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

[kentar]

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?

[Dillsue]

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. 

Edited September 30 by Dillsue

[kentar]

Understandable, but i've got fan heaters as backup and the immersion heater is taking care of the hot water. I'm pulling the trigger by the end of the month, which I think is ample time to decide on a plan.

[Dillsue]

8 hours ago, kentar said:

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?

A large emitters surface area is what works best with a HP so that it can work at low temperatures. UFH gives a large area without being intrusive but radiators can give a big enough surface area too. Retro fitting UFH is more involved, intrusive and costly than radiators so it's up to you what you go for depending on your preference for the type of emitter versus your budget. If you've already got usable plumbing for radiators then that's a plus for rads.

 

Hopefully you already know what's involved in installing wet UFH so you'll appreciate that it's pretty disruptive to install. If youve sized your rads to operate at low temperature, the plumbing involved in swapping from rads to UFH is a small proportion of the work to install UFH.

 

The other consideration is to look at your insulation and draught proofing before you spec a new heating system.

[JohnMo]

Cylinder for a 4 bed do 250 to 300L

 

1. Unless mad set on UFH don't, do radiators with a  design temp of 35. You will get push back from installers, but your running costs will be very good.

2. No

3. Wall mounted fan coils, these would give summer cooling as well as heating and smaller than 35 Deg radiators. But more expensive.

 

A2A heating system (Aircon) would replace the whole wet system, but you would need to think about DHW heating as separate thing.

 

4. Costs

Do not accept a system design that includes 

Glycol - it requires a higher flow temp and more pumping power to move it about the system compared to water.

Buffer, volumiser is ok to achieve 20L water per kW output of heat pump.

Thermostat in every room, so a zoned system - needs to be fully open single zone ideally.

Third party control, need to manufactures controls only.

 

Suspect most your plumbing will get replaced.

[richi]

I second @JohnMo’s suggestion: Consider air-to-air. I’ve recently installed a pair of 5kW Toshiba Haori splits—one at each end of the house. So far so good and my modeling suggests it'll cope with the coldest winters we've seen in this old house since we moved in 10+ years ago (and summer cooling without having to worry about condensation problems with fan coils).

 

I expect to be a bit better than cost neutral, but even if I don't save money, I still needed to replace the 50-year-old boiler, the oil tank and a couple of manky rads.

 

Caveat: I’m replacing an oil fired system, which is roughly 150% of the running cost of gas.

[marshian]

46 minutes ago, richi said:

 

I expect to be a bit better than cost neutral, but even if I don't save money, I still needed to replace the 50-year-old boiler, the oil tank and a couple of manky rads.

 

Caveat: I’m replacing an oil fired system, which is roughly 150% of the running cost of gas.

I was under the mistaken belief that oil fired boiler heating was the cheapest of the fossil fuels?

 

I’ve always assumed the downsides were storage, higher cost of oil boilers and paying up front for the oil (compared to paying after usage for gas)

 

clearly I was wrong?

 

 

[richi]

Oh, wait, I think you're right, sorry. My memory of the gas price is bogus.

A good price for bog standard 28s kerosene delivered is 55p per litre. A litre has a touch more than 10 kWh of energy. however, the best oil boilers are less efficient than the best gas boilers, because they don't modulate.

[Gone West]

1 hour ago, richi said:

Oh, wait, I think you're right, sorry. My memory of the gas price is bogus.

A good price for bog standard 28s kerosene delivered is 55p per litre. A litre has a touch more than 10 kWh of energy. however, the best oil boilers are less efficient than the best gas boilers, because they don't modulate.

An oil boiler is cheaper to run than a gas boiler. There are also modulating oil boilers.

 

https://sapphireheatsolutions.co.uk/

 

[Beelbeebub]

Oil boilers are currently cheaper to run than gas but the price tends to be more volatile when world events happen around oil producing nations....

 

 

[JohnMo]

But the OPs non condensing one won't be cheaper than anything to run. Plus it's at least 30 years old.

[marshian]

3 minutes ago, JohnMo said:

But the OPs non condensing one won't be cheaper than anything to run. Plus it's at least 30 years old.

 

Wasn't the point being made........................

