UFH - validate my thinking plus some questions

[Bigdeadbadger]

Evening All,

 

Was looking for collective thoughts on my ideas for my UFH.

Yes, I have quotes from most of the major players, just wanted some second opinions.

 

About the house first:  380m2 in total, 210m2 on the ground floor.

Planning UFH on ground floor only with only towel rads in the 4 upstairs en-suites.

House is 220mm SIPS panels throughout including roof.  Triple glazed.  Aiming for < 2 AH (I know not super fantastic, but trying to be realistic based on time I've had).  House will have MVHR.

Foundation is an insulated raft 250mm concrete + 65mm screen on top.  Raft has 250mm EPS and external Insulated upstand.

Heat will be via Gas Boiler (its available right outside my house and building warrant was done in 2023).

Location - central Scotland

I'm happy that I didn't attach the pipe to the slab during pour as there is no way the cowboys that put up my kit would not have blasted nails through it.

 

Main system points are:  looking at minimum of  14 ports, some places up to 15 to accommodate the number of loops and volume of pipework.

Would be 16mm pipe on 150mm spacing

Because of the number of ports, it's looking like 2 manifolds.  And therefore 2x pumps.  Any things I should be aware of there? does it make balancing the system horrendous? or just more time?

With what will be 1200m or so of pipework, is there any benefit to fitting a buffer tank.

 

Am I missing anything? or creating future headaches?

 

I'm clear that warm up / cool down time will be long.  Trying to just run the house at a steady temp.

 

Edited yesterday at 18:59 by Bigdeadbadger

[JohnMo]

2 hours ago, Bigdeadbadger said:

Heat will be via Gas Boiler

Our new build was completed with gas boiler, soon after moving in solar gains were painful. Bought a cheap ASHP to allow cooling via UFH. A year later the gas was disconnected, and ASHP used for heat and cool. Was cheaper then gas, no gas standing charges for one, using a time of use tariff, some solar and battery, makes ASHP as cheap as chips to use.

 

I would drop down to 200mm centres and run from a single manifold (our floor area is 192m2 and we have 7 loops on 300mm centres).  Run direct from a good boiler, so no pumps or mixers.

 

UFH design, you need it room balanced by design, as far as possible, so room loops need to output room heat loss and each room receive its heat loss via the UFH. This makes balancing easy, I have all loops fully open at the flow meters and let the ASHP decide the flow it wants or needs.

 

So operation can be two ways, WC or simply a fixed flow temp and a thermostat to limit, a single thermostat will do or a few, but don't bother with manifold actuators, so one thermostat needs heat all loops come on. Get an under floor wiring centres with some bells and whistles, so the room thermostats are actually temperature sensors and the can doo heat and cool, for when you install the heat pump.

 

Boiler get a system boiler, not heat only one.  Run PDHW (priority domestic hot water), this drive two temperatures from the boiler, one for heating the other for DHW heating. Get a heat pump cylinder, nothing else. Reheat times super quick and best boiler efficiency.

Edited yesterday at 21:15 by Nickfromwales
Typo fixed :)

[Bigdeadbadger]

Thanks @JohnMo
 

some more questions - see you’re in NE Scotland and still had solar gain issues - that’s a bit concerning for me - was there something specific about your amount of glass? 
 

I have ~26m2 south/southwest facing glass. Although I do have an overhang 
 

 

[JohnMo]

We have a 6m overhang, but its the westerly afternoon sun in spring and autumn that gets in, can get really hot.  Today sun all day solar gain from a low sun, so a benefit.

 

So a cylinder with 3m2 coil as used by heat pumps, used with a gas boiler are great, either low flow temp or super fast reheat. Or you could do a cylinder heated by a heat pump, as in an all in one unit

Edited yesterday at 21:16 by Nickfromwales

[Nickfromwales]

59 minutes ago, JohnMo said:

I would drop down to 200mm centres and run from a single manifold (our floor area is 192m2 and we have 7 loops on 300mm centres)

315mm of 'slab' to heat......

 

2 hours ago, Bigdeadbadger said:

Heat will be via Gas Boiler (its available right outside my house and building warrant was done in 2023).

I think you should reconsider this, as cooling via your slab wold be quite a nice thing for the summer months.

