Throwing out oil boiler - tell me if I'm mad

[benf]

Just a sanity check.

This is for a house in scotland, 2 storeys, sandstone, internal 9.5 x 8.5 metres and storey heights 2.8 and 2.5m. 2 bathrooms, 2 shower rooms.

Currently, there's an ancient oil boiler and ancient oil tank. No mains gas.

I want a heating system that will still be viable in 20 years time, so I don't favour oil at all. I've considered heat pumps and thrown that idea out too because it's a listed building, not suitable for underfloor heating.

I had a few ideas, sent them to a company that provides buffer tank systems and between us, this is what we have come up with:

Multi-fuel stove in kitchen 21kW max of which 14kW is water boiler gravity feed to Tank 1.
A 48kWh fan-assisted storage heater in the hall
2 x 500 litre stainless steel buffer tanks
4 x custom immersion heaters able to heat to 95C (3 on Econ-7 only)
Pumped heat transfer between the tanks
Central heating rads running off Tank 2
Hot water off Tank 1 from plate heat exchanger then anti-scald valve

and the calculations:
online boiler sizing calculators give me a figure like 56kW. That sounds like a lot to me, as currently, we're in a bigger house with a 30kW gas boiler. I don't want the system to be capable of T-shirt conditions in winter, so I'm deliberately under-sizing.

My peak demand estimate: 8 hours at 30kW + 8 hours at 10kW
Total 320kWh per day.

To meet the demand: Stove 10 hours at 21kW = 210
plus 47 and 48 from the buffer tanks and storage heater respectively makes a total of 305. It's a fraction under but I have a plan just in case: while doing the re-wiring, we will put cables in for additional storage heaters in bedrooms and install them later if needed.

Not everybody would like it because to get sufficient heating for winter, you have to run the stove. However, it suits us fine. So, is there a gotcha somewhere?

 

[dcawkwell]

You need to do a boiler sizing check on the building first. Roughly 1.5kw per radiator. You have done it wrong a house doesn't need 56kw.
There are calculators on-line.
Ensure house should is well insulated etc.

Best solution will then be
air source heat pump with either an lpg boiler as a booster or oil boiler
as a booster. Air source and ground source can drive normal radiators
they don't have to be connected to underfloor heating.

Air source heat pump should be able to do the house most of the year
with second boiler coming on as a booster.

Forget stove you will spend all your time filling it with fuel.

 

[benf]

I'll try to find another boiler sizing calculator. I admit I was very surprised by 56kW.

According to Energy Saving Trust, 80% of installed air source heat pumps fail to reach a COP of 2.6, so that's more expensive than direct heating using Economy 7. Also, I understand that with a heat pump for central heating, if not using underfloor, then you need oversized radiators. For aesthetic reasons, I want to keep the existing rads in this house.

 

[dcawkwell]

I'll try to find another boiler sizing calculator. I admit I was very surprised by 56kW.

According to Energy Saving Trust, 80% of installed air source heat pumps fail to reach a COP of 2.6, so that's more expensive than direct heating using Economy 7. Also, I understand that with a heat pump for central heating, if not using underfloor, then you need oversized radiators. For aesthetic reasons, I want to keep the existing rads in this house.

Works fine here.
Forget energy saving trust. If you want you can run the air source on economy 7 and build up a buffer of hot water ready to go first thing in
the morning. Rest of the day the air source runs on peak electric.
Air source is certainly cheaper for me than lpg boiler I have.
Radiators are fine. Go for doubles with twin convectors rather than singles.

 

[benf]

Ah! The rads are cast iron. I doubt we'd get permission to change them to modern ones and we wouldn't want to.

Heat pump plus buffer is one that I had considered earlier. I threw it out because it's considerably more expensive to buy/install and, because of not having oversized rads or underfloor, it would need supplementary direct heating to get the water up to a decent temperature.

 

[D_Hailsham]

I'll try to find another boiler sizing calculator. I admit I was very surprised by 56kW.

Use the Whole House Boiler Size Calculator, it's produced by the Building Research Establishment. I did a quick calc on the figures you gave, assuming single glazed windows and minimum insulation and only got 27.5kW!! Good loft insulation will reduce this by 5kW.

 

[D_Hailsham]

Ah! The rads are cast iron. I doubt we'd get permission to change them to modern ones and we wouldn't want to.

You can find out what their output is from Guide to Radiator outputs. Just be aware that the outputs quoted are based on a mean water temperature of 75C; the standard now is a mean of 70C. There are conversion factors at the back of the book.

 

[benf]

Thanks. That was the boiler size calculator that I had used, but I must have got something badly wrong on inputting the figures. I tried again and got a figure a bit below the one you got. So, that's a lot more sensible and is close to the figure for heat demand that I was actually working on as I wasn't believing the high figure!

I had a look at the rad outputs link and I recon we have 12 rads but I can only gauge the size of a few of them from photos as the house is 400 miles away from me just now. I guess they're mostly between 1 and 1.5 kW.

So, maximum central heating output will be about 15 or a little more. The spec of the stove (yes I'm still keen on the multi-fuel thing, I forgot to mention that the village is in the middle of a forest) says it can run 12 radiators, which I took with a pinch of salt, but if it really sends out 14 kW then presumably it can. Though I also know that the burning in a stove is cyclical so it doesn't produce its max output all the time.

It leaves a gap to reach the 27kW that the house should need and that should be met by direct heat from the stove plus heat from the storage heater in the hall.

One thing my wife worries about is how well the heat from the storage heater will permeate the house from the hall. The hall is a straight wide corridor from the front door to the back of the house with rooms off on both sides.

One thing I hadn't thought about is the over-high temperature of radiator water! If the norm is 70 and water in our store is 95, that's a problem innit! Obviously we could change to standard immersion heaters which are limited to 75. But that reduces the amount of stored heat substantially. Is there a cunning way to get the water in the rads at a safe temperature when the tank temperature is so high?

 

[simond]

What you think regarding radiators really doesn't matter.

If you put an ASHP in, your MCS installer must size the emitters as per MCS stipulations which were recently updated.

Failure to do this will cause your installation to be exempt from RHI and any future FiT.

By my count that is 4 abbreviations so far! Governments just love making everything impossibly obtuse and complicated.

I want to add an ASHP to my oil boiler, but to get the incentives and meet MCS it is just too difficult and expensive. Most will decide the same.

 

[benf]

Simon: ASHP wasn't on my shopping list anyway. So there's no MCS installer involved and fewer TLAs (Three-letter acronyms) and ETLAs (Extended three-letter acronyms).

My idea is to keep the main hardware of the existing heating system which includes some classic radiators, but change the source of heat to something more futureproof and in tune with the locality.

I need to know if I'm mad though, or more usefully, whether it has a chance of working.