Oil to ASHP. Can I? and do my numbers workout?

[pacemaker1000]

Large house with 90% underfloor heating, just 3 small rads hardly used.

current oil consumption 2500 litres per year of which I estimate 700 hot water. Switched water to electric so working on 1800 litres for heating used 6 months of year in UK climate or 300 per month and 10L a day?

boiler is 17 years old so assuming 80% efficiency and read 1L oil approx translates to 8kw of energy so 10x8=80kw?

So I need an 10kw ASHP on for 8 hours or 16kw for 5 hour’s continuously?

ASHP has a ratio of 4to1 so cost would be 20kw a day 

 

excuse if this is garbage 😁

 

[Nickfromwales]

18 minutes ago, pacemaker1000 said:

Large house with 90% underfloor heating, just 3 small rads hardly used.

current oil consumption 2500 litres per year of which I estimate 700 hot water. Switched water to electric so working on 1800 litres for heating used 6 months of year in UK climate or 300 per month and 10L a day?

boiler is 17 years old so assuming 80% efficiency and read 1L oil approx translates to 8kw of energy so 10x8=80kw?

So I need an 10kw ASHP on for 8 hours or 16kw for 5 hour’s continuously?

ASHP has a ratio of 4to1 so cost would be 20kw a day 

 

excuse if this is garbage 😁

 

Hi.

Not garbage, but missing some key points to better calculate your comparison  

A CoP of 4:1 would be achievable if the flow temp to the heating emitters is very low, but with rads in the mix you'll likely be up much higher. When the temp goes up, the CoP goes down, and it goes down again when winter removes a significant amount of heat energy in the air.

Lots of things change these numbers, such as the amount of insulation under the UFH, the fabric and ventilation heat loss of the house ( how many kWh of heat are required to be input to keep the house at 21oC when it's bitterly cold outside ) and more.

The oil will be a difficult beast to replace, especially in a home which is not efficient and highly insulated / excellent glazing etc, so maybe consider going for a Grant Hybrid ASHP with an integral oil burner for the bitter parts of the year when the HP cannot quite keep up.

LINK  

[pacemaker1000]

Thanks never knew about hybrids.

think I’m pretty well insulated. Boiler packed in once and house stayed warm for days. I thought I could use my existing energy figures and just replace boiler KW's with electric?

[pacemaker1000]

Hybrid looks good as a backup but doubt the 5k grant would allow it.

going back to original post. If a need 20kw a day, which would be best? Small pump flat out or biggest I can get?

I have 4hrs of cheap off peak electric so would make sense to run continuously during this period so biggest would be cheaper to run

 

only three small rads that hardly used as in attic space 

[Nickfromwales]

6 hours ago, pacemaker1000 said:

Thanks never knew about hybrids.

think I’m pretty well insulated. Boiler packed in once and house stayed warm for days. I thought I could use my existing energy figures and just replace boiler KW's with electric?

Yes but you hadn't reduced the 4:1 figure which I would imagine is optimistic for heating via rads plus UFH, if keeping existing rads.

 

6 hours ago, pacemaker1000 said:

I have 4hrs of cheap off peak electric so would make sense to run continuously during this period so biggest would be cheaper to run

 

only three small rads that hardly used as in attic space 

4 hours on Octopus? What will you do before that kicks in? If the system has already heated up via 'other' the HP will have very little to do, so deffo smaller HP going flat out imo. Noise for the neighbours may be a consideration, if you have any?

Can you introduce a sizeable buffer tank? You could use those 4 hours to heat the system plus the volume of water in the buffer, then for the HP to shut off after the 4 hours expires. The heating would then consume the stored energy before needing to draw off oil again.

If your home is very well insulated and has such low heat loss characteristics, then it sounds like this would ( could ) be a good option. Any HP will suffer short-cycling if the house performs very well and the heat demand is very low and steady ( eg after it has been initially brought up to 'comfort' temp ).  

