Hi @EmmCee and welcome to the forum.  I don't know much about pellet boilers but have just had an ASHP installed in a house with no previous central heating which also had a stove with a back boiler, but for hot water only.

If you have central heating now, why can't some of the the existing pipes be used for an ASHP?  You may need some new radiators and you will need some new pipework because the ASHP will be in a different place and you'll probably need a new hot water cylinder. But you may be able to re-use the pipes connecting the radiators. 

Our supplier took the same approach as yours and routed everything through the attic.  Our house is single storey and this made things a lot easier.  Underfloor pipes were impractical and we didn't want to bury pipes in the walls either. This means we are left with a lot of exposed pipework.  The installers ran a lot of it above the skirting boards and where we needed drops from the attic into rooms, made them in the least visible position.  However, we do have exposed copper piping somewhere in each room.  We might disguise/cover some of it, make a feature of it or just live with it. It doesn't look that bad and it's worth it to us to have decent heating.  We have disconnected the back boiler and will remove the pipework for it when we get around to it.

As you say a wood pellet stove is going to involve your storing and shifting several tonnes of pellets a year and it will be several bags per day on a cold winter's day and none in the summer.  On the other hand an ASHP is labour-free but can be expensive in colder weather/climates.  Theoretically pellets should be similar in price per kWh to an ASHP in running costs but it depends on your house and you also need to consider what grants are available to you. 

Good luck and please feel free to ask more questions.  BTW it's good that you have found three ASHP suppliers to quote; many folks south of the border are struggling to find suppliers.

Hi there,

I stand corrected, but all three companies advised against using the existing underfloor pipework because it is so old and is of the wrong bore for effective use of the ASHP. I was also concerned about the exposed pipework primarily because of the aesthetics. The ASHP system is already very expensive and I'm reluctant to spend 1/3 of our budget for something that looks unattractive. 

The main problem with the pellet stove is, indeed, lugging the bags - particularly when we are trying to plan ahead and are already both pensioners 🤔 

@EmmCee,

I'm not an expert but I don't think there is anything special about the pipes an ASHP needs other than there may be issues if you mix pipe bores but even that is solveable. Having said that I can understand your ASHP, or other system installer being nervous about using old, unseen pipes. It's your choice about the aesthetics but there must be a lot of older houses where retro-fitted heating has meant having exposed pipes. 

If you are determined not to use fossil fuels and don't want storage heaters then another option is air to air heat pumps, which are similar to air conditioners but also produce heat.  There are one or two folk on here either have them or are thinking about it. @Derek-M is one, although I think he uses it as a supplement to gas.  They are a lot cheaper than the ASHPs you've been looking at although they have their own issues and (at least in England) there is no financial help with them.  Might be worth a Google though. 

Hi EmmCee,

It is difficult providing advice with limited information, but I will try to give a few suggestions.

When you say you have a multi-fuel boiler, what fuels can it use? Why do you need to replace it?

I am assuming that you do not have a mains gas supply?

I have read recently about Hydrotreated Vegetable Oil (HVO). Have you considered an oil boiler?

How well insulated is your property? Do you have any idea of your heat loss figures?

Pipework is not often the defining factor when ASHP's are installed, since the primary purpose of the pipework is to transport the water from the heat source (boiler, ASHP) to the heat emitters (radiators). If the pipework is of reasonable size then there should be no problem. Even with smaller bore pipework a slightly larger pump can quite often resolve any difficulties. The defining factor is more often the size and output rating of the radiators, since ASHP's work more efficiently at lower water temperature.

I suppose the other factors in your case would be where you could site an ASHP and where the connection to your present central heating system could be made.

Please ask further questions if you need clarification.