Heating system for an ICF house with UFH

[Nelliekins]

Ok, so we have already made a number of mistakes on this build of ours...

 

1. Not enough insulation for the floors (because it's an uninsulated slab)

 

2. Open vented gas boiler already purchased and hung on the wall

 

 We have a largish house (just under 300m2 over 3 floors) with UFH everywhere @ 150mm centres. Concrete floors in basement and ground floor, and chipboard upstairs (on top of 50mm EPS70 into which we have cut channels for the UFH pipes). The UFH is 16mm Pex-al-pex with a manifold per floor, connected in parallel using 22mm plastic pipe. Total pipe length is approx 2.2km.

 

Originally, I planned to have a TS provide both DHW and CH via the UFH. However after reading dozens of posts on here, I figure I can probably do better for less money (which is kind of important now that our self build mortgage is the size of GDP for a small third world country).

 

My plan is to use the gas boiler and a 3kw immersion heater (powered by our 5kw PV system using an iBoost diverter). They would heat a 140l indirect cylinder (that i happen to have) directly to e.g. 75C. I would use a 100kw PHE to extract the heat from the cylinder (also directly, not via the coil) and provide mains pressure DHW via a manifold (to provide centralised isolation).

 

So the first question is this: is my 140 litre cylinder big enough for our DHW requirements?

 

Realistically, we need either 2 showers or 1 bath and 1 shower at a time. If it takes 10 minutes to fill a bath, that's fine by me. 

 

For the UFH side of things, I have no clue. The plan is to keep the house at a constant 20C all the time. I thought of using the coil in my cylinder to provide the heat, but I doubt it'd keep up with the heat load...

 

From memory, our space heating requirements are roughly 3.6kw and our ventilation losses are 2kw. The latter will be further mitigated by the use of our Renovent Excellent 450 Plus MVHR unit with its own 1kw heater built in, of course. 

 

Help!!! ? 

 

How do I work out if the coil can produce enough heat for the UFH? Or if the cylinder needs to be larger to support the additional load of the UFH? If needed, I can always add another cylinder, can't I?

 

@Nickfromwales you said "A problem shared"... ?

 

Neil. 

[Nelliekins]

Oh, and a feature request for the eventual solution... we would like to be able to bypass the heated buffer tank and circulate cold water through the UFH to achieve even a tiny bit of cooling in the summer... the 6m wide bifolds are unshaded at present and face South!

 

I presume this would simply require a bypass valve between the heat source and the buffer tank?

 

Here's a sketch of what I was thinking of...

DHW Design - draft 1.pdf

[le-cerveau]

Don't know about the coil however as as you cylinder is primary water, you can just take the UFH direct from the tank.  There is no need for a buffer as that is to reduce short cycling is a boiler/ASHP but you already effectively have one, your tank is effectively a TS as it is primary water not potable.

 

How are you going to cool the UFH, where will the cold water come from?  With your buffer, you would just raise everything to the same temperature and delay the temperature raise but not cool it, you need to inject 'cold water' at some point.  This is usually via an ASHP, unless you intend to just run mains cold water (water bill) through it?

[PeterW]

Do you have the boiler spec and the total heat loss requirements for the house..??

 

140l isn’t going to cut it - sorry....  I’d be looking at 250 litre UVC at this point or potentially a pair of Sunamps however that is an expensive capital outlay and I’m not sure about your budget. 

 

Cooling with mains cold wont work - you’ll lose the inhibitor out of the system. A small cheap ASHP would help but it’s not going to help your SAP / DER score so I would suggest it’s left as an after build additional element.  I would put a small buffer in the system to hydraulically separate the UFH and the cooling / heating sources. 

[Nelliekins]

1 hour ago, le-cerveau said:

How are you going to cool the UFH, where will the cold water come from?  With your buffer, you would just raise everything to the same temperature and delay the temperature raise but not cool it, you need to inject 'cold water' at some point.  This is usually via an ASHP, unless you intend to just run mains cold water (water bill) through it?

 

I figured that if we used a 2nd tank as a buffer, and bypass the coil (which is the heat source) from the 1st tank, then that would at least distribute the solar gain heat throughout the entire UFH system. The system is closed, so there wouldn't be "new" water being introduced. 

 

I don't suppose there is an equivalent to an immersion heater that cools the cylinder is there? 

 

24 minutes ago, PeterW said:

Do you have the boiler spec and the total heat loss requirements for the house..??

