Hybrid boiler

[Jilly]

Still trying to chose CH and DHW within all the constraints of my conversion and conservation area. Builder is recommending I have wet UFH to future proof. 

 

Has anyone got any experience of the Sime Murrelle Revolution 30  gas (it would have to be LP) and a small heat pump (which confusingly doesn't need to have an external box) so is recommended by the energy saving trust. I'm doing this to get through BC now with adequate heat. 

 

I want to have future options so think I need a thermal store to allow for possible solar thermal, may be woodburner and back boiler, GSHP if they become cheaper, or compost heat is I become a hippy. Does such a thing exist? `I've read you have to spec in advance. Am I correct?

[SteamyTea]

How much insulation has be suggested to go under the UFH slab?

[Jilly]

70mm has been specified (there is 150mm to play with between the old slab and the FFL). there might even be a little more. I am hoping to squeeze 100mm in and a low profile UFH if we go that way. 

[SteamyTea]

150mm minimum.  Any less, get radiators fitted.  No point heating the mud under the house up.

[Mr Punter]

I don't think Ground Source HP offer many advantages over Air Source.  Air Source is far more widely used so will probably be a much better bet in terms of future availability and innovation.

Heat pumps are zero carbon, subject to electricity generation method.

 

LPG is a pain and I would not recommend.  You are held to ransom by the suppliers.  It is expensive and emits CO2.

 

Solar thermal seems a waste of time compared with solar PV, which is getting better all the time.

PV is zero carbon.

 

Woodburner is toxic and polluting and expensive if you don't have a free supply of wood.

[Jilly]

2 hours ago, Mr Punter said:

I don't think Ground Source HP offer many advantages over Air Source.  Air Source is far more widely used so will probably be a much better bet in terms of future availability and innovation.

Heat pumps are zero carbon, subject to electricity generation method.

 

LPG is a pain and I would not recommend.  You are held to ransom by the suppliers.  It is expensive and emits CO2.

 

Solar thermal seems a waste of time compared with solar PV, which is getting better all the time.

PV is zero carbon.

 

Woodburner is toxic and polluting and expensive if you don't have a free supply of wood.

I'm not going to have an air tight house as its a conversion,  both types of pumps seem expensive, and I'm expecting I'll need a secondary heat source. 

I'm running out of money fulfilling planning conditions.

I will get solar something in the future.

I'm disappointed wood is out of fashion, I've got a free source...but need planning permission for the chimney, so can't do it right now anyway. 

 

 

[Nickfromwales]

Use a cheap ASHP to provide the bulk of your heating, with oversized radiators if UFH is a no-go. You can fortify this with an in-line electric boiler to boost the flow temps in December/ January and mothball it when not needed. Similar running costs to LPG when you compare holistically, and zero pita to install / own / service / maintain ( plus no dragging big bottles around ). 
If you have 150mm to play with, then an overlay system Link over 100mm of insulation may just get you through, but the more insulation the better of course. These retrofit UFH systems typically utilise a higher than preferable flow temperature, when using an ASHP, so a design heat calculation to work to will be paramount before being ‘advised’ any further.

[Dave Jones]

4 hours ago, Mr Punter said:

I don't think Ground Source HP offer many advantages over Air Source.  Air Source is far more widely used so will probably be a much better bet in terms of future availability and innovation.

Heat pumps are zero carbon, subject to electricity generation method.

 

LPG is a pain and I would not recommend.  You are held to ransom by the suppliers.  It is expensive and emits CO2.

 

Solar thermal seems a waste of time compared with solar PV, which is getting better all the time.

PV is zero carbon.

 

Woodburner is toxic and polluting and expensive if you don't have a free supply of wood.

 

ground source don't suffer the same as ASHP which stop working/produce tepid output when its very cold. 

[SteamyTea]

1 hour ago, Dave Jones said:

 

ground source don't suffer the same as ASHP which stop working/produce tepid output when its very cold. 

The installation price difference allows for this in the running costs.

For the week or two a year that they underperform from specification (if it is an MCS it should be no more that 4 days a year), supplementary heating is used.

 

You really have a problem with ASHPs don't you.

[Jilly]

16 hours ago, Nickfromwales said:

Use a cheap ASHP to provide the bulk of your heating, with oversized radiators if UFH is a no-go. You can fortify this with an in-line electric boiler to boost the flow temps in December/ January and mothball it when not needed. Similar running costs to LPG when you compare holistically, and zero pita to install / own / service / maintain ( plus no dragging big bottles around ). 
If you have 150mm to play with, then an overlay system Link over 100mm of insulation may just get you through, but the more insulation the better of course. These retrofit UFH systems typically utilise a higher than preferable flow temperature, when using an ASHP, so a design heat calculation to work to will be paramount before being ‘advised’ any further.

