£5000 sounds like a lot of money to me for a simple replacement of an oil boiler. I paid that for a new oil system boiler (not a combi) and a new bunded oil tank (it was a replacement for a wood pellet system so we had no oil tank) plus associated pipework.

I looked at ASHP but it would have cost about £12K and as I had already had RHI for the wood pellet boiler I could not claim that a second time of course. In any case I was very sceptical that an ASHP would be man enough for our system as we also have 15 radiators with 10mm microbore pipework.

I think you will struggle with an ASHP if you are in Scotland and you say that only some of the house is well insulated.

More experienced folk than I will undoubtedly have something to say shortly!

Even at £3700 that seems like a very expensive boiler. I realise you want an external version but a quick scan on the web suggests around £2500 for a 25kW jobbie.

I bought my new oil boiler as soon as the 7 year RHI period finished on my wood pellet boiler*. This was because I was nervous that HM Gov would suddenly announce a ban on any new installations of oil boilers and I would have been forced to go down the ASHP route.

*The reasons why I gave up on the wood pellet experiment have been well documented elsewhere on the GBF. Basically it was a pain in the backside and it was no surprise really when the manufacturer went bust about half way through the 7 years!

I am also thinking of changing my CH + DHW boiler for an A2W heat pump. My boiler is a 40kW gasifying wood burner and whilst different from your oil burning combi I think that some of the same principals apply.

My problem was the same - what size heat pump is needed.

FWIW I hold the opinion that any building can be heated with a heat pump - it just needs sizing correctly. If it is not sized correctly it will be more expensive to run than a correctly sized HP. This is not to say that a poorly insulated house won't be expensive to heat, just that if the HP is not properly sized it will be more expensive than otherwise.

So

Having asked a similar question here I came to the conclusion that without knowing what my heat load is it would be impossible to ascertain the correct size HP.

The heat load will be need to be known on a daily basis so that the coldest day can be managed. Determining the heat load was difficult (impossible) with a wood burner by measuring the fuel input. In the same way it will be difficult to determine your daily heat load by measuring the oil used, especially as the DHW comes from the same boiler.

The solution I came up with was to install heat meters on the system and then keep a log of the daily heat demand. My system supplies 2 detached houses (side by side, 2M gap) and provides DHW for both. Each house has a heat meter on the CH. I also noted the daily min /max temps

Not surprisingly the biggest heat load occurred on the coldest day with a heat load of 81kWh and 70kWh respectively.

Advice from here is that a storage tank is used for DHW and some HP hours should be set aside to heat the DHW i.e don't try to heat the DHW at the same time as running the CH.

Also advice from here was to plan to run the HP for long periods rather than blast the house with heat for frequent short periods.

Adding the maximum heat loads together gives a daily load of 151 kWh, which over 18 hours is 8.4 kW / hour. Then apply a safety factor of say 20% gives a HP of 10kW output.

Then comes the radiator sizing. Conventional wisdom says when you go to a HP the rads. will need to be increased in size to account for the lower flow temps. I found this to be correct !!!

I run my system at 60 deg. flow. To test the performance at HP temps. I reduced the flow temp. progressively until the system failed to perform as required. The results were that just adequate performance was achieved with a flow temp of 50 deg. but 45 deg. was insufficient. So bigger rads will be needed with a change to a HP. However I found that I can move the larger rads to replace smaller ones and by shuffling the rads. around I could minimise the number of new rads. needed. (The system has a total of about 18 rads.).

So the conclusion I came to is that to replace my 40kW gasifying wood burner would need a 10kW output heat pump and the radiators will need upgrading to suit.

I will be quite happy if those here who know better take apart my reasoning as needed.

Posted By: mitchino15 radiators, plus two woodburners

Your house sounds a similar size to ours and our oil boiler is 30kW IIRC

we got a quote from Kensa for a GSHP which was propsed as 15kW single phase (no 3 pH here)

But it really does come down to the heat loss of the house, we had to get a room-by-room heat loss calculation done and it wasn't easy because of different ages of the building as it had been extended at different times over the last 200 years.

I can send you the quote if it is of any use to you for interest. it was about £15k supply only.
We would have to increase radiator sizes as well.

At the moment we have left space in the field for the 5 x slinkies that are required (we did 2 trenches for the barn covnersion)

While we are slowly working our way around the house upgrading the insulation.

Posted By: mitchinoWillInAberdeen, interesting to note that you think running the heat pump is more expensive than running an oil boile

No sorry, thats not what I meant, the opposite! I wasn't clear enough.

3000l of oil per year is costing about £2000/yr, so £20k over a ten year lifetime (depending what happens to world oil prices), so much more significant than the purchase cost of the boiler, and more uncertain.

You'd get about 25000 kWh/yr of useful heat out of that oil. To get the same heat with a heat pump would need to buy 8000kWh/yr of electricity, costing £10-£15k in the long run over 10 years (depending what tariff you are on, and whether electricity prices continue falling with renewables). So maybe £5-£10k cheaper than the oil bill, depending on future prices.

So is worth spending £5-£10k extra upfront on a heat pump compared to oil boiler, plus whatever grants.

If you have an EPC for your house, call some installers, they can look up your approx heat demand online and give you a ballpark cost over the phone, if you decide to go further they will come and survey in detail.