[ReedRichards]

I replaced an oil boiler with an ASHP; one company I approached (Grant) initially suggested a hybrid boiler but it wasn't necessary, not remotely as I ended up with a 12 kW heat pump.  Don't be put off by nay-sayers.  However don't confuse your seasonal average energy use with the amount you will use on a cold winters day.  My average oil usage was 59 kWh per day (2100 l of oil per year).  My average ASHP usage (all-radiators) is about 19 kWh per day but the biggest 1-day usage (that I noticed) was 54 kWh.

 

A COP of 4 seems a tad optimistic; you would have to operate the radiators at the same low temperature (35 C ???) that your UFH requires so the ones you have would need to be replaced by radiators with a very much larger surface area.

 

Most people with UFH use it for long periods because it is very slow to heat up and cool down (so if the interior cools it takes a long time to warm up again).  Be guided by the mode with which you operate your oil boiler at present.  If you can run it for 4 hours then turn it off for the rest of the day you'll be able to do the same with your heat pump.  I doubt this will work and it's too warm outside now to do the experiment.  If you want your UFH to take-in and store a lot of heat you will need to wear thick-soled shoes much of the time as your floor will be too hot for bare feet (I suppose you could use a huge buffer tank but my system won't heat the buffer tank alone, there has to be a thermostat demanding heat).  And heat pumps run less efficiently at higher output water temperatures so you can only dump a lot of heat quickly by sacrificing performance.  All this considered, my vote is for small pump flat out, except most of the time it won't be.  

 

Your heat pump will only need its full output capacity on cold winters days; most of the time it will be under-employed.  Heat pumps generally can't modulate down to a small fraction of their full capacity so the MCS accredited approach is to match the size of the heat pump to the 99.6% requirement of the heating with margin for the hot water and defrosting.  If you are looking to get the £5k grant you may well be guided along this direction.  On the other hand people on this forum who do a DIY install seem to favour over-capacity.  If you get an MCS-accredited installer they should do a heat loss calculation and work out the size of heat pump you need; just check whether the calculation is based on intermittent or continuous use  

 

Edited May 12, 2022 by ReedRichards

[ReedRichards]

27 minutes ago, Nickfromwales said:

Any HP will suffer short-cycling if the house performs very well and the heat demand is very low and steady ( eg after it has been initially brought up to 'comfort' temp ).  

 

Any heating system will cycle unless:

1. It can modulate its output (to as little as necessary)
2. It knows the both the desired room temperature and the actual room temperature ("Load Compensation").

Oil boilers do not modulate so will come on until either the room thermostat is satisfied or the return water temperature gets too high and causes a cut-out.  ASHPs can modulate (but not as much as gas boilers).  Some can do Load Compensation; other cannot.

 

Short cycling is when the normal cycles are so short in duration that wastage of energy that occurs when the heating system is cranking itself up has a significant impact on overall efficiency.  With an oil boiler there is not much that you can do about it.  Heat pumps generally use weather compensation to reduce the amount of short cycling but if you oversize your heat pump for the demands of your house then it will run in shorter cycles.    

[joth]

8 hours ago, pacemaker1000 said:

boiler is 17 years old so assuming 80% efficiency and read 1L oil approx translates to 8kw of energy so 10x8=80kw?

So I need an 10kw ASHP on for 8 hours or 16kw for 5 hour’s continuously?

ASHP has a ratio of 4to1 so cost would be 20kw a day 

 

excuse if this is garbage 😁

 

Very close, your maths seems good but your units are garbage 😅

There's a serious difference between kWh (energy) and kW (power) and you can't interchange them. 

 

 

LMFTFY:

 

boiler is 17 years old so assuming 80% efficiency and read 1L oil approx translates to 8kWh of energy so 10x8=80kWh?

 

So I need an 10kW ASHP on for 8 hours or 16kW for 5 hour’s continuously?

