Where to start? Designing a heating system for our retrofit/extension

[Iceverge]

23 hours ago, lookseehear said:

Thermal Store for Heating and Hot Water
I'm considering installing a large thermal store (around 400-500L) to handle both heating and DHW while keeping the existing oil boiler.

 

Fine, beware for DHW you're roughly halving the capacity of the store Vs an UVC. Eg a 500l TS will only deliver roughly half the amount of hot water a UVC will at the same temp. They work well with high temperature water supply though. I put a 250l Maxipod in my parents house. 

 

On 07/10/2024 at 13:36, lookseehear said:

We’ll be installing solar PV, but likely without a battery for now. Excess solar energy can be diverted to the thermal store for heating and DHW

 

Good idea. Water is a cheap battery. Beware you may not have quite enough solar in shoulder months to achieve the 60ish Degs a TS needs to work. An UVC meanwhile will work right down to 40 DEG. 

 

On 07/10/2024 at 13:36, lookseehear said:

So, we can use cheap electricity to 'charge up' the thermal store overnight at roughly the same cost as heating with oil.

 

Fine for DHW, however unlightly to do much for space heating. Beyond early morning. 

 

On 07/10/2024 at 13:36, lookseehear said:

• We plan to install large radiators or FCUs in the bedrooms and avoid UFH in areas without insulation.

• Main Living Space
  In the upstairs living area, we’re thinking of large radiators, and perhaps plinth FCUs in the kitchen to provide an extra heating boost if needed.

• Bedroom in New Extension
  For the new extension, we’ll install wet UFH. However, instead of embedding it in screed, we might use an overlay system on top of a well-insulated slab. This would allow the room to cool overnight and heat up quickly when needed, making the UFH act more like a radiator - any pros/cons here?

• Bathrooms
  We have two downstairs bathrooms (en-suite and main bathroom) in the older part of the house. I’d like to install UFH for extra comfort underfoot.
  With 100mm of PIR insulation and 50mm of screed, head height shouldn't be too compromised. A wet overlay system over 100mm PIR might be the best option here.

 

Rads everywhere. As large as you can fit. Simple cheap, all on the same zone. 

 

Put electric UFH mats in the bathroom on a timer for comfort. 

 

In my parents house the solid fuel cooker will often get the store to 70ish Deg + after which the rads will kick in to dissipate the excess heat. They'll be running at maybe 30deg which is far more comfortable than the regular "hot" rads you get. My point being, if you oversize your rads and run them at a low flow temperature then you'll have lots of the benefits of UFH without the complexity of integrating a new and old system. 

 

On 07/10/2024 at 13:36, lookseehear said:

External Insulation: I’m tempted to install 150mm of EPS around the foundation downstand (about 600mm deep) to reduce heat loss through the floors over time. This wouldn’t go around the entire property but could cover about 50%.

 

Good plan. If you were just to add 10mm of PIR under the floor though it'd make a world of difference. I did a thermal model here for someone ages ago. 

 

There's no mystery with internal insulation. Just make sure it can "DRY" more than it gets "WET". 

 

1. Eliminate bulk water. Gutters, chutes , drains leaks driven rain etc. 

 

2. Regulate internal humidity with mechanical ventilation. MVHR DCV and PIV is the hierarchy. 

 

3. Don't build layers in manner that will trap moisture with materials of very low permeability, PIR, foil, polythene etc.

 

4. Stop air taking internal moisture where it should go via air leaks. 

 

Lime and hemp is good, but over sold IMO. A mild sand cement wall parge and tapes to the windows and floor for Airtightness. A battened layer of mineral wool for insulation and plasterboard would be fine. Airtighess is the key here. 

 

 

 

 

 

 

 

[Iceverge]

Your wife is very very right. 

 

It's a house not a branch of currys. nothing more frustrating than being unable to work the controls. It's more expensive, more lightly to break etc. 

 

Single zone heating on a timer or single thermostat. All rads no plinth heaters or fan coils or UFH.  

 

UVC for DHW only. No messing with banking heat for space heating. It doesn't really work unless you have thousands of litres. 

 

dMEV to every wet room.

 

Electric UFH in bathrooms on a switch or timer. 

 

Fastidious attention to Airtightness. 

 

As much insulation as you can fit. 

 

As much PV as you can fit. 

 

No home automation or connected nonsense. It's out of date as soon as it's installed and everyone who is not Geek hates it. 