A rule of thumb is take your annual energy (25000kWh) and divide by 3000. So you need 8kW of heat. Then round up to the next standard size model, probably 12kW.

The first thing you need is a proper heat loss survey - any supplier that doesn't offer that - walk away.

System design is critical.

Posted By: WillInAberdeenThen round up to the next standard size model, probably 12kW.

Over sizing a HP can be problematic as it often then cannot modulate down and starts cycling. There is an argument for the sizing to be marginal and accept that on the occasional very cold spells, you need some direct heating to augment it.

HeatGeek are getting a really good rep.

Add in proper monitoring (not just the HPs own monitoring) so you can independently monitor the HP and the COP you are getting. Accept that you do need to monitor performance and tweak the settings on the HP to get the best COP; it is not fit and forget.

Do lots of research on the different HPs available. Data Access (API), Modulation, standby power draw, weather compensation etc.

You may also need Planning consent for the HP - that takes time so if the system is on its last legs, you need to get a shift on!

Posted By: WillInAberdeenIf your installer is MCS registered then they have to install a bigger heat pump which MCS can guarantee will heat the house toastie warm in all weathers without augmentation

I wonder if it's possible to find an installer who is MCS-certified who is wise enough to recognize the problem and do a [smaller] non-compliant installation after doing an MCS survey? And if, what are the cost implications?

New heatpumps should do that automatically (called "weather compensation") - they should constantly be seeking the lowest possible flow temperature that is just warm enough for the radiators/ufh to output enough heat for the prevailing weather. New gas boilers may also do this. Low flow temperatures = better efficiency.

They calculate this using an algorithm which the installers call a 'curve'. Much happy geekery is possible, to tweak the best curve for each house, it's not a case of fitting a box with the default settings, hence new skills needed for installers and customers.

As further layers of geekness, you might want the rads to run warmer first thing in the morning to get the house warmed up quickly, then cooler thereafter. Or run warmer during off-peak electricity prices and cooler at other times.

The TRVs on each radiator or UFH loop might have conflicting ideas about how warm that particular rad should be, so some installers like to remove the TRVs so that the weather compensation is fully in control of the whole system. But then sunshine in one side of the house will overheat those rooms, and the EPC assessor will note that TRVs are absent.

I would be happy to retain some smaller radiators and so let the system raise the flow temperature during the coldest days, I would accept a trade off of radiator size against electricity consumption. But again the MCS installers have to comply with standards that discourage this kind of trading off, they have to make the rads big enough so that the flow temperature stays lowish and no subsequent owner complains about electricity consumption.

Posted By: WillInAberdeen"weather compensation"

it did come with a sensor for that as standard, but I didn't want the plastic box on the outside of my barn and also didn't want to run the cable. So I am the weather compensator!

Posted By: WillInAberdeen... and using tomorrow's internet weather forecast instead of today's temperature measurements from the plastic wall box. I don't know if this is happening in practice yet!

For one of the more front running brands it surely doesn't. They have a WiFi breakout box but it does not do much aside from error reporting and enabling the user to set the temperature from their phone app.
The Nest thermostat has been looking up temperatures instead of measuring itself since version 1.

Internet lookup of weather forecasts could enable weather controllers to look ahead and adapt the heating regime to avoid over- or undershoots for buildings that are slow to react, think high thermal mass and/or well insulated...

Posted By: WillInAberdeenbut that is specifically not allowed by the MCS standard MIS 3005-D.

MCS is a pile of stinking excrement.

Posted By: WillInAberdeenHeat pumps don't need planning consent

In Scotland, perhaps, but in E&W they do (usually).

Posted By: WillInAberdeenand the EPC assessor will note that TRVs are absent.

Better to simply open wide (or fit Wi-Fi ones that can adjust).

Personally, I'd always fit the HP to a tank so heating the tank is independent of the house heat demand. You can also then manage the return flow temp more accurately. Then take off the tank for the heating demand.

https://www.planningportal.co.uk/permission/common-projects/heat-pumps/planning-permission-air-source-heat-pump/

Posted By: bhommelsFor one of the more front running brands it surely doesn't. They have a WiFi breakout box but it does not do much aside from error reporting and enabling the user to set the temperature from their phone app.
The Nest thermostat has been looking up temperatures instead of measuring itself since version 1.

Internet lookup of weather forecasts could enable weather controllers to look ahead and adapt the heating regime to avoid over- or undershoots for buildings that are slow to react, think high thermal mass and/or well insulated...

This is true, but local temperatures can still vary by a few degrees (enough to throw out a heating curve). I'm sometimes surprised at the temperature difference between forecast temp and the readings my boilers weather comp is reading (checked also against another outdoor thermometer that has a display in the house - living in a house that's a constant temperature through winter I find I have to check outdoor temp when deciding what to wear if leaving the house )

Posted By: Simon StillReally? It's just an outdoor thermometer and I think ours is less than 20cm square.

Unfortunately yes.
The box is small, discreet, grey plastic. The barn is stone.
There are no cables, plastic pipes, antennas, or any other thingamajigs on the outside. Only Galvanised guttering & downpipes. Roof windows are frameless, flush-fitting. Windows and doors are painted timber.

OCD

Not really on the north gable end. The gutters & downpipes are on the east and west sides of the barn.