 

Boiler is a WB 30i regular boiler. The heat loss from fabric is around 3.6kW and 2kW from ventilation losses (assuming 1ACH - we are hoping to improve on this though) 

 

24 minutes ago, PeterW said:

140l isn’t going to cut it - sorry....  I’d be looking at 250 litre UVC at this point or potentially a pair of Sunamps however that is an expensive capital outlay and I’m not sure about your budget. 

 

Cooling with mains cold wont work - you’ll lose the inhibitor out of the system. A small cheap ASHP would help but it’s not going to help your SAP / DER score so I would suggest it’s left as an after build additional element.  I would put a small buffer in the system to hydraulically separate the UFH and the cooling / heating sources. 

 

Yeah, the budget wouldn't cope with the SAs I suspect. 

 

I wasn't thinking of cooling with mains. I was thinking of a second tank as a UFH buffer, and bypassing the heat source - which would as you suggest hydraulically separate the UFH from the DHW. At worst, we can use the UFH buffer to distribute solar gain a bit, and at best we could actively cool the UFH buffer (must be possible somehow?)

 

Neil

[PeterW]

The only way to actively cool is to lose the heat out of the building - easier said than done if you are keeping the heat inside the fabric and you’re also talking about wanting cooling when ambient air is +23/24c which is near impossible with a water source. 

 

With a 2kw ventilation heat loss, have you got MVHR in there ..?? If not, plan now as that could sort your 10% SAP saving in very quick order. 

 

If  you are tight on capex then look at a big UVC and a buffer with a a coil in it - use a standard indirect hot water tank if needed and then hook a cheap ASHP to it which will allow you to cool the slab in summer but also if you add an immersion into that buffer it will allow you to use as a secondary dump for excess PV.  

[newhome]

I’m not technical at all so just my two penneth. If you have a tight budget I would forget the cooling and put in the best heating solution you can afford. I don’t have cooling in my house. It was too hot for a handful of days this summer (23 degrees in some rooms). I simply used a fan and open windows to address. 

[Nelliekins]

35 minutes ago, PeterW said:

The only way to actively cool is to lose the heat out of the building - easier said than done if you are keeping the heat inside the fabric and you’re also talking about wanting cooling when ambient air is +23/24c which is near impossible with a water source. 

 

With a 2kw ventilation heat loss, have you got MVHR in there ..?? If not, plan now as that could sort your 10% SAP saving in very quick order. 

 

Hi Peter. As mentioned in the OP we have a fairly large Brink Re450+ MVHR unit, with a 1kW postheater built in. The 2kW figure is based on 1ACH, which we are hoping to improve on quite a bit. 

 

Quote

If  you are tight on capex then look at a big UVC and a buffer with a a coil in it - use a standard indirect hot water tank if needed and then hook a cheap ASHP to it which will allow you to cool the slab in summer but also if you add an immersion into that buffer it will allow you to use as a secondary dump for excess PV.  

 

Any recommendations on ASHP? I understand that monoblocs can be used for cooling as well as heating, so that might work very well for the UFH. I presume we would never get close to the dew point on the floor? 

 

I am not a fan of UVCs (don't know why either - just a personal thing), which is why I was heading down this path of DIY heat Bank in the first place... What's the actual heat energy requirement for the DHW system? I had thought that a 140l cylinder, heated by a 30kW gas boiler (and potentially a 3kW immersion as well) would replenish heat as fast as it was cooled by the loop through the PHE... Am I wrong to think that? Sorry - I don't know enough of the math to work it out yet! 

 

[ProDave]

Who produced your heat loss figures?  What temperature is the total heal loss figure at?  and where does 2KW heat loss via the mvhr come from? I would expect that to be a lot lower. 

 

You need accurate dependable figures to start with.

 

The 140L thermal store is WAY too small.  Don't forget that with a thermal store the water in it starts to cool down as soon as you start drawing water, so typically you need a larger thermal store than you would if using an UVC instead.  Another argument for a larger cylinder is the small one would not have much capacity for storing excess solar PV, and to be viable you need to be aiming for near 100% self usage.

[Nelliekins]

14 minutes ago, ProDave said:

Who produced your heat loss figures?  What temperature is the total heal loss figure at?  and where does 2KW heat loss via the mvhr come from? I would expect that to be a lot lower. 

 

You need accurate dependable figures to start with.

