C on the water temp.

Given I have loads of spare electricity, this feels like the best thing to do.

Question:
Does this sound like a bad idea? Anything wrong with the mix of the two?

 

[Johntheo5]

What is your heating demand now, roughly?.

 

[crappy]

Wife demands far more than I...

Not sure how to answer your question. In kw?
The boiler is, I believe, 25kw.

 

[Johntheo5]

kwh if possible, per annum or litres of oil/annum if oil fired.
How many rads have you got.

I have kept a very accurate history of my own house heating (&HW) energy usage for years and years. I have a SE (non condensing) 20kw Firebird oil fired boiler. I have averaged exactly 1474 L/year (over the last 10 years) of kerosene ~ = 15108kwh/year, ~ = 12086kwh/year Net in a 4 bed semi, with heating on 16 hours/day.
Highest consumption say from Jan to March is 87kwh/day or 70kwh/day, net.
You can use this as a rough template of what you might use.

If you were to install a HP with a COP of 2.8 (280% "efficiency") then the HP compressor elec consumption should be, 70/2.8, 25kwh/day.
question is, how much excess solar generation will you have available in those winter months?, charging your battery at 7.5p/kwh is attractive vs my last oil price at 8.1p/kwh, don't know what gas prices are, if you have a gas fired boiler. Of course you would then require a 25kwh battery pack, about the capacity of the first generation Nissan Leafs!!.

Personally, I wouldn't go down that road but spend the £10k/£15k conversion costs on upgrading your house insulation, then look at your energy usage after this and make your decisions then.

I see you have a oil boiler.

 

[crappy]

Interesting calcs.

I "top up" oil 3 times a year and always 1500-2000L @ 79p/L

Rads:
3 x 1.4m doubles in the large open spaces and master bedroom.
5 x 600mm bathroom and utility room dump ones
4 x 600mm double along hallway
3 x 800mm doubles in bedrooms
2 x 800mm doubles in lounge
2 x 800mm doubles kitchen and tv room

The boiler is a Heatslave 20/25.

Given the house is a converted (and listed) (in 1999/2000) barn, it is quite leaky.
I've done everything I can on windows (they're newish, double-glazed)
The house does not get cold in winter but that's because I through fire at it

I do actually have 27KWh of battery which runs the house just fine.
On a dank day in Jan/Feb, the panels hold things neutral and does not really discharge the battery other than when the sun goes down.
At this point, I can run like this (with no heating yet) for 2 days without topping up.
Overnight, if required, I can charge them by 90% overnight between 23:30 and 05:30.


Two main reasons I want to change:
1. The boiler is 22 years old. Last winter it broke down at least once per month. Soon it's going to be unecconomical to repair.
2. I've invested in PV and Batt to do this so need to see it through

I converted my garden annex - installed a 5KW HP (air-air). It's bloody brilliant. takes 10 minutes to heat the rooms up. During start-up it draws around 1.2KW and quickly settles to 300ishW and continues to throw warm air out. See below. (Yes, I had insomnia so woke up early this morning)

Right, my thinking!
1. Do I go water-water and, if needed, upgrade the rads to be bigger? (Starts getting more expensive).
2. As 1 but backfill with air-air?
3. Start with air-air and see how it goes and gettison the boiler at some point.

 

[vulcancontinental]

The Carrier Aquasnap 290 can hit 75 degrees but like all heat pumps keep it as low as possible. Insulate, carry out an accurate heat loss then decide.

 

[Johntheo5]

Interesting calcs.

I "top up" oil 3 times a year and always 1500-2000L @ 79p/L

Rads:
3 x 1.4m doubles in the large open spaces and master bedroom.
5 x 600mm bathroom and utility room dump ones
4 x 600mm double along hallway
3 x 800mm doubles in bedrooms
2 x 800mm doubles in lounge
2 x 800mm doubles kitchen and tv room

The boiler is a Heatslave 20/25.

Given the house is a converted (and listed) (in 1999/2000) barn, it is quite leaky.
I've done everything I can on windows (they're newish, double-glazed)
The house does not get cold in winter but that's because I through fire at it

I do actually have 27KWh of battery which runs the house just fine.
On a dank day in Jan/Feb, the panels hold things neutral and does not really discharge the battery other than when the sun goes down.
At this point, I can run like this (with no heating yet) for 2 days without topping up.
Overnight, if required, I can charge them by 90% overnight between 23:30 and 05:30.


Two main reasons I want to change:
1. The boiler is 22 years old. Last winter it broke down at least once per month. Soon it's going to be unecconomical to repair.
2. I've invested in PV and Batt to do this so need to see it through

I converted my garden annex - installed a 5KW HP (air-air). It's bloody brilliant. takes 10 minutes to heat the rooms up. During start-up it draws around 1.2KW and quickly settles to 300ishW and continues to throw warm air out. See below. (Yes, I had insomnia so woke up early this morning)


View attachment 304043




Right, my thinking!
1. Do I go water-water and, if needed, upgrade the rads to be bigger? (Starts getting more expensive).
2. As 1 but backfill with air-air?
3. Start with air-air and see how it goes and gettison the boiler at some point.

Thats a very interesting graph of the HP power consumption, what reduces/controls the power, is there a variable speed fan somewhere??.