 

ASHP has a ratio of 4to1 so cost would be 20kWh a day 

 

 

 

 

 

[Fly100]

1 hour ago, joth said:

 

 

 

ASHP has a ratio of 4to1 so cost would be 20kWh a day 

 

 

 

 

 

 

 

Sanity checking this, is the 20kWh worked out from the the ampage drawn by the unit ? as you have to consider there maybe some modulation too

[pacemaker1000]

Thanks for all the great replies. Good to know it is feasible to use an ASHP with my setup.

should clarify all my UFH is in concrete so heat retention is excellent hence running during cheap hours. Would probably set all room stats to 22c during this time.

 

[joth]

1 hour ago, Fly100 said:

 

Sanity checking this, is the 20kWh worked out from the the ampage drawn by the unit ? as you have to consider there maybe some modulation too

 

To clarify, the OP doesn't own a ASHP yet so this is not empirical data from measuring an installed system. The OP's method is to calculate the output heating energy demand per day (80kWh) then divide by the COP (assumed a hypothetical COP=4) to get the primary energy demand of 20kWh per day.

This is the integral of voltage * current over the day. so it could be 10amps at 200V for 10 hours, or 1 amp at 20,000V for an hour, or any other mix. It would depend on the specific ASHP selected and installation environment.

 

(obviously 20kV heat pump is ludicrous but the point is for this stage of the paper exercise, the specific current and voltage don't matter, so long as the integral of power over the day can meet the goal)

 

Edited May 12, 2022 by joth

[SteamyTea]

6 hours ago, ReedRichards said:

With an oil boiler there is not much that you can do about it.

Why don't oil burners modulate.

My car is a diesel, modulated extremely well.

[HughF]

1 hour ago, SteamyTea said:

Why don't oil burners modulate.

My car is a diesel, modulated extremely well.

Very good question.... It's probably because the pump is a continuous displacement, instead of the continually variable displacement of an injection pump or common rail injector.

[pacemaker1000]

Spoke to installer who says rough rule of thumb is 50watts per square meter

seems about right

 

suggested a 10kw pump as sufficient and would work harder than bigger one. Be more efficiently too.

 

RHI grant will cover cost of pump and expects installation to be around £2000

as oil boiler is old and be circa £2000+ To replace it seems a no brainer?

Edited May 12, 2022 by pacemaker1000

[Wil]

Another oil to ASHP user here. Although I ended up with 2. Big ones. The second has been mostly redundant since March but was very much in demand Dec, Jan, Feb. Still awaiting my PP to do the renovations to unlock the fabric improvements I hope to make. WBS in the main living room (also coldest least insulated room) as a backup/ top up.

 

House loss anywhere between 50 and 200W/m2 for something that wasn’t built from the ground up to be energy efficient so you’d be a little on the lower side with that assumption… 

 

W

[ReedRichards]

Me 2100 litres of oil per year.     @pacemaker1000 2500 litres of oil per year. 

Me 12 kW ASHP.                          @pacemaker100010 kW ASHP (suggested)

Me MCS heat loss calc.                @pacemaker1000 Installer uses rule of thumb

Me New DHW cylinder                 @pacemaker1000 Not mentioned so presumably uses old cylinder with immersion heater.

 

 

[joth]

4 minutes ago, ReedRichards said:

Me 2100 litres of oil per year.     @pacemaker1000 2500 litres of oil per year. 

Me 12 kW ASHP.                          @pacemaker100010 kW ASHP (suggested)

Me MCS heat loss calc.                @pacemaker1000 Installer uses rule of thumb

Me New DHW cylinder                 @pacemaker1000 Not mentioned so presumably uses old cylinder with immersion heater.

 

 

 

OP says water already switched to electric and implies the ASHP would only be for space heating.

 

I assume this installer's 10kW guess is just for guideline pricing? If they do an MCS supply and install they're obliged to do the room by room calcs so would in theory find any shortfall and upsize the unit then. But I personally would not have any confidence in that 50W/m2 guess so would get some more quotes first!