Cheap ASHP? ?

[Nickfromwales]

https://www.ebay.co.uk/itm/Generation-6-Samsung-Air-Source-Heat-Pump-16kW-Capable-of-65-C/223765085822?epid=1960183766&hash=item3419703e7e:g:5nkAAOSwzDFb4ddf

[joe90]

18 hours ago, Dave Jones said:

ground source don't suffer the same as ASHP which stop working/produce tepid output when its very cold. 

Not necessarily,  mine does not, I still get 48’ DHW and same to use in my buffer tank blended down for the UFH!!! it may have defrosted a few times when the dew point is reached, but that is more about RH rather than temp.

[ProDave]

19 hours ago, Dave Jones said:

 

ground source don't suffer the same as ASHP which stop working/produce tepid output when its very cold. 

What do you call "very cold"?

 

My 5kW LG ASHP has continied to heat my house and DHW when it was -14 last night and now at a daytime high of -4.

 

No doubt the COP is not as good and it has needed to defrost a couple of times, but it has not stopped working.

[Judd]

Hi Jilly

For UFH if you use a flow screed then 70mm insulation is fine. I converted a barn 6 years ago with recommended 70mm insulation and the screed can be laid at only 40mm deep, which means in places it’s 25mm thick above the pipework. Depending on the flow screed this is within the prescribed limits of the product. Obviously you cannot do this with a standard dry screed, so clearly a flow screed is more expensive. I believe some of the newer screeds also allow for direct polishing...I had a micro cement finish put on mine, and this I would NOT recommend.

[Judd]

Hi Jilly

One further thing on these newer screeds....you have to completely adhere to the spec with both accurate depths and drying times/methods...that did add time & complexity to getting it done 

[PeterW]

70mm insulation doesn’t meet building regulations under a floor unless it’s something like Aerogel. Minimum with PIR and a square building is 85mm. 

[Nickfromwales]

46 minutes ago, PeterW said:

70mm insulation doesn’t meet building regulations under a floor unless it’s something like Aerogel. Minimum with PIR and a square building is 85mm. 

Which is still shite.  

[A_L]

1 hour ago, PeterW said:

70mm insulation doesn’t meet building regulations under a floor

 

????...........Part L1B requires 0.22 for floors (Table 2 page 16) .......7m x 7m floor with 70mm PIR is 0.21, better if larger. But I would go for 100mm or more (U=0.17 or better)

[ProDave]

11 minutes ago, A_L said:

 

????...........Part L1B requires 0.22 for floors (Table 2 page 16) .......7m x 7m floor with 70mm PIR is 0.21, better if larger. But I would go for 100mm or more (U=0.17 or better)

Take a moment to work out what that actually means.

 

If your UFH temperature is running at 30 degrees and it's 0 degrees outside, your 7 by 7M floow will be losing 7 * 7 * 30 * .21  = 308 watts of heat through the floor all the time.

 

I don't know the total heat input required for that 7 by 7M room. but if I applied that to my house, it would mean nearly a third of the heat I put in went straight out through the floor.

[pdf27]

6 minutes ago, ProDave said:

Take a moment to work out what that actually means.

 

If your UFH temperature is running at 30 degrees and it's 0 degrees outside, your 7 by 7M floow will be losing 7 * 7 * 30 * .21  = 308 watts of heat through the floor all the time.

 

I don't know the total heat input required for that 7 by 7M room. but if I applied that to my house, it would mean nearly a third of the heat I put in went straight out through the floor.

It isn't 30°C though, there's a very long time delay (several months unless you have a lot of groundwater flowing) between the air temperature and soil temperature under a slab. 150W is more realistic - essentially this is the same reason that GSHPs get higher COPs than ASHPs in winter.

That level of insulation is still rubbish and will cost you a lot over the lifetime of the insulation, but it's important to get the numbers right.

 

On 10/02/2021 at 18:28, Dave Jones said:

ground source don't suffer the same as ASHP which stop working/produce tepid output when its very cold. 