 

 

[Nickfromwales]

52 minutes ago, Iceverge said:

UVC for DHW only. No messing with banking heat for space heating. It doesn't really work unless you have thousands of litres. 

Yup. For heating, via thermal storage, you can directly heat the circulating water with a big immersion and use that to bolster/reduce oil consumption (basically by it being seen as an electrical inline heater of sorts), and that will have some benefit of introducing cheap there direct electricity to space heating, but for a domestic heating scenario in a poorly insulated 2-storey home the least capacity TS I’ve installed for bulk stowage is 2600L. That was storing at 85°C to be anywhere close to being useful/effective and to have sufficient sustain.

A 300-500L storage tank heated to 85°C would be drained to a useless temp in an hour or two. 

[sharpener]

25 minutes ago, Nickfromwales said:

A 300-500L storage tank heated to 85°C would be drained to a useless temp in an hour or two. 

 

Yes. We have a very specific use case which is heating the bedroom rads at 0700, so before we get up but after the end of the cheap rate, the 270 litres was calculated with just that in mind.

 

The plan was to do the same before we go to bed, but now that Cosy has an extra cheap window 2200 - 2400 we don't need it so it can be re-purposed for living room rads over the 1600 - 1900 peak.

 

I think part of the OP's plan was to use the TS as somewhere to amalgamate all the various sources of heat, it is still valid for that. And also to prevent the oil boiler short cycling though I would question whether that is really necessary, ours used to fire 3 or 4 times an hour for 5-10 mins, I don't think they mind that unlike a HP. But unless a proper condensing type they object to low flow temps as any condensate will then rot the flue.

[Gone West]

2 hours ago, lookseehear said:

I love this! I can have a look back through your posts to see if there's anything relevant, but I'd love to hear in particular why you decided on hempcrete vs another type of insulation for the solid walls.

 

There isn't much on here that I've posted, just a thread about TS v UVC and a thread about what to do about old stone walls and IWI. I'm not going to go the whole hog on IWI just something to help raise the temperature, a bit, of the surface of the old stone walls.

 

 

[ReedRichards]

4 hours ago, sharpener said:

And also to prevent the oil boiler short cycling though I would question whether that is really necessary, ours used to fire 3 or 4 times an hour for 5-10 mins, I don't think they mind that unlike a HP

So say it's 3 times an hour for 5 minutes, that would be a duty cycle of 25% On, with an On time of 5 minutes.  Why do you think a heat pump would mind that?  

[JohnMo]

1 hour ago, ReedRichards said:

On time of 5 minutes.  Why do you think a heat pump would mind that?  

My heat pump instructions state

 

The choice of regulation is fundamental to optimise the efficiency of the system and to reduce the number of starts/hour in order to achieve the correct thermodynamic balance and the correct lubrication of the mechanical parts in the compressor, it is recommended to choose them in order to have a continuous operation of the heat pump of at least 10 minutes.

 

Found with mine, running less than about 10 minutes, result in the best part of no heat being added to the house. So it is now set to run no less than about 15 minutes.

 

Short running is likely to use lots of energy and not do much heating of the house. Especially when the heat source is located outside.

[ReedRichards]

10 hours ago, JohnMo said:

Short running is likely to use lots of energy and not do much heating of the house. Especially when the heat source is located outside.

That sounds a bit non-physical.  If you are using lots of energy then you must be heating something, even if it's with a COP of near-one.  And if it's not inside then it must be outside.  If it's outside, as you seem to think then what is happening?  Is some motor inside the heat pump getting very hot?  Do those mechanical parts inside the compressor need more lubricant?  Your symptoms "best part of no heat added to the house" sound more like a fault than regular behaviour.

[JohnMo]

1 hour ago, ReedRichards said:

That sounds a bit non-physical.  If you are using lots of energy then you must be heating something, even if it's with a COP of near-one.  And if it's not inside then it must be outside.  If it's outside, as you seem to think then what is happening?  Is some motor inside the heat pump getting very hot?  Do those mechanical parts inside the compressor need more lubricant?  Your symptoms "best part of no heat added to the house" sound more like a fault than regular behaviour.

I have zero issues with the heat pump. A heat pump behaviour is different to a boiler, its output is a lot different. Most heat pumps are sub 10kW, most oil boilers are 4x the output.