 

The 140L thermal store is WAY too small.  Don't forget that with a thermal store the water in it starts to cool down as soon as you start drawing water, so typically you need a larger thermal store than you would if using an UVC instead.  Another argument for a larger cylinder is the small one would not have much capacity for storing excess solar PV, and to be viable you need to be aiming for near 100% self usage.

 

The heat loss figures were actually offhand remarks from our EPC guy. He did a "back of the fag packet" job with the U values and areas for each element (roof, floor, walls, glazing) for the 3.6kW value and IIRC it was for 20C inside with -5C outside. It actually seemed a really simple calculation, so I might try to replicate it... 

 

The 2kW value was the ventilation loss based on volume of air in the house and 1ACH. That figure will hopefully decrease, because we are aiming for 0.5ACH or less, and the MVHR will further reduce the heat lost. Plus it has 1kW of post heating built in. 

 

I was assuming that we would store/use excess PV in 4 separate places:

 

- in the DHW cylinder using an immersion

- in the UFH cylinder using an immersion (summer only) 

- in the MVHR unit (for the postheater) 

- in a solar battery (to be added later) 

 

So how do I size the DHW and UFH cylinders? If the 140l isn't adequate for the DHW, would it be adequate for the UFH buffer? 

 

Would a Sunamp unit be a sensible solution for the DHW, with the 140l cylinder for the UFH buffer? Or should I stick to getting another (larger) cylinder? 

Edited December 28, 2018 by Nelliekins
Added comment re sunamp

[PeterW]

As @ProDave says - thermal store needs to be bigger rather than smaller as it’s acting as not just your DHW buffer but your heating buffer too in this instance. I would bite the bullet and add in a much bigger TS, lose the PHE and pump as that’s just things to go wrong, and use an in tank coil. 

 

I would add a pair of immersions - one 1/3 up and one 2/3 up and use them to offset the PV or even use on E7 as backup. 

 

Other option is to add a second “solar coil” at the bottom of the TS and link to the ASHP and the UFH. - would give low grade heat to the tank from PV driven ASHP and you could divert this to the UFH for heating, and then isolate it for cooling.  

[ProDave]

Search out the heat loss spreadsheet that @JSHarris has produced, download it and enter all your dimensions, U values etc and you will get a proper idea of heat loss.

 

And the ventilation loss is a red herring, that would be the heat loss if you just ventilated with simple extract fans, but you are using mvhr so probably 90% of that "heat loss" will be recovered.

[Jeremy Harris]

Here's a link to the heat loss calculator spreadsheet, just download it, rename it to change the .txt suffix to .xls (the forum software has a problem with .xls files) and open it in Excel, LibreOffice Calc or something similar: Heat loss calculator - Master.txt

 

If you also want to look at your UFH efficiency etc, then this spreadsheet might be useful, too: Floor heat loss and UFH calculator.txt

[Nickfromwales]

Ok.

This is getting beyond complicated. Lets cut through the thicket and get to basics.

 

First off. Have you actually ascertained that you need active cooling or is it a comfort requirement only? If so, my 2 penneth;

 

Keep your gas boiler as backup and add the cheapest ASHP you can get your hands on. ( Also you referred to this as an open flue boiler, which I don't believe it is. Could you please clarify that it does indeed vent to atmosphere via a coaxial 100mm flue through the wall? ) Use the ASHP to provide background heat from PV when required, and to provide cooling when the sun is causing trouble. FYI, you do not drive cooling into a buffer tank, you drive that directly to the slab as you don't benefit from a cold tank ;).

 

Switch to gas when the sun is poor / infrequent so any PV generation goes towards your base loads, lights, plug in loads etc. Best of both worlds then, as you'll be fully utilising PV and burning cheap gas when necessary. I wouldn't suggest this tbh but as you've already gone and bought and fitted a gas boiler its now an exercise to make the most from what you have.

 

I'd recommend just adding a Sunamp, mostly for size but also for simplicity, and feed that with pre-heated cold mains water from the buffer DHW PHE. That way you can use the ASHP to provide uplift, use PV to fortify both the buffer and the SA, you can reduce the size ( and cost ) of the SA, and you can keep the existing buffer tank + PHE ( as I assume these are also already bought? ).

 

You said earlier that you don't like / want and UVC as you wanted this to be a DIY hot water install. WRONG. When you heat a cylinder ( unless its an open / vented TS ) and that cylinder has a volume of 15L or more, it requires a G3 installation and sign off + ongoing annual G3 inspection. Exactly what type of cylinder do you currently have ?

