Air Source Heat Pump in Stone House

[Bemak]

Anyone have any experience of installing a heat pump into an old house of stone wall construction? I'm still undecided on the heating system for a refurb. The existing walls are 600mm thick and I'm just curious if a heat pump would work considering the walls would almost act like a heat sink. I'm not looking to dry line the walls as I want to keep them breathable so I was just going to remove the existing render internally and just repoint the stonework with lime. 

 

Just curious if anyone has tried a similar approach to see how they're getting on.

[Mr Punter]

If the walls are dry and properly pointed they will have a bit of thermal resistance anyway.  The floors, roof and windows also play a big role in thermal efficiency.  You can use a heat pump but if the house is poorly insulated and not fairly airtight it will cost a fortune to run.

[Bemak]

The plan is to upgrade all windows and doors to triple glazed and to insulate the attic when redoing the roof. The installation of the windows will provide the opportunity to improve the air tightness. 

 

In theory it should work, even if the render was left on internally. But theory is one thing - just interested to hear of experiences

[Redbeard]

Hi,

 

Have you done a heat-loss calc? What's the worst-case scenario at 21 internally and say -5 externally (or whatever parameters you want to work to)? If less than 16 or 17kW then you are OK for a single-phase ASHP.

 

You probably know you can keep the walls breathable and improve the insulation using rigid wood-fibre, cork or (with a rather worse lambda value) hemp/lime. However if you simply want exposed stone walls then you won't want to cover them with anything. Is the existing render lime or sand/cement based? If lime and not too cracked you could treat it as a parge (air-tightness) coat for wood-fibre.

[TonyT]

Do a heat loss based on the proposed new windows etc,

don’t miss this bit out!!

[Roundtuit]

I think I'd invest in airtightness test before making a decision.  It will help you identify where the leaky bits are and decide whether new windows will make enough improvement.  Thinking about it, unless you're fitting mvhr, you'll have to have trickle vents, so there will maybe be limited airtightness benefit...

[Bemak]

27 minutes ago, Roundtuit said:

I think I'd invest in airtightness test before making a decision.  It will help you identify where the leaky bits are and decide whether new windows will make enough improvement.  Thinking about it, unless you're fitting mvhr, you'll have to have trickle vents, so there will maybe be limited airtightness benefit...

 

That is very true. I'm reluctant to go down the OFCH route even though I know it would work very well. I was considering a timber stove and solar panels that would supplement an oil boiler - maybe that is the best in terms of flexibility. 

 

At the moment the house has timber single glazed windows that are ancient and falling apart - so I will definitely do the windows regardless. It's not a cold house mind even with the single glazing. 

 

 

[Bemak]

1 hour ago, Redbeard said:

Hi,

 

Have you done a heat-loss calc? What's the worst-case scenario at 21 internally and say -5 externally (or whatever parameters you want to work to)? If less than 16 or 17kW then you are OK for a single-phase ASHP.

 

You probably know you can keep the walls breathable and improve the insulation using rigid wood-fibre, cork or (with a rather worse lambda value) hemp/lime. However if you simply want exposed stone walls then you won't want to cover them with anything. Is the existing render lime or sand/cement based? If lime and not too cracked you could treat it as a parge (air-tightness) coat for wood-fibre.

 

I haven't done a calc yet. Good idea though. There is no heating in the house at present.

[Bemak]

2 hours ago, Redbeard said:

You probably know you can keep the walls breathable and improve the insulation using rigid wood-fibre, cork or (with a rather worse lambda value) hemp/lime. However if you simply want exposed stone walls then you won't want to cover them with anything. Is the existing render lime or sand/cement based? If lime and not too cracked you could treat it as a parge (air-tightness) coat for wood-fibre.

 

Ya I've looked into all this. I actually really like the idea of hemp etc. Having thought long and hard about it, and considered the significant cost associated with same, I've decided to leave the external render alone. It's in very good shape apart from a few isolated issues - so I'm going to try and resolve them and see how it goes. Internally the house is pretty dry considering it's rarely used. I'm going to remove some of the internal render on the gable side at ground floor level which has always been damp to see what's what. Will make a call on it then. I'll probably patch internally and see how it goes. I'm hoping that a french drain or some form of perimeter drainage will resolve a lot of the damp issues at GF level. I'll replace the flashings around the chimneys and replace the existing concrete barges and possibly reroof as well. Needs to be done regardless but I've a funny feeling it will resolve a lot of the dampness issues.

 

In an ideal world of course I'd remove the render internally and externally; dry line internally with hemp and rerender with inside and out with lime but my budget would be spent at that and there's plenty of other things that need doing.

[Iceverge]

I remember reading somewhere, maybe one of those Scottish documents, that old stone wall have significant thermal resistance but only if they're not damp. 