[kentar]

On 01/10/2025 at 07:59, Dillsue said:

A large emitters surface area is what works best with a HP so that it can work at low temperatures. UFH gives a large area without being intrusive but radiators can give a big enough surface area too. Retro fitting UFH is more involved, intrusive and costly than radiators so it's up to you what you go for depending on your preference for the type of emitter versus your budget. If you've already got usable plumbing for radiators then that's a plus for rads.

 

Hopefully you already know what's involved in installing wet UFH so you'll appreciate that it's pretty disruptive to install. If youve sized your rads to operate at low temperature, the plumbing involved in swapping from rads to UFH is a small proportion of the work to install UFH.

 

I have a basic understanding of what's involved in retrofitting UFH. I have wooden floors donwstairs which are to be ripped out at some point. If I was doing it now I would opt for UFH from the start. But let's say I wanted to do change from radiators to UFH, what exactly is involved? And what design considerations should I make now to ensure a smoother transition? Is it as simple as unplugging a rad and plugging in the pipe for the UFH?

[kentar]

On 01/10/2025 at 09:06, JohnMo said:

Cylinder for a 4 bed do 250 to 300L

 

1. Unless mad set on UFH don't, do radiators with a  design temp of 35. You will get push back from installers, but your running costs will be very good.

2. No

3. Wall mounted fan coils, these would give summer cooling as well as heating and smaller than 35 Deg radiators. But more expensive.

 

A2A heating system (Aircon) would replace the whole wet system, but you would need to think about DHW heating as separate thing.

 

4. Costs

Do not accept a system design that includes 

Glycol - it requires a higher flow temp and more pumping power to move it about the system compared to water.

Buffer, volumiser is ok to achieve 20L water per kW output of heat pump.

Thermostat in every room, so a zoned system - needs to be fully open single zone ideally.

Third party control, need to manufactures controls only.

 

Suspect most your plumbing will get replaced.

 

1. What's the basis of your objection to UFH?

2. See my comment above.

3. Would you be able to elaborate of wall mounted fan coils? Isn't that air conditioning?

 

What would be the difference in running cost between a conventional CH system vs A2A aircon?

[kentar]

On 01/10/2025 at 13:20, richi said:

I second @JohnMo’s suggestion: Consider air-to-air. I’ve recently installed a pair of 5kW Toshiba Haori splits—one at each end of the house. So far so good and my modeling suggests it'll cope with the coldest winters we've seen in this old house since we moved in 10+ years ago (and summer cooling without having to worry about condensation problems with fan coils).

 

I expect to be a bit better than cost neutral, but even if I don't save money, I still needed to replace the 50-year-old boiler, the oil tank and a couple of manky rads.

 

Caveat: I’m replacing an oil fired system, which is roughly 150% of the running cost of gas.

 

That's an interesting take. Are you able to compare the efficiency of your oil fired system to a non-condensing boiler? If it doesn't cost any more to run than my current boiler, it would be something i'd be keen to look into.

[kentar]

20 hours ago, JohnMo said:

But the OPs non condensing one won't be cheaper than anything to run. Plus it's at least 30 years old.

 

The boiler is 20 years old. The boiler design however would be older as they stopped manufacturing it in 2003.

[JohnMo]

1 hour ago, kentar said:

have a basic understanding of what's involved in retrofitting UFH

Suspended floor you need lots of insulation as downwards heat loss is huge. If you have a solid floor you either insulate very well or do zero insulation and use the ground as a buffer. But the zero insulation needs to run the full heating season to be efficient.

 

Lots of insulation is a U value circa 0.15 or better

 

1 hour ago, kentar said:

What's the basis of your objection to UFH?

To do well is difficult as a retrofit. To radiator is just easier.

1 hour ago, kentar said:

Would you be able to elaborate of wall mounted fan coils

Fan coils uses water as the heating medium Aircon uses refrigerant. But similar otherwise. But an ASHP will do heating, cooling and DHW.aircon just heating and cooling, not DHW.

 

1 hour ago, kentar said:

What would be the difference in running cost between a conventional CH system vs A2A aircon?

Depending on lots of factors, like flow temperature, but pretty similar or very different.

[JohnMo]

1 hour ago, kentar said:

Are you able to compare the efficiency of your oil fired system to a non-condensing boiler?