 

 

 

[ReedRichards]

1 minute ago, joth said:

 

OP says water already switched to electric and implies the ASHP would only be for space heating.

 

 

Yes, I read that but is it the right thing to do?  You would have to run the immersion heater solely on some cheap night-rate tariff to achieve a comparable running cost to using an ASHP to heat the water.   

[pacemaker1000]

Yes hot water on cheap tariff. Enough for three showers and a bath at moment for 75p a day.

This maybe changed in future to solar electric 

[ReedRichards]

59 minutes ago, pacemaker1000 said:

Yes hot water on cheap tariff. Enough for three showers and a bath at moment for 75p a day.

This maybe changed in future to solar electric 

 

But this might be about 30p a day if you were using a heat pump on that cheap tariff, saving 45p a day about £165 per year.   

[pacemaker1000]

Waiting for clarification from installer but may get away  with 10kw which given current grant would be free! 
slight cost if need bigger pump

seems a no brainer if it works as it’ll cost £2k in oil and probably same if need new boiler soon due to age.

installer not sure if HAVE to include hot water too to get grant, which could always disable, as only need 34c for underfloor heating

[ReedRichards]

59 minutes ago, pacemaker1000 said:

 

installer not sure if HAVE to include hot water too to get grant, which could always disable, as only need 34c for underfloor heating

 

Yes but your heat pump does not operate at one fixed temperature (unless it is extremely primitive).  When it needs to heat the hot water it will ramp up the temperature to 55 C and you will get a COP in the range 2.5 to 3.0.  The rest of the time it can give you 34 C (or less on a warm day) and hopefully you will get that COP of 4 that you aspire to.   So you have a choice between heating your hot water with a COP of 1 or a COP of 2.5 to 3.0.  I cannot understand why you want to do the former. 

[HughF]

1 hour ago, pacemaker1000 said:

Waiting for clarification from installer but may get away  with 10kw which given current grant would be free! 
slight cost if need bigger pump

seems a no brainer if it works as it’ll cost £2k in oil and probably same if need new boiler soon due to age.

installer not sure if HAVE to include hot water too to get grant, which could always disable, as only need 34c for underfloor heating

Remind me why you're bothering with the grant funded install again? You do realise that you can do the heatloss calcs and hp sizing yourself, buy a unit for about £3k and just pay a plumber to fit it (or do it yourself)? Saving yourself probably 5-7k in the process...

[pacemaker1000]

Sorry Hugh I don’t understand why you’d query getting the grant?

ASHP cost is around 4-5k as don’t get vat back on direct purchase. Plus install would be at least £2k, so £7 in total?

with grant it’s completely free or at most just the difference in a 10 and 12k pump….no brainer?

 

re other comments:

 

I would run hot water on whatever is the cheapest which would probably be Pump during cheap electric? During this time in winter I would take advantage of cheap electric to heat all rooms to about 22*C, or at least best it could do as concrete will then retain heat during most of the day and Pump would kick in when needed. So question would be whether Pump could cope with DHW and heating during this short period.

 

of course in Summer HP is not needed if using alternative ways for the DHW. This would mean he ASHP would be turned off completely for 6 months and don’t know if this would be a bad thing so using it for hot water may be necessary?

[joth]

You originally mentioned running 10kW HP for 8 hours each day. This won't work in cheap rate window!

Likewise 16kW for 5 hours may work in economy 7 but rules out octopus Go (4 hour at 7.5p/kWh). So you'll need to go as large as you can if you want to optimize for cheap rate electricity

 

Fwiw we have a 8kW ecodan (160m2 passivhaus) and run it for 3:30 or less each day on heating and then 30mins DHW to fit into Octopus Go cheap rate. On cold days I have automation that sets the UFH flow temperature very high then backs it off as the in-screed thermistor heats up, to ensure I get maximum utilisation during the cheap rate (even though this reduces COP, it works out cheaper due to the tariff)

Edited May 13, 2022 by joth