 

A low temp heat source like an ASHP works well with a very insulated house that is continually heated. A strategy of intermittent heating would be better with a house of high heat loss. 

 

If you get a period of steady temperatures, overnight probably with no wind,  you could plug in a few electric rads and see what the house temp stabilises at with all internal doors open.

 

Then you'd know exactly your house heat demand.

 

Eg if it was 10 deg outside, 15 deg inside and you had 4kW worth of rads on full blast then you would have a heating load of 800w/deg for that condition of windiness.

[ReedRichards]

There is a myth, oft perpetuated that an ASHP requires a well-insulated house.  ASHP are not magically different from boilers, stoves and other forms of heating so the only circumstance where an ASHP would not be suitable is if its heat output could not match the requirements of the house.  You can easily get ASHPs with up to 18 kW of heat output; I don't doubt there are bigger ones for the commercial market.  And if the heat requirements of your house are large then it will be expensive to heat whatever form of heating you choose.  

[TonyT]

I use a 72kW one at work for a big building..

 

 

I have similar style house, currently has has has Gas CH, wood stoves and solar panels.

 

all helps, but the most important part was insulation.

as I redid rooms i stripped the lath and plaster and popped in 100mm kingspan, put ROCKWOOL slabs under the suspended timber floor and between some ground floor/first floor ceiling voids to top up the Victorian deafening.

 

i enjoy modernising the house 

[Bemak]

3 hours ago, ReedRichards said:

There is a myth, oft perpetuated that an ASHP requires a well-insulated house.  ASHP are not magically different from boilers, stoves and other forms of heating so the only circumstance where an ASHP would not be suitable is if its heat output could not match the requirements of the house.  You can easily get ASHPs with up to 18 kW of heat output; I don't doubt there are bigger ones for the commercial market.  And if the heat requirements of your house are large then it will be expensive to heat whatever form of heating you choose.  

 

Very good point. If I pair the HP with solar it's not a bad setup

Edited August 13, 2022 by Bemak

[Iceverge]

Good Point @ReedRichards

 

If you are heating any building continuously it doesn't matter what kind of heating system you have as long as the heat provided exceeds the heat demand. However unless you have a very low demand house this will be prohibitively expensive and wasteful of energy. 

 

Response time will becomes important for most houses but especially those with poor insulation and intermittent heating.

 

ASHPs work most efficiently when the flow temp is as low as possible, therefore to have an efficient system you would need really huge radiators ( or fan assisted rads) to have both a low flow temp and quick response times. Maybe an oversized A2A multi split would be better. 

 

 

[saveasteading]

We are currently converting such a building, with 600mm of granite, plus some ashlar sandstone.

There are tested figures for these walls, and they are better than the heat loss programs allow proportional to thickness. 

However they are not remotely close to building standards. Scottish warrant requires insulation to certain numbers 'as far as reasonably practicable'. That does not allow bare stone and no insulation, unless it is a listed building with very special walls.

Heat loss is not related to the source of the energy, so ashp makes sense...we are doing this. 

Obv the floor is warmed more than with space heating,  so floor insulation must be paramount.*

As mentioned, dampness in the wall will increase heat loss. Read up on pointing.

 

As above, if you were to have no insulation on the walls then you will need a lot of heat to warm the walls before the space becomes comfortable. And it will be expensive.

 

* an insulated floor requires 100mm minimum insulation under a screed. Beware the effect of this on your works esp excavation ,(and wall stability) and existing door heights.

 

[saveasteading]

18 minutes ago, saveasteading said:

There are tested figures for these walls,

From a very quick look, the tested value for a 600mm stone wall in good condition is an r value of 1.0. So that is like 40mm of mineral wool or 20mm of PIR.

Or a U value of 1.3, which is much better than the published figures for a solid brick wall (2.0) but nowhere near a relaxed conversion target of 0.3, or a modern house target of 0.18.

 

Thus the walls are a very good start to the insulation of a building, but not remotely enough for modern expectations.

[SteamyTea]

33 minutes ago, saveasteading said:

insulated floor requires 100mm minimum insulation under a screed

And more.

W/m².K. 

The K becomes important with older buildings, so to keep the W at a sensible level, a lower ∆K is wanted.

Only real choice is thicker insulation.

 

 

Edited August 14, 2022 by SteamyTea

[Bemak]

1 hour ago, saveasteading said:

We are currently converting such a building, with 600mm of granite, plus some ashlar sandstone.

There are tested figures for these walls, and they are better than the heat loss programs allow proportional to thickness. 

However they are not remotely close to building standards. Scottish warrant requires insulation to certain numbers 'as far as reasonably practicable'. That does not allow bare stone and no insulation, unless it is a listed building with very special walls.

Heat loss is not related to the source of the energy, so ashp makes sense...we are doing this. 

