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Author Topic: High Temp heat pump vs Oil boiler for radiators  (Read 5192 times)
texasholden
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« on: April 16, 2020, 11:38:00 AM »

Hi all,

Firstly I hope you all are keeping well at the present time.

I don't want to bore you but might be relevant so background info...

We have a 300ltr TS connected to a WBS and with backup 3kw electric elements x3. The setup is a very basic one I did myself a few years back (with both inspiration and much needed advice coming from this forum for which I am very grateful) and overall it's worked very well for us.

However, having now married and being lucky enough to have a healthy baby daughter we are rapidly outgrowing our rather modest stone cottage. This said, recently it became an option that we might purchase the neighbouring cottage (even more modest than ours) and the two combined would be our forever home.

The cottages are both basic although I would upgrade the glazing and insulation wherever possible. This said I can't see that wet UFH is an option (I did look into it when renovating our cottage but there aren't really any foundations to speak of so digging down a foot or so to add the necessary insulation wasn't feasible, hence why I opted for rads, so I'm thinking I'd likely go the same way with next door, which leads me to whether I would combine rads with:-

1) High temp air to water heat pump running at say 55C
2) Oil tank / boiler

My question is whether there are heat pumps that can genuinely provide 55C hot water at a reasonable COP given ambient temps around 0C for much of the period of peak consumption. I suspect that I'm looking at annual consumption of circa 10k kwh via the oil / heatpump.

Any advice welcome
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Countrypaul
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« Reply #1 on: April 16, 2020, 12:57:28 PM »

Have you got any figures for the amount of expected heat required at -3C outside? Are you looking at 10K KWh as the heat for the comined home or just the new part?
Have you done any modeling in say Excel using figures from Degree days?

You indicate that you want a flow temperature of 55C (from either a HP or Oil boiler) - can you explain why you believe you need that temperature?

One feature of using a HP that many adopt is to use much larger output radiators running at a lower circulation temperature hence the question of why 55C.

I am assuming you want a single combined heating system that would heat what is currently 2 properties but will become one.

By using larger radiators at a lower temperature you might find that you can easily reduce the circulation temperature and make a HP more effective. You might find that the modeling then indicates you could heat the whole place using say 40C circulation except for maybe 10 days a year. Those 10 days could then be supplemented by either your WBS, or additional direct electric heating (as for example, you already have in place, or even oil filled electric radiators).

Since you have a WBS, I assume the TS is open vented, is your CH system also open vented? The reason for asking is that if pressurised you have to use a Heat Exchanger which introduces some inefficiency and might make life more complicated if the HP (or OB) requires a pressurised system.

In any modelling, you should also consider the effect of using E7 or TOU tariffs for the electricity.

Does the TS supply DHW plus CH? If your family grows, it may be that the TS is not large enough to cope with both and a separate DHW tank may be preffered (use the TS to prewarm the DHW?).

Don't overlook the cost of servicing for an oil boiler - from my exerience they are not trouble free. I lived with one for 25yrs and the first time I discovered that plastic tanks fill with water was when the pump ceased due to getting water in and corroding. The plastic tank I had did not have any drain valve to I have to siphon off the collected water every year. If the boiler was on when the tank was filled then debris would get stirred up (algal growth at the water/oil interface I believe) this would be drawn in the boiler (it would be too fine for the first filters) but block the filter in the pump and that in the nozzle.  Whilst mice and rats (I lived near a farm in the country) tended to have a go at the insulation, more than one engineer who serviced it commeneted I was lucky (or perhaps they were) as they got used to cleared diced mice from the fan units at other locations!

Have moved house and now have a HP, but not had it long enough to give any useful feedback on it. It will need an annual service if you get RHI for the HP though.
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RIT
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« Reply #2 on: April 16, 2020, 02:25:28 PM »

A lot of background info can be found at (or linked from)

    https://energysavingtrust.org.uk/renewable-energy/heat/air-source-heat-pumps

As Countrypaul has already noted an output temp of 55C is something of a target as it means you are expecting a very large uplift during the coolest time of the year.

One thing you should do is try and get a quote or 2, these will give you some firmer figures to work from, for both an HP and oil-based solution.