 

Is it a Heb home? Or independant?

[Bigdeadbadger]

Thanks too @Nickfromwales

 

independent - all on my own some.

 

reluctance to go ASHP is upfront cost and pain of dealing with change to my building warrant - my local council have been a source of much pain. 
 

 

[marshian]

9 minutes ago, Bigdeadbadger said:

Thanks too @Nickfromwales

 

independent - all on my own some.

 

reluctance to go ASHP is upfront cost and pain of dealing with change to my building warrant - my local council have been a source of much pain. 
 

 

 

Can always "upgrade" to ASHP later??

 

What do you think your heat loss will be at design temp because that's going to determine what min boiler output you need and the HW cyl size and HW demand is going to determine the maximum boiler output? (unless HW is lower than the heat loss)

 

 

[Nickfromwales]

16 minutes ago, Bigdeadbadger said:

reluctance to go ASHP is upfront cost and pain of dealing with change to my building warrant - my local council have been a source of much pain. 

Fit it and say feck all. I doubt anyone other than the next door neighbour, if you've pissed them off, would ever pull you up over it.

 

If they do, just go for a retrospective amendment to get it approved. This country is an ass!!!!

[Nickfromwales]

17 minutes ago, Bigdeadbadger said:

independent - all on my own some.

Good man.

 

Have you considered AeroBarrier to get you to <0.6ACH? One we just had done started out at 1.2 ACH and a few hours later was sub 0.2.

[Bigdeadbadger]

If i can read my SAP calcs correctly 

 

fabric heat loss = 194 W/K

 

Averaging for seasonality, energy for water heating is ~200KW/H per month 

 

5 bed house, 3 kids - all be it boys 😬. So was thinking 400l HW cyl

does that help? @marshian

 

[Bigdeadbadger]

19 minutes ago, Nickfromwales said:

Good man.

 

Have you considered AeroBarrier to get you to <0.6ACH? One we just had done started out at 1.2 ACH and a few hours later was sub 0.2.

No - had not - although it looks interesting. Sounds like radweld for cars 😊

 

also budget is under a lot of strain, and still almost all internals to get done. 

 

closest I have (appreciate it’s not as comprehensive as what you’ve just shared) is using the spray on airtightness paint from pro clima for some tricky junctions. 

Edited yesterday at 22:08 by Bigdeadbadger

[Nickfromwales]

24 minutes ago, Bigdeadbadger said:

closest I have (appreciate it’s not as comprehensive as what you’ve just shared) is using the spray on airtightness paint from pro clima for some tricky junctions. 

Won't scratch the surface of all the places you don't even know you have air leaking out of.

 

25 minutes ago, Bigdeadbadger said:

also budget is under a lot of strain

Use the force.

[marshian]

23 minutes ago, Bigdeadbadger said:

If i can read my SAP calcs correctly 

 

fabric heat loss = 194 W/K

 

Averaging for seasonality, energy for water heating is ~200KW/H per month 

 

5 bed house, 3 kids - all be it boys 😬. So was thinking 400l HW cyl

does that help? @marshian

 

 

If my mental maths is right I'd say your water heating is going to need more poke than your space heating so get a boiler which will go as low as possible for space heating (Viessmann 200 will go down to 2kWh)

 

Then size the boiler to to recharge a 400 litre cylinder with a heat pump coil in the shortest time possible (so 400 litres from 20 to 55 needs 17 kWh of energy) to do that in ~30 mins would need a boiler capable of throwing 32 kW at it)

[Nickfromwales]

17 minutes ago, marshian said:

so 400 litres from 20

If it gets to that low a recovery temp, the cylinder stat is on the floor and the cylinder is sat on it?

[marshian]

11 minutes ago, Nickfromwales said:

If it gets to that low a recovery temp, the cylinder stat is on the floor and the cylinder is sat on it?

 

Mine gets that low every damn day..............

 

Cold water in is less than 12 Deg C right now

[JohnMo]

3 minutes ago, marshian said:

 

Mine gets that low every damn day..............

 

Cold water in is less than 12 Deg C right now

But your cylinder average temperature most will not. My cylinder has the thermostat 1/3 up and it can read 7 degs, after a large draw off, but the water coming out of the cylinder is close to 50. So mean temp is quite a bit hotter.

