Seeking advice wrt modernising the heating system on a 100+ year old property.

[Dillsue]

6 hours ago, kentar said:

I'm going to throw a spanner in the works... say I went with a gas system boiler and new rads with the idea of moving to ASHP at a later date. What accommodations would I have to make now to allow for this? 

 

On 30/09/2025 at 23:40, Dillsue said:

If your boiler is now defunct, my advice would be to have a look on ebay or marketplace and get a secondhand replacement to repair or replace what you've already got. That will buy you time to answer all your questions and work out what you need/want rather than rushing into throwing an ASHP under pressure to get the heating back on. 

There's only you that can decide on any prep mods for an ASHP. All the previous posts have started to scratch the surface on the things you need to consider before you make the decisions. The problem you have is many of those posting will have their own preference so what one person says is right for them isn't necessarily the right thing for you, or others, the size of rads being a classic example.

 

I want the best system efficiency so have designed for a low flow temp and large radiators as I'm not bothered about large rads but dont want the cost and disruption of removing floors to install UFH. Others don't want or can't have large rads so are running with smaller rads/higher flow temps and living with higher running costs but lower installation costs. Neither option is right for everyone.

[Andehh]

Unless you're going mad with insulation, I really don't think an ASHP makes sense in this sort of property. 

 

Why not replace gas, for gas? 

[Dillsue]

2 hours ago, Andehh said:

Unless you're going mad with insulation, I really don't think an ASHP makes sense in this sort of property. 

 

Why not replace gas, for gas? 

We've got friends living in a large solid walled 1800s house and their ASHP works well for them. They've got insulated UFH, fairly recent DG and deep loft insulation. No IWI or EWI so they've not gone mad with insulation but are still happy.

[JamesPa]

On 30/09/2025 at 23:19, kentar said:

My house is a 4 bed semi, early 1900's and is solid brick construction. Can't say how old the plumbing system is, but safe to assume pre 80's. An ASHP will be replacing the now defunct gas boiler and an unvented 200/250l hot water cylinder will also go in, but i'm unsure of what to do about the heating system. 

 

The current system uses a non-condensing boiler with a vented hot water cylinder. It lacks hot water pressure. The shower is fed using a pump to combat this but it runs the 140l tank dry within 10 mins.

Half of the rads need replacing as they're extremely old. One rad is definitely full of sludge and barely gets warm even after a flush. The only saving grace is that all pipes to rads are 15mm.

 

I've heard people claim that underfloor heating is best with ASHP, but also that it's difficult to retrofit to older properties. 

 

1. Should I go for underfloor heating or radiators?

2. Can radiators be changed easily to underfloor heating at a later date, or does it need to be designed for it from the off?

3. Is there a third (or fourth) option?

4. Any other considerations?

For my money and to answer your questions

 

I agree with the advice from @JohnMo and @Dillsue

 

1. UFH re rads is a decision for you based on personal preference and disruption.  I presume you currently have rads so rads will be less disruptive.  UFH ultimately should be cheaper to run and of course takes up no space.  It is generally more comfortable than rads, but ASHP (ie low temperature) heating even with rads is substantially more comfortable thyan traditional high temperature heating.  If you go with rads try to design for a FT of 45 or less (you will struggle to get much less).

2. If you are going UFH Id do it now not later.  Doing it later means you suffer disruption twice for little gain

3. A mix of rads and fancoils or, if house is reasonably open plan, air to air.  I had one fancoil fitted (because a rad really wouldn't) when I had my ASHP was installed, and wish I had a few more.  With fancoils, UFH you can do light cooling if your house needs it, which you cant with rads.  A2A of course does full aircon.

4. (i) It really depends on the house loss.  Ultimately it will need a survey but if its currently reasonably warm most of the day (whether or not you actually heat it) you can get a rough idea from oil consumption.  Take you annual litres consumption, multiply by 10 to convert to kWh, then by the assumed efficiency (perhaps 75% if its a very old oil boiler)  This gives you an estimate of kWh/year needed to keep the house warm..  Divide by 2000 to get an upper estimate of loss, divide by 3000 to get a lower estimate of loss, result in kW.  Suggest you do this and post the result for further comments.  You should anyway be doing this IMHO to sense check any survey results, which can be wildly inaccurate (my survey was more than double actual loss, and 50% over appears to be not unusual)

    (ii) If the house is really lossy then A2W ASHP may not be a good choice unless you plan to upgrade insulation.  This is NOT because A2W ASHPs cant cope with the loss, they can, but because ASHPs work best when operated 24x7 or thereabouts.  In the vast majority of houses that makes sense, even if they are only occupied part time.  However in very lossy, low thermal mass houses (ie houses with, essentially, the thermal characteristics of a tent) point and time of use heating makes more sense.  A2A isnt a bad match for this!

 

Hope that helps address your original questions.

Edited 18 hours ago by JamesPa