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Victorian Semi Retrofit / Extension ASHP and UFH Advice

 
Air Source Heat Pumps (ASHPs)
Last Post by Mars 1 month ago
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Matthew_W
 Matthew_W
(@matthew_w)
Active Member Member
Joined: 2 months ago
Posts: 5
Topic starter 08/03/2025 10:55 pm  

As I said in the introduction thread my partner and I are planning to extend our Victorian semi.  We are wondering whether it would also be a good time to replace our 14-year-old combi boiler with a heat pump system.  We have planning permission but yet to complete detail drawings or engage builders, so we hope the right time to be thinking about making sure the building could work with low temperature heating.  I hope this post hasn't got too long but there is so much to get straight.  Any advice or guidance is welcome!

The house is currently about 120 square metres with the extension planned to take us up to around 160.  Typical  layout with a wing extending out the back of the main house.  The original house had solid walls.  The kitchen wing has been previously extended, in the early 90s so the kitchen and the bathroom above have a mix of solid walls and insulated cavity.  The extension will fill in the internal corner between the original house and the kitchen and add a two storey extension on the gable end, so will cover some of the solid walls, but some will remain and several chimneys, so I think the heat load will be quite high.  I need to make the plunge and get some installers in to do calculations on this: hopefully working from plans will not be an obstacle.

Looking at the particular features of the house and our plans leaves me with a number of questions.

I am wondering what to do with the suspended wooden floors downstairs and whether underfloor heating is a sensible choice for these rooms.

The back room, which will end up being part of the open plan kitchen-diner and having no external walls has a suspended floor over a shallow void.  The boards are, I think, original and we would like to retain.  Could I lift the boards, insulate, fit UFH and re-fit?  What other options would I have?

The front room and hall are over a cellar.  Front room currently insulated with kingspan boards, the hallway just has a lath and plaster ceiling in the cellar.  I've seen some UFH spreader plate systems that are said to be suitable for fitting from below.  Might they be suitable for these two rooms?  Is there anybody on the forums that has experience with these sorts of systems, good or bad?

The kitchen has a direct-to-earth floor.  We will be extending this room significantly, so I think we want to go for a screeded under floor system over insulation here.  Anything in particular to be aware of with the existing areas?  I guess we would ask our builders to excavate out to get the depth for insulation and the screed with the UFH pipes.

For new rooms upstairs, what are the relative merits of underfloor heating against fitting radiators?

In terms of plumbing layout, I am thinking that the plant room could be in our new utility room with the heat pump itself standing out the back of this.  It would be below a window: does that lead to any particular considerations?  We will be too close to the neighbours to put the heat pump at the side of the house.  We have quite a large bathroom so I think I would site the hot water cylinder in there.  That would be upstairs from the plant room but with reasonably straightforward pipe runs.  Cylinders for heat pump systems look huge: are structural calculations required for the joists that will be supporting the cylinder?

Matthew


   
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 JamesPa
(@jamespa)
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Joined: 2 years ago
Posts: 2315
09/03/2025 10:35 am  

When you are renovating is always a good time, as it makes any installation much easier and you can plan properly.  In terms of the detail I can only comment on two aspects of this:

 

UFH vs Radiators:  UFH takes up no space and can generally be run at a lower temperature than radiators (typically 30-35 rather than 45 or sometimes even more).  This significantly improves the efficiency of an ASHP.  With radiators run at a design temperature of 45 you should see about the same running cost as gas, maybe a bit lower with UFH run at a design temperature of 30-35 it is 20-30% cheaper

Heat pump cylinders:  because we typically store water at 48C rather than 60C heat pump cylinders do tend to be a little larger than cylinders for boiler based systems, however its also the case that they tend to be designed for a more luxurious lifestyle (just because time and expectations have moved on).  200l is quite common, 300l not uncommon, it depends obviously on your usage (you get people who want 6 off 10 minute showers at 20l/min in quick succession!).  I haven't heard of structural calculations being done but I suppose it may be necessary in some cases and certainly if the cylinder is going mid span.  Domestic flooring is I believed designed for ~150kgf per sq m, a 200l tank weighs 200kg, so would exceed the loading.  I have a 200l cylinder and no calcs were done, but it replaced an existing cylinder the joists underneath are 200mm and the span at this point is only about 1m, so it was obviously going to be OK.

I dont know anything about the detail of alternative UFH arrangements, hopefully someone else will.

There is a good guide to ASHPs here  which you may want to read.  Note particularly the parts that say to avoid external controls, in particular 'smart' controls which are generally very bad for ASHPs.  With 120sq m you are looking at one zone (max 2 - one up and one down) directly connected to the ASHP run, almost certainly, fully open loop on weather compensation.  

 

Hoe that helps, feel free to ask as many questions as you want.

 

 

 

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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Matthew_W
 Matthew_W
(@matthew_w)
Active Member Member
Joined: 2 months ago
Posts: 5
Topic starter 12/03/2025 11:05 pm  

Thank you.

I've got a couple of leads courtesy of Mars, and requested design consultation via the Heat Geek web portal, so I'll see What some experts say about underfloor heating options and surely generate some more questions.


   
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Mars
 Mars
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Posts: 3112
12/03/2025 11:24 pm  

@matthew_w good luck with the quotation and proposal process. I’m looking to some updates and see how your ‘local’ Heat Geeks fare.

Buy Bodge Buster – Homeowner Air Source Heat Pump Installation Guide: https://amzn.to/3NVndlU
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