Obv the floor is warmed more than with space heating,  so floor insulation must be paramount.*

As mentioned, dampness in the wall will increase heat loss. Read up on pointing.

 

As above, if you were to have no insulation on the walls then you will need a lot of heat to warm the walls before the space becomes comfortable. And it will be expensive.

 

* an insulated floor requires 100mm minimum insulation under a screed. Beware the effect of this on your works esp excavation ,(and wall stability) and existing door heights.

 

 

That's interesting - what are you doing in terms of insulating the walls? I'm reluctant to dry line with an insulated PB because that will just cause me more issues down the line. I'm also reluctant to hack off the internal sand and cement render and replacing with hemp/insulating lime render because the cost is prohibitive and would kill the project for us. The internal render is actually very good and dry for the most part.

[Bemak]

2 hours ago, Iceverge said:

Good Point @ReedRichards

 

If you are heating any building continuously it doesn't matter what kind of heating system you have as long as the heat provided exceeds the heat demand. However unless you have a very low demand house this will be prohibitively expensive and wasteful of energy. 

 

Response time will becomes important for most houses but especially those with poor insulation and intermittent heating.

 

ASHPs work most efficiently when the flow temp is as low as possible, therefore to have an efficient system you would need really huge radiators ( or fan assisted rads) to have both a low flow temp and quick response times. Maybe an oversized A2A multi split would be better. 

 

 

 

Fan assisted rads are interesting - they'd probably provide a better sense of immediate heat within the room too rather than standard rads. Plus the space saving is nice too

[Bemak]

For reference we currently live in a 70s house of solid block construction. Previous owners lined the walls with insulated PB (30mm insulation). We have the temp set to 19 degrees in winter. That's what we consider comfortable. 

 

[jayc89]

42 minutes ago, Bemak said:

 

That's interesting - what are you doing in terms of insulating the walls? I'm reluctant to dry line with an insulated PB because that will just cause me more issues down the line. I'm also reluctant to hack off the internal sand and cement render and replacing with hemp/insulating lime render because the cost is prohibitive and would kill the project for us. The internal render is actually very good and dry for the most part.

 

We're currently renovating a solid brick house. For the most part the property's bone dry. Where we did have problems was around the cellar and the room directly above it which had been tanked internal and externally in sand/cement (there was some lime mixed in too but very little), coupled with a leaking soil pipe, it was a bit of a soggy mess. 

 

Any sand and cement currently on the property might not be causing problems yet, but if/when it starts to crack and allow moisture in, it will. 

 

Internal wall insulation on solid brick builds is a pretty divisive subject, on here and the wider community, apparently a new paper is due to be released next month which may or may not help in reaching a consensus here.

 

 

[saveasteading]

3 hours ago, SteamyTea said:

And more.

Yes in theory. But with this type of building the foundations are probably very shallow. We had area already exposed, ie the stone walls were 100mm above the visible ground, thus sitting on 100mm of exposed vertical ground.

It would be very easy to destabilise the walls..indeed there are tales of them falling down during such works. And yet existing lintels may dictate the finished floor level.

 

1 hour ago, Bemak said:

what are you doing in terms of insulating the walls?

Cassettes of insulated timber positioned 25mm inside the masonry. That was  fine on paper (or screen) but the reality is of non planar walls and so the void varies and more floor is lost.

 

1 hour ago, Bemak said:

Fan assisted rads are interesting -

Yes that works. I designed that, with ashp,  into our own office building 15 years ago. The effect is indeed quick, and can be reversed for cooling.  But the ducts are large and obtrusive.

[Bemak]

Whether I go heat pump or oil boiler, the heat loss is going to be the same so it will be more expensive to heat regardless. I assume that the heat pump would still be cheaper to run than a kerosene boiler in this context particularly if I marry the former with PV?

 

 

[saveasteading]

2 minutes ago, Bemak said:

I assume that the heat pump would still be cheaper to run than a kerosene boiler in this context

According to recent discussions on here, only mains gas would be cheaper, prior to the current price increases and concerns.

There is lots of good info on bh on this, including quite recently about ashp with PV., and that is what we will be doing.

 

Just thought though, further to saying that ducted heated air works... it worked fine in a 10 hour office scenario, but maybe not so well in domestic, with varying room conditions.

[Bemak]

49 minutes ago, saveasteading said:

According to recent discussions on here, only mains gas would be cheaper, prior to the current price increases and concerns.

There is lots of good info on bh on this, including quite recently about ashp with PV., and that is what we will be doing.

 

Just thought though, further to saying that ducted heated air works... it worked fine in a 10 hour office scenario, but maybe not so well in domestic, with varying room conditions.

 

I wouldn't go ducted, but rather those fan assisted radiators. I hadn't come across them before. Must look back over other recent threads as you suggest - it's been well covered