Hello Everyone, I am doing up a big old semi-detached stone walled house, that currently has an oil fired boiler. Gas is not an option, and there is no garden to put an ASHP and no possibility for solar thermal. I would like to move away from Oil, and like the idea of a wood pellet boiler, but my understanding is limited. I think what i want is a small pellet boiler and large thermal store to heat 200m2 and power up to 4 showers. However there will be times when the only requirement will be hot water for 2 people, would this setup be sensible in that case? Any advice or pointers to professionals/resources would be greatly appreciated, I keep going round in circles! I've looked at the hot water cylinders with integrated ASHP that can be indoors, are these sensible? Or just using a completely separate system for 1 shower. Is it possible/desirable to use two different thermal stores?
Thanks

If you are going for thermal stores (TS) then it is generally advised to separate central heating and hot water (DHW) - so 2 TS.

I would not contemplate a wood pellet boiler. They are expensive, the fuel is bulky to store and unless you can buy in bulk the fuel can be expensive and for reasonable delivery lorry access is needed. And then there is the environmental impact of burning wood (air pollution).

A quick dip into the internet came up with the following - 'A basic wood pellet boiler will cost you somewhat in the range of £4,000-6,000,' more of course if you want a fully automatic one and then you have the storage demands of the fuel. A wood pellet will also need a suitable chimney which may be an additional cost (probably a liner as a minimum) assuming you have a chimney in the location where the boiler needs to go.

I would suggest a gas boiler running on LPG or propane so a standard gas boiler will fit with the burner jets changed. (And spend the several thousands of pounds left over on additional insulation) The gas for a gas boiler can be supplied by standard gas bottles. A couple of47kg cylinders can be fitted with auto change over to ease use.

If you need 4 showers running together then a DHW tank will be needed as a combi won't supply 4 showers but this is no problem with a gas boiler and the set up is not much difference in price to a combi boiler (over here that is)

LFM: I changed from oil to wood pellets about 6 years ago. I have enjoyed the benefits of RHI payments and will continue to do so until next September. With hindsight I would never have had a wood pellet boiler and would never advise anyone to consider it unless there really is absolutely no alternative! I admit my perspective may be somewhat biased by the bad experience I have had. The main reasons would be:

The purchase price and installation cost is very high. Without the RHI scheme I don’t think many would have been sold at all.

You will find that the number of firms able/willing to service the boiler or get spare parts is minimal (assuming the original installer is still around that is – many, like mine, have gone to the wall as has the boiler manufacturer) and probably miles from where you live so they can charge hefty mileage charges before they lay a finger on the boiler. Choose your installer and boiler carefully!

Unless you are prepared to pay megabucks for the magic “self cleaning” wood pellet boiler you will find that you will need to clean and maintain the boiler very frequently. That may or may not be an issue for you. I am ok with it as I am reasonably practical, but if anything happened to me, my wife would not be able to cope with it.

We moved away from oil for two reasons – one, to get away from use of fossil fuels and two, to avoid the huge cost swings that occur in the price of heating oil, usually related to political events rather than for any technical reason. In reality the cost of heating oil up until the present virus pandemic was the same as it was 6 years ago. (In fact it dipped significantly between 2015 and 2018!). In the meantime the cost of wood pellets continues to creep up without any real justification as far as I can see. As we consume a relatively small amount of pellets in the grand scheme of things (approx 3 tons p.a.) we have no clout whatsoever with the supplier and are at their mercy when it comes to price increases.

Sorry to be the harbinger of doom but please think very carefully before going down the wood pellet route. When the RHI payments stop next autumn we are seriously considering reverting to oil, which probably 90% of the local residents use where we live as we are not on the gas grid. I am hoping that there will be some dramatic breakthrough in ASHP technology which will lead to a quantum jump in COP values and also if the demand for ASHP increases then the cost should hopefully come down!!

Thank you so much for a ll the responses, I was waiting for an email alert so didn't realise I had any! sorry for the silence.

<blockquote><cite>Posted By: owlman</cite>Why do you need a garden for ASHP?</blockquote>
To locate the outdoor unit? I have no outside space at all and can't pt anything on my walls without planning, and I'm next door to a listed church.

@Peter_in_Hungary
thanks for the advice re twp thermal stores, although you and everyone else does seem to be pushing against Wood pellets. I was worried that i was going for a tech that i liked the sound of rather than the most sensible one.
With regards to Propane and LPG, I can't have anything outside, I have a basement. is it wise to store that much gas in a basement? I don't know but it seems like something I would want to avoid.

@WillinAberdeen
I will only have control of one shower, I'm planning on renting out the rooms,so making people take turns to shower may be tricky. The oil tank is in my basement. does that eliminate the propane tank? Ican't have anything outside at all.

@Jeff B
Thanks for this, I don't want to make a very expensive mistake!

@Artiglio
Looks like my dreams of a pellet boiler will remain just that :-(

LFM - I didn't realise that space was such a critical factor in your case, in which case I would have added storage space to my list of negatives! As we have a largish back garden I invested in a large storage silo to store the pellets, which are delivered in a large lorry and blown into the silo through flexible tubing. It is large - approx 3 metres high and 1.5 metres diameter and holds 3 tons of pellets (one year's supply). Even if I hadn't gone that route I would have had to have a pallet load of bags delivered three times a year (1 ton per pallet).

As Peter says, it looks like a new oil boiler is favourite. This is probably the way I will have to go fairly soon but maybe keep the wood pellet system in tandem as well, to have both options.

For the rented room DHW, instantaneous electric hot water could be a solution. Thereby reducing the need for such a huge hot water storage facility, plus maybe a smaller boiler, and only cold water feeds to each of the rooms.