 

You don't have lots of energy, a 6kW heat pump running at full load for 10 mins generates a max 1kWh of heat. Getting 60L of water from 21 deg to 28 is 0.5kWh, plus heating a bunch of pipes, a heat exchanger etc. That 1kWh is very quickly consumed, before any real useful work is done.

 

So a typical heat pump running for 10 mins looks like this

Key points are

Compressor starts at 22.5 degs return temp

At 10 mins the flow temp has increased to 28.6 degs and return temp is up to 24.8 deg.

Actual heat output after 10 mins is only 0.852kWh, so basically heating the system, and not much heat is going into the house.

 

 

[ReedRichards]

2 hours ago, JohnMo said:

Actual heat output after 10 mins is only 0.852kWh, so basically heating the system, and not much heat is going into the house.

 That's good data.  But after 10 minutes you have now warmed up all the water inside your house to 26.7 C average.  So even if everything was turned off at that point you would still have heat going into the house; that doesn't stop.  You will still derive benefit from all parts of the heated system that are inside the insulated fabric of the building; that energy spent heating the system was not wasted, as you seem to imply.

[JohnMo]

2 hours ago, ReedRichards said:

You will still derive benefit from all parts of the heated system that are inside the insulated fabric of the building; that energy spent heating the system was not wasted, as you seem to imply.

Not saying it's wasted, but it not enough to be properly useful. It better to have a slightly longer run, the remainder run time is all useful heat. Its getting the correct balance between heating stuff up and heating heating stuff up and adding heat to the house

 

a few runs at similar average outside temps, only change from first and send two runs was a slight change to compressor stop hysteresis to get a longer run. The compressor start hysteresis remained the same

 

7.8 deg OAT, short runs (10 to 15 mins duration), 9.1kWh electric 44.2kWh heat delivered - short runs (space heat CoP 4.86)

7.5 deg OAT, longer runs runs (15 mins plus duration), 5.7kWh electric 27.8kWh heat delivered - longer runs (space heat CoP 4.89)

slightly colder day at 6.8 deg OAT, again longer (35 mins plus duration) runs, 6.8kWh electric 33.8kWh heat delivered - longer runs (space heat CoP 4.97)

 

On all occasions above is run full WC mode, house remains at a stable 20.5 degs. Difference between 7.8 and 7.5 deg day is a reduction of 40% electrical use. Which is huge. CoP stays similar, CoP increase on the 6.8 deg day, is due to slightly cooler day leading to longer run times and less standby time.

 

As point to note I did find similar reductions in energy use when I optimised the gas boiler when running very low flow temps, once I added the heat meter and started to understand what was happening.

 

 

[lookseehear]

On 08/10/2024 at 13:52, Iceverge said:

Rads everywhere. As large as you can fit. Simple cheap, all on the same zone. 

 

Let's just say for a minute that I don't want huge radiators in all the bedrooms, what would the next best option be? How oversized is oversize? While space isn't massively at a premium, I feel that placement is incredibly important if you're putting in oversized rads so as not to be in the way of furniture.

 

On 08/10/2024 at 13:52, Iceverge said:

Put electric UFH mats in the bathroom on a timer for comfort. 

 

My brother in law and my in-laws have this, and both have said that it doesn't work as well as they'd hoped. If you don't have a regular daily routine (our weeks can be quite varied) then a timer doesn't work, and I'm imagining that it's slow to heat up and expensive to run. What's the real life experience like of using these?

On 08/10/2024 at 13:52, Iceverge said:

Good plan. If you were just to add 10mm of PIR under the floor though it'd make a world of difference. I did a thermal model here for someone ages ago. 

 

I like this idea - I'll try and factor this in.

 

On 08/10/2024 at 13:52, Iceverge said:

A mild sand cement wall parge and tapes to the windows and floor for Airtightness. A battened layer of mineral wool for insulation and plasterboard would be fine. Airtighess is the key here

 

Can you explain in a little more detail? By parge do you mean brushed on? Do you then mean battens screwed directly to the wall, with mineral wool filling the whole gap between wall and plasterboard? If the airtight layer is the parge coat, doesn't this risk condensation behind the plasterboard on the wall?

 

If walls are already plastered and in a good state of repair I assume battens directly to this, mineral wool fill and regular plasterboard on top? This would be cheap and easy. Any issues with the original walls being painted (from a breathability perspective)?

 

Thanks for all the advice so far.