[marshian]

13 minutes ago, JohnMo said:

But your cylinder average temperature most will not. My cylinder has the thermostat 1/3 up and it can read 7 degs, after a large draw off, but the water coming out of the cylinder is close to 50. So mean temp is quite a bit hotter.

 

Really depends on cyl size and how much has been depleted - I was taking worst case scenario in a house with 5 people 

 

I have 117 Litre Cyl (2 people) 4 showers a day (two morning and two evening sees the top of the tank at 38 and the bottom at 12 - I can guarantee you that the average cyl temperature is not 26 Deg C (best guess would be 10 L at 38 and 107 litres at 12 would be an avg temp if you mixed it of 16 Deg)

 

@Bigdeadbadger would be in more HW trouble if his three kids were girls

[Iceverge]

11 hours ago, Bigdeadbadger said:

No - had not - although it looks interesting. Sounds like radweld for cars 😊

 

also budget is under a lot of strain, and still almost all internals to get done. 

 

Don't bother with a boiler or ASHP and get Aerobarrier instead. 

 

Find £100 for a willis heater and a 3Kw Immersion for now. 

 

Gather data for a year or two and then fit the exact ASHP that suits your house. You can also bide your time on eBay and display models this way and pick up a bargain. 

 

 

[JohnMo]

8 hours ago, Iceverge said:

3Kw Immersion for now. 

A direct invented cylinder is about the 1/3 the price of an indirect cylinder. Time of use tariffs make it a viable option. Plus you could use a smaller cylinder charged to a higher temperature if you needed more capacity. So 300L v 400L. Not many years ago our 5 bed house had a 120L cylinder - how things change

[Iceverge]

300l direct UVC here. 2x 3kW immersion. Water stored at 70deg on TOU tarriffs. 

 

Does a family of 5 ok. Can store 14kWh before the tap water gets cool. 

 

We use 9kWh daily for 5 in the house. Need to run it an extra hour in the evening if inlaws stay. 

 

Annual cost is €420.

 

 

 

 

 

 

 

[Iceverge]

Out of interest if we were to switch to an ASHP at stores 50deg 300l would only store about 7kWh. We'd need daytime dearer electric to top up. 

 

That would leave 7kWh heated at 13c/kWh at a COP OF 2.5 = 37c

 

2kWh @ 24c/kWh @ COP of 2.5 = 19c

 

Total 56c/Day. Vs €1.17 now. 61c/day saving and an ESHP is about €2400. 

 

Payback would take over 10 years by which time I would worry the ESHP may need replacing . 

 

 

Just like when we built I don't think there's a case for a heat pump for DHW alone when you can avail of reasonable night rate electric. 

 

 

 

 

[JohnMo]

37 minutes ago, Iceverge said:

Out of interest if we were to switch to an ASHP at stores 50deg 300l would only store about 7kWh. We'd need daytime dearer electric to top up. 

 

That would leave 7kWh heated at 13c/kWh at a COP OF 2.5 = 37c

 

2kWh @ 24c/kWh @ COP of 2.5 = 19c

 

Total 56c/Day. Vs €1.17 now. 61c/day saving and an ESHP is about €2400. 

 

Payback would take over 10 years by which time I would worry the ESHP may need replacing . 

 

 

Just like when we built I don't think there's a case for a heat pump for DHW alone when you can avail of reasonable night rate electric. 

This is interesting, I recently switched from ASHP heating DHW to direct immersion, have I noticed a step change in running costs? - no.

 

Our heat pump is fair distance from cylinder (a good 20 to 25m), it reports a CoP of over 3, but reality of kW of heat actually delivered to cylinder and the efficiency of a heating element in the water, means electric costs isn't much different, between heat pump and immersion. Immersion slightly more expensive, but not really noticeable.

 

The other thing is PV, we get clipping (reduced output, even in February) of generation based on export limits, so using more electricity at midday is better than loosing it via clipping. Which with 3kW immersion is easy to manage with a simple timer. 

 

Cylinders there could be £6-700 difference in price between a direct and heat pump indirect cylinder - that buys a lot of electricity. On good tariff that's about 1.5 years of free DHW just on cylinder price.