Air to Air is very attractive, of course no water heating then but you can get a stand alone HP for this. If you go fully air to water then you should run the HP water temperature at say 45C to achieve a reasonable COP, this will give HW at 40C, enough for most purposes and then just boost the cylinder temperature to 60C once per week for legionella protection, you can use a electric heating element to do this, will only consume ~ 4.7kwh/week to boost a 200L cylinder from 40C to 60C.
The rad heating downside of running with say flow/return temps of 45C/37C is that the rad output is only 32% of the rated output.
Other considerations with air to air is defrosting, how is this done?.

Maybe just start, as you suggest with air to air and see how it goes. How woukd you get the air to different rooms?, stand alone units?.

 

[crappy]

The Carrier Aquasnap 290 can hit 75 degrees but like all heat pumps keep it as low as possible. Insulate, carry out an accurate heat loss then decide.

Very interesting - thanks. Challenge will likely be, that 75 is in ideal conditions. See below.

Thats a very interesting graph of the HP power consumption, what reduces/controls the power, is there a variable speed fan somewhere??.

It's how the inverter ones work. Variable speed fan and compressor. As it catches up with life, it backs off considerably. Very efficient.

Other considerations with air to air is defrosting, how is this done?.

This is a challenge.
The 5kw one I have in the annex was interesting when super cold (-12 we saw). Anything below 2C saw the same reaction to varying degrees.

The unit would do as its told and try make lots of heat. Very quickly it would frost over, go into a defrost cycle and try again. This was a 10-15 minute cycle. During this time you'd either get no air inside or very tepid air.

The trick I found was to tease it into working. Instead of setting to 22C, set it to 11 (the annex (used only as an office) cools to below 10 at night... Converted barn )
Then 12 then 15 then 20 etc. It was, to be honest, a pain. I don't want that in the house.

Where HP is used for underfloor heating, I don't you get or realise this problem exists due to the thermal mass of the floor.

To a large extent, I don't think I will because I won't let it drop to a point where it needs all the juice. Watch this space...

Maybe just start, as you suggest with air to air and see how it goes. How woukd you get the air to different rooms?, stand alone units?.

I think I'll go air to air and keep the boiler to start. With the above in mind, at least if it struggles I won't be cold.

 

[vulcancontinental]

Very interesting - thanks. Challenge will likely be, that 75 is in ideal conditions. See below.


75ºC at -5ºC. The unit will do it but the COP will not be as good as it would be at say +5ºC

 

[crappy]

Have you seen that in practice and for prolonged periods? I ask because my annex one can operate down to -7.5. Yes, it operates but only just!

 

[Johntheo5]

The unit would do as its told and try make lots of heat. Very quickly it would frost over, go into a defrost cycle and try again. This was a 10-15 minute cycle. During this time you'd either get no air inside or very tepid air.

Where does the unit get the heat from to do a defrost??.

Where HP is used for underfloor heating, I don't you get or realise this problem exists due to the thermal mass of the floor.

No problem with low temps for UFH, I was referring to using existing rads which will only emit that 30% or so of rated heated at 45C/37C/20C conditions.

Have you got a link to that air to air unit please.

 

[crappy]

Defrost, I believe (and chimes from what I observed) is the outdoor unit going into reverse. It warms the radiator up slighlty to, well, defrost.

I noticed another unit which has an optional 3 or 9kw resistive heating element to aid cold weather. I could not work it out from the specs but the ideal situation here would be to modulate that heat just to keep it on the cusp of freezing over.

Link to my current unit: https://www.orionairsales.co.uk/pan...17000btu-r32-a-wi-fi-install-pack-15274-p.asp

I'll likely end up getting 7kw or 9kw unit (I think they have a bigger outdoor unit) and never run it at full tilt in cold weather to avoid the freezing.

 

[woodpeckerx]

I "top up" oil 3 times a year and always 1500-2000L @ 79p/L

Oil is 55p/L at the moment (+5% VAT). What area/volume are you heating, that usage sounds enormous unless you have 500m2 property?

I moved from a late 1960s 170m2 detached house, cavity wall insulated and loft insulation that was way below current regs, my 20 year old oil boiler with a Riello RDB buner never consumed anywhere near 2000L in a year, the house was always hot and used it for HW in the summer. For the 20 years I was using it the maintenance cost was zero.

I know you've invested in solar now but there's a high probablity there will be failures within the 10 years it takes to break even, the cheap and nasty capacitors used in the inverters swell and fail far too often.

Personally I would replace the oil boiler and insulate the house with the money. I'm now on gas due a house move but would switch back to oil at the drop of hat. I've looked at various *eco* systems and concluded they will all be a money pit and highly likely to have failures way before they break even and may never even break even. Oil is by far the cheapest heating option and ~half the current price of gas per kwh at todays prices.

 

[vulcancontinental]

Have you seen that in practice and for prolonged periods? I ask because my annex one can operate down to -7.5. Yes, it operates but only just!

No, only the certified test figures. What's the refrigerant in your annex unit, who is the manufacturer?

 

[Johntheo5]

Defrost, I believe (and chimes from what I observed) is the outdoor unit going into reverse. It warms the radiator up slighlty to, well, defrost.

Its a air to air unit? so what have the rads got to do with it?.