There are two issues here:

1. Stopping working - they'll have a rated minimum temperature, for instance for Panasonic it's -20°C. Coldest temperature ever recorded in the UK is -27°C, so if you're in an exceptionally exposed location it may be an issue. For 99.9% of the population they'll never see anything that cold though.
2. Tepid output is a sizing issue, not one inherent to the technology. **Some** ASHPs can produce higher outputs at higher temperatures, and if somebody hasn't read the manual correctly and installs one which can only provide the required 15kW at air temperatures above 10°C performance isn't going to be good enough.

 

On 10/02/2021 at 11:32, Jilly said:

I want to have future options so think I need a thermal store to allow for possible solar thermal, may be woodburner and back boiler, GSHP if they become cheaper, or compost heat is I become a hippy. Does such a thing exist? `I've read you have to spec in advance. Am I correct?

The only way that GSHP is going to be cheap is if someone installs a shared ground-loop system in front of your house. If that happens the only infrastructure you'll need is a couple of pipes from it to the heat pump location. If that doesn't happen, it'll stay seriously expensive unfortunately.

A thermal store isn't likely to be a good match here unless you absolutely want to heat with wood (where it's required for safety reasons). Because of the way they work you need to store the heat in them significantly warmer than your hot water temperature needs to be, which in any case is the hottest water in the system. That really cripples the heat pump performance, you're much better off segregating your domestic hot water and space heating requirements as a result.

Compost heating is a thing, but not practical on a domestic scale - you're looking at the best part of 100 tonnes of compost to provide a decent flow rate of hot water.

[Temp]

We have 80mm PIR under our UFH and wish we had more.  We have 21mm engineered wood flooring on top and in our first winter which was very cold we had to crank up the flow temperature to 50C to get enough heat out of the floor. Fortunately we could do that as we have an oil boiler.  

 

 

[SteamyTea]

1 hour ago, pdf27 said:

isn't 30°C though

No, but probably never less than 20⁰C different, except where the flow goes into the room.

So you could say that area has a higher loss, maybe a minimum of 25⁰C difference.

 

Another way to look at is to calculate the floor losses with conventional heating, which may have a mean temp different of 12⁰C. Then work out how much extra insulation thickness is needed so that an UFH system matches those losses.

[pdf27]

3 minutes ago, SteamyTea said:

No, but probably never less than 20⁰C different, except where the flow goes into the room.

So you could say that area has a higher loss, maybe a minimum of 25⁰C difference.

 

Another way to look at is to calculate the floor losses with conventional heating, which may have a mean temp different of 12⁰C. Then work out how much extra insulation thickness is needed so that an UFH system matches those losses.

I think that's a little pessimistic.

"Depth" is a pretty good proxy for distance from outside air here, so the rims of an unheated room will get that cold but the centre will be significantly warmer. Add in the impact of the losses and the fact that soil is a pretty poor thermal conductor with a lot of heat capacity, and I wouldn't be surprised to see 15°C at the centre of a room where the floor is at 30°C and the insulation is at or about building regs minimum.

The floor losses approach is a good one though, and easy to do: essentially you're modelling the amount of insulation that is required to have a temperature difference of (UFH Flow Temperature) - (Room Temperature) across it for the design heat loss of the room.

1. For an example, assume 80 W/m2 of heat required with the mean water temperature at 35°C.
2. "Radiator" floor U-value is the building regulations minimum (0.22 W/m².K) : assume the ground is at 5°C so that's 3.3 W/m² lost we want to maintain with underfloor heating.
3. Lambda value for PIR is 0.02 W/m.K.
4. 3.3 W/m2 = (Lambda/thickness) x (35-20)
5. 3.3 = 0.02 x (1/Thickness) x 15
6. Therefore you need an extra 90mm of PIR to keep heat losses the same comparing radiators to underfloor heating in this particular scenario. That's probably a significant overestimate - you gain from the insulation even when the heating isn't at full power - but should give some idea of the impact of running warm floors.
7. If you keep the minimum building regulations value and accept higher losses, you're effectively doubling your losses to ground (+3.3 W/m²).

[SteamyTea]

40 minutes ago, SteamyTea said:

So you could say that area has a higher loss, maybe a minimum of 25⁰C difference

That was not really reflecting what I was trying to say (was in a queue and only had half a brain on it).

 

Thinking about it, and taking @pdf27's calculations into account, it may be best to have the 'hottest' part of the UFH pipework as close to the centre of the building as possible, not around the exterior walls.

wall.

Not sure how much it would save, but when you start to get to extremely low energy numbers, a small saving becomes more important.

[A_L]

3 hours ago, ProDave said:

Take a moment to work out what that actually means.

It was more a matter of what is practical/possible/necessary for the OP.