Non condensing oil boiler is circa 60 to 70% efficient. A modern condensing boiler circa 90% in a well setup system. A simple single zone ASHP around 400 to 450% efficient. But flow temp is king with ASHP.

[Dillsue]

4 hours ago, kentar said:

 

I have a basic understanding of what's involved in retrofitting UFH. I have wooden floors donwstairs which are to be ripped out at some point. If I was doing it now I would opt for UFH from the start. But let's say I wanted to do change from radiators to UFH, what exactly is involved? And what design considerations should I make now to ensure a smoother transition? Is it as simple as unplugging a rad and plugging in the pipe for the UFH?

If your designing for a low flow temp then yes, just move the pipes from the rad to the UFH.

[Beelbeebub]

Be aware that the flow temps in UFH are lower than even a fairly low temp rad system - so 35C is a typical maximum flow temp for UFH - but 35C is about the lowest maximum flow temp for a rad system before the radiator sizes start getting silly.  Basically it is possible to run much lower flow temps (and hence higher efficiencies) with UFH. That said, you can still get very good efficiencies with rads, it's just that UFH has the potential to be even more efficient.

 

One thing to watch out for if you have an old building is that the maximum power output of UFH before you start hitting impractical floor temps might not be enough if you house is particularly drafty or poorly insulated.  Effectively you need to cook your feet in order to stay warm.

 

On the other side, if you have a tile/concrete floor, which is best for UFH and you ar every well insulated the temp of your floor might have to be so low that the floor feels cold to bare feet, even though it's warm enough to keep the room at 22C.  My old house need a temp difference of about 2C down to near freezing, so our 20C rooms meant the floors were 22C, which felt cold to bare feet.  It only became comfortable to walk on in bare feet well below zero!

[marshian]

1 hour ago, Beelbeebub said:

Be aware that the flow temps in UFH are lower than even a fairly low temp rad system - so 35C is a typical maximum flow temp for UFH - but 35C is about the lowest maximum flow temp for a rad system before the radiator sizes start getting silly.  Basically it is possible to run much lower flow temps (and hence higher efficiencies) with UFH. That said, you can still get very good efficiencies with rads, it's just that UFH has the potential to be even more efficient.

I don't agree

 

I've got T22 rads - they replaced T11 rads in a 1980's house - they are the same size as original - just thicker - They are not silly sizes. My WC flow temp starts at 25 deg C and ends at 35 Deg C at -2.5 OAT

 

It's perfectly possible to run UFH flow temps thro rads and still keep a house at temperature

[sharpener]

On 03/10/2025 at 09:25, Beelbeebub said:

but 35C is about the lowest maximum flow temp for a rad system before the radiator sizes start getting silly

 

Disagree, we had to raise the design flow temp for our HP to 47.5C to avoid using a massive T33 in one bedroom using all available space on one wall. Big lossy house like the OP's. Apart from this one rad 45 would have been doable, but not 35.

[Beelbeebub]

On 03/10/2025 at 10:33, marshian said:

I don't agree

 

I've got T22 rads - they replaced T11 rads in a 1980's house - they are the same size as original - just thicker - They are not silly sizes. My WC flow temp starts at 25 deg C and ends at 35 Deg C at -2.5 OAT

 

It's perfectly possible to run UFH flow temps thro rads and still keep a house at temperature

The flow temps quoted are usually the design flow temp, ie at minimum Pat. So in your case the flow temp would e described as "35C" even though it only hits that for a few days a year. 

 

Trying to heat you house at - 2.5 outside with (say) 30C flow would be tricky as the DT would likely only be a few degrees (mean rad temp is half way between flow ad return so for 30C flow, it would be 27C ave temp for a DT of only 7C at which point rads output about 15% the rated output)

 

My point is that UFH flown temps are usually limited to avoid damaging floor finishes. This maximum varies but can be in the low 30's which is about where the sensible minimum flow temps are. 

 

My house runs low 30's and my k22 rads are barely warm to touch but it manages to stay warm(ish). 

 

But if I had a leakier house I doubt it would work. 

[kentar]

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? 

[Nickfromwales]

53 minutes 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? 

Rads twice the size, basically. Plus bigger pipe sizing for distribution to rads.