Do you have unused attic space? If so it might be an option to install an ASHP in it with discreetly vented air ducts, perhaps under the eaves.

It's generally considered a bad idea. I suppose if you did some roof redesigning you could create a "cutout" roof balcony and then a condenser would be possible.

<blockquote><cite>Posted By: owlman</cite>What is wrong with going all electric.</blockquote>

Owlman - I'm not sure if your question is directed at me or the OP? I would love to have an ASHP but simply can't afford it now. If I had not used my RHI "allowance" on the biomass system then I could go for that but having used said allowance I am therefore not entitled to another bite of that particular cherry which is fair enough! An oil boiler with new bunded storage tank would be about £3K and I could do most of the installation work myself, together with my brother-in-law who is a plumber with the necessary oil boiler installation qualifications. I don't want to go down that route but I don't have much option.

By all electric do you mean an electric boiler? If so, no, too expensive to run.

@ Jeff B
I have difficulty understanding the logic of Air to Water ASHP.
Namely why heat relatively low temp water to circulate it around a property with all the installation costs and material usage it entails, with either, high capacity radiator/emitters or ripping up floors to install UFH.
To my mind is simply easier to heat the air and let it waft around the property i.e. Air to Air ASHP. It's relatively easy to install, it has a quick response time, It's cheap to run with excellent COPs, and it can be used for Summer cooling, ( for free if combined with PV ).
Based on my own experience of the past year, I think wet CH systems are outdated.

P.S. I've just been comparing electricity usages for 2018/19 (pre-ASHP) and 2019/20. Still a couple of months to go but looking very good.

There are two benefits of Air to water ASHP that come to mind...
1) you can heat domestic hot water.
2) for space heating you can run the heat pump at a much lower temperature compared to Air 2 Air and achive a higher COP.

for 1) the cost benefit may be marginal - you may only get a COP of 2 for DHW (at a pinch 2.5 - 3 in the summer)
for 2) shifting heat by air means either moving the air over fins relatively fast (noise, drafts) or hotter air with a corresponding high output temperature on the heat pump and a lower COP.

But it's not cut and dried - I went for UFH and ASHP - but Air to Air could have been a solution - in my house the heat in each room balances out so a central Air 2 Air fan complex could have worked - with the money saved I could have paid for solar PV and heated the water most of the year for free. But those fan units do look big.

For a big stone wall house I would be worried that the any sort of ASHP will be able deliver the kind of heat required - It may have to work hard, be noisy and expensive if the flow temp can not be brought down. It depends on the heat output required when it's -5C outside for a week.

My ducted A2A system has a COP of up to 4.81on heating mode. The air movement is unnoticeable as is the noise from the indoor unit located above the sitting room in the loft space.
True, with A2W DHW is useful feature,--if you want stored hot water, but as you say it doesn't come without strings attached, in the form of a reduced COP. It begs the question whether its better to have the higher A2A COP and heat water by some other means e.g. solar. I don't think anyone has crunched the numbers though.
P.S.
Incidentally when I was referring to the OP and "going all electric" I wasn't thinking of heat pumps which he already said was out of the question, just other forms of electric space heating in his newly refurbished well insulated property top be.

Posted By: owlmanMy ducted A2A system has a COP of up to 4.81 on heating mode.

I'm curious what temperature it is heating the air to?

I suppose it is recycling air rather than heating fresh external air, so it can heat as much as it wants?

Posted By: Jeff B

Ah, I didn't realise you had a ducted system. I hadn't considered that as a possibility - I was going on the assumption that I would have to have the usual wall mounted internal fan units. Did you install your own system? I presume the in-coming warm air is via ceiling grills? Do you also have an MVHR system?

Yes, I did the design, purchase, and install myself, with input and commissioning from my "F" Gas fitter at appropriate times.

The system is for the space heating of a large 60M2+ sitting room, with a large attached, pitched roof, sun-room separated by 4M wide fold and slide doors, plus a kitchen. I have two output and two filtered input grilles, one of which draws input air from the top of the sun-roon, - so it's often a little pre-warmed, even in winter. It also serves to circulate/pull air into the sunroom when the sliding doors are open.
Positioning of the three main sitting room grilles was dictated to some extent by the loft structure above but with an eye to best circulating the warm air, although ultimately an educated guess, as was ductwork sizing
The house is slightly unusual as it's built on a gentle slope with a large internal Gable wall separating two distinctly different ends of the house. the rear facing aforementioned sitting room etc has a trussed roof above, fortunately huge, but still a bit of a struggle with ducting.
The front of the property has a traditional purlin and rafter construction above bedrooms hall and bathroom. So this is my next foray into ASHPs and may be a different approach, with a second condenser/compressor.
Throughout all of this past years experience I still have my log fired Biomass boiler which serves the front of the house albeit very sparingly as the warm ASHP air permeates into the hall way and a little beyond.
I'm anxiously waiting for mid June, to get a full electricity usage figure but so far I've used less than the previous year, and all other living aspects have remained broadly the same.
The other real bonus has been Summer cooling, which runs for free by my Solar PV.
All in all I'd never have another wet CH system and that includes UFH.

P.S.
I don't have MVHR

Posted By: owlmanI suppose if you did some roof redesigning you could create a "cutout" roof balcony and then a condenser would be possible.

I'd also suggest looking into this. If necessary to obscure it due to the proximity of the church, covering over the 'balcony' with architectural louvres (eg https://langandfulton.co.uk/product/italia-100-roof-top-screening/) may be acceptable.