Another critical item is going to be your location as that will impact the oil costs as well as your electricity costs. For electricity, the key issue is can you have a smart meter installed. It is likely that the lowest cost tariffs for electricity are going to be tied to smart meters, so knowing if you can have one installed will greatly affect your calculations.

The cost of heating oil at the moment is very low, but it would be a mistake to use the current cost in your calculations, you can see average historic costs for the last few years at
 
      https://www.boilerjuice.com/heating-oil-prices/


One other thing to consider is to mix and match a bit. A home wide HP has the issue that you are trying to size for the largest, most used room. So you end up with needing a high flow temp or some very large radiators. An alternative could be that you also install a ASHP to support the heating of just that room so that the main system can operate at a lower temp.
« Last Edit: April 16, 2020, 02:31:47 PM by RIT » Logged

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ecogeorge
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« Reply #3 on: April 16, 2020, 03:57:17 PM »


This said I can't see that wet UFH is an option (I did look into it when renovating our cottage but there aren't really any foundations to speak of so digging down a foot or so to add the necessary insulation wasn't feasible,
can you elaborate why not ??
I dug down in an old cottage with no foundations , damp proof membrane -concrete pad -75mm insulation -75mm screed with pipes in.
bring the membrane up the walls to where plasterboard will stop . result is lovely warm floor and damp eliminated.
George
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texasholden
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« Reply #4 on: April 16, 2020, 04:35:48 PM »

Thank you both for the input - much appreciated and I will take a look at the links over the next few days but meantime to quickly clarify a couple of questions...

Yes, I am thinking of combining the heating systems (my current is indeed a vented TS supplying DHW via a PHE with a CH pump kicking in when store reaches 70C).

In terms of why 55C, it's really down to unfettered wall space being at a premium, so even at 55C with some rough calcs based on double panel convectors where sensible I'd probably be able to achieve only about 18C at zero outside - which is probably fine - as the aim is to take the chill-off and as commented I can always top up with WBS or electric.

Thanks for the advice on water ingress and oil prices. I've noticed electric and gas tariffs have taken a dive due to the collapse in crude - so I suspect current prices are a long way below the long term trend!
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texasholden
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« Reply #5 on: April 16, 2020, 04:59:07 PM »

Re the UFH George, might be my misunderstanding but when we renovated I had to replace the existing concrete which was hodge podge and damp. Indeed I did put in a much needed membrane at the time but building control insisted on underpinning the building if I went down more than 100mm (the inspector took great delight showing that the stone walls only went about 100mm below ground level and were directly on mud!

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ecogeorge
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« Reply #6 on: April 16, 2020, 07:26:58 PM »

Understand , i didn't bother building control -they are busy people! bike
George
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benseb
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« Reply #7 on: April 16, 2020, 09:03:04 PM »

We had an ASHP fitted last year. Reasonably insulated.

We were putting a new floor down in the kitchen and hall so added UFH and then upgraded the reds to triple column reds in the lounge.

The difference is amazing. We could run the kitchen/hall at 35c. But the lounge needs 45c. The difference doesnít sound much but itís the difference between the heat pump being on say 50% of the time vs it being on 100% of the time.

I then purchased a FLIR One thermal camera. It showed our insulation wasnít particularly well installed in places.

Weíre now going to pull the ceiling down and add kingspan properly. We may do some of the internal walls too

Moral to the story, donít underestimate how much the higher temps will affect your bill. Itís the main factor so do everything you can to insulate first. It may save you money on the heat pump too
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rogeriko
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« Reply #8 on: April 17, 2020, 12:24:58 AM »

ASHP are only good for modern highly insulated buildings. I wouldn't even think about putting one in an old stone cottage. Electricity is about 20p per kwh oil is currently about 3p per kwh. Unfortunately if you want heat burn something, dont forget your electricity mostly comes from someone else burning something.  (Drax burning every tree in Wales quite unnecessarily just to get the govt subsidy)   stir stir
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brackwell
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« Reply #9 on: April 18, 2020, 09:38:39 AM »

You cant economically run a ASHP at 55C in a old cottage.   What would i do ?

I would keep and extend the oil boiler and rads to the new property but run off a different tank from the HW or no tank at all.

I would then do everything to cut the use of oil. 

 Fit a air to air source HP to take some of the load particularly in the shoulder months.

Go on to Time of Use (TOU) leccy tariff for HW and poss of using oil filled/storage heater for the coldest rooms in winter.

Insulate and insulate again will pay for itself if you stay there and the comfort levels will greatly improve. Consider inside/outside wall insulation.  Plant trees /fence to reduce prevailing wind and chill factor.

When considering new windows consider increasing the size of South facing ones to increase solar thermal gain.

I would consider further reducing my running costs by going on to TOU tariff by Octopus so the oil filled rads/storage heaters could take on power at night at cheaper rates than burning oil.

Ken
 

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Countrypaul
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« Reply #10 on: April 18, 2020, 11:17:24 AM »

You cant economically run a ASHP at 55C in a old cottage.   What would i do ?

I would keep and extend the oil boiler and rads to the new property but run off a different tank from the HW or no tank at all.

I would then do everything to cut the use of oil. 

 Fit a air to air source HP to take some of the load particularly in the shoulder months.

Go on to Time of Use (TOU) leccy tariff for HW and poss of using oil filled/storage heater for the coldest rooms in winter.

Insulate and insulate again will pay for itself if you stay there and the comfort levels will greatly improve. Consider inside/outside wall insulation.  Plant trees /fence to reduce prevailing wind and chill factor.

When considering new windows consider increasing the size of South facing ones to increase solar thermal gain.

I would consider further reducing my running costs by going on to TOU tariff by Octopus so the oil filled rads/storage heaters could take on power at night at cheaper rates than burning oil.

Ken
 



He has no oil boiler at present.
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brackwell
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« Reply #11 on: April 18, 2020, 11:35:18 AM »

Oops!

Well in that case i would fit one. You need something you can nearly totally relie on eg. Beast from the East, Ill in bed, returning from hols etc.   Then if its never used thats great.

I would not use the oil to do the HW in summer -far too inefficient.
 
Ken
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nicos
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« Reply #12 on: April 18, 2020, 11:38:51 AM »

To give an idea of what COP you might be able to achieve we've been running our ASHP with a flow temperature of 60 degrees all winter with a COP between 2.3 and 2.4
It's been a very mild winter here in the midlands so that's probably as good as it would ever get but I thought some real world figures for running an ASHP at a high flow temperature would be useful for you.
The house is very large and a mixture of old (1830s single brick) and new, it still has rads sized for the old gas boiler and currently meets the hot water demand of 8 people. This winter we used an immersion heater a bit and lit the log boiler a handful of times but the rest of the demand was met by the 8.5kW heat pump! We have a 2300 litre thermal store which helps but I've still been very surprised. We got the ASHP to assist the log boiler but due to a busy winter at work last year I didn't lay in adequate firewood stocks so the ASHP has been doing all the heavy lifting. Possibly not the most economic solution but it's given us a good idea of what the system will and won't do.
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benseb
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« Reply #13 on: April 19, 2020, 11:50:16 AM »

I bet thatís cost a bomb Nicos?

We have a 260sqm house, run our ASHp between 39 and 45c

Our elec bill (inc cooking every day etc) hit £380 in Dec/Jan. about £80 of that is non-heating.

So over a year works out about £1500 for heating. Hate to think what that would be like at 60c
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nicos
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« Reply #14 on: April 20, 2020, 09:32:34 AM »

Well yes I was expecting to have to remortgage the house to pay the electricity bill but once again I was pleasantly surprised! The heat pump cost about £250 per month to run in December and January so by the sounds of it not that different from yours and definitely cheaper than if we had to buy logs for the boiler. I don't know if it's the big thermal store which makes the difference?
 
With these cool, sunny conditions nearly all the heating and hot water is currently being covered by our PV generation with just a little cheap rate overnight to make sure there's hot water in the morning so with a bit of luck our annual heating bill won't be too eye-watering. Also the house is effectively split into three so we split the bills three ways which helps a lot.

I have acquired as much firewood as I could fit in over the winter so should be able to experiment with running the heat pump at a lower temperature next winter. But then again it's very tempting to just leave it turned up and not have to spend quite so many hours processing firewood!


 
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