 

10 hours ago, Nelliekins said:

@Nickfromwales you said "A problem shared"...

?

 

 

[Nelliekins]

1 hour ago, Nickfromwales said:

Ok.

This is getting beyond complicated. Lets cut through the thicket and get to basics.

 

First off. Have you actually ascertained that you need active cooling or is it a comfort requirement only? If so, my 2 penneth;

 

No need just a suspicion at this time... 6m wide bifolds facing south could lead to overheating of the kitchen, but I don't know this to be the case at present... 

 

1 hour ago, Nickfromwales said:

Keep your gas boiler as backup and add the cheapest ASHP you can get your hands on. ( Also you referred to this as an open flue boiler, which I don't believe it is. Could you please clarify that it does indeed vent to atmosphere via a coaxial 100mm flue through the wall? ) Use the ASHP to provide background heat from PV when required, and to provide cooling when the sun is causing trouble. FYI, you do not drive cooling into a buffer tank, you drive that directly to the slab as you don't benefit from a cold tank ;).

 

The WB Greenstar Classic 30i boiler has a telescopic 100mm flue vented out through the side wall, already fitted. 

 

Re cooling, got it. 

 

1 hour ago, Nickfromwales said:

 

Switch to gas when the sun is poor / infrequent so any PV generation goes towards your base loads, lights, plug in loads etc. Best of both worlds then, as you'll be fully utilising PV and burning cheap gas when necessary. I wouldn't suggest this tbh but as you've already gone and bought and fitted a gas boiler its now an exercise to make the most from what you have.

 

The cost of the boiler was low enough, and the PV was never intended to provide all the heat energy when I started this project. If I had known more about the likes of TS, and Sunamp units when we started, it would have been a different matter... 

 

1 hour ago, Nickfromwales said:

I'd recommend just adding a Sunamp, mostly for size but also for simplicity, and feed that with pre-heated cold mains water from the buffer DHW PHE. That way you can use the ASHP to provide uplift, use PV to fortify both the buffer and the SA, you can reduce the size ( and cost ) of the SA, and you can keep the existing buffer tank + PHE ( as I assume these are also already bought? ).

 

We have the 30kw open vent boiler, a 140l indirect cylinder with 3kw top immersion, and a 100kw PHE. 

 

1 hour ago, Nickfromwales said:

You said earlier that you don't like / want and UVC as you wanted this to be a DIY hot water install. WRONG. When you heat a cylinder ( unless its an open / vented TS ) and that cylinder has a volume of 15L or more, it requires a G3 installation and sign off + ongoing annual G3 inspection. Exactly what type of cylinder do you currently have ?

 

We have a 140l RM indirect cylinder, which I was planning to use as a vented direct cylinder (by just ignoring the coil) with a tank above it. The cylinder would be either for primary heating (UFH buffer) or for use with the PHE to produce DHW. 

 

1 hour ago, Nickfromwales said:

?

 

Yep that's how I feel right now too! ?

[Nickfromwales]

Ok.

There is no way I'd put an open pipe, high temp ( therefore high loss ) cylinder in my new house. Just goes too far against the progression of mankind, sorry. 

All you need to do here is provide a better medium for transferring PV and gas into DHW, purely because 140L is just under half of the size of TS you'll need IMO.

 

Somethings gotta give, so I shall stick with my previous recommendations, to fortify the existing tank with an ASHP ( only because of PV and the side effect of gaining cooling which is nice to have at the end of the day ) so you can keep that at a lower temp = lower losses, transfer ASHP > DHW to provide much of your bathing at max CoP and from 'free' PV generated electricity where available, thus giving you a lot of very cheap / free background heating and DHW, and retain what you have.

 

The alternative is to bin the 140L tank cylinder and replace with a 350-400L unit which will give much more sustain at lower set temperature. It would give you the single cylinder solution you seek ( and remain DIY ) and have sufficient size to absorb all of your excess PV.

[Nickfromwales]

Oh yes, you said open vented not open flued. My bad, scan reading again

 

Classic System aka sealed and pressurised  

Classic Regular aka 'heat only' / open pipe / open vented

 

Which one do you have?

[Nelliekins]

2 minutes ago, Nickfromwales said:

Oh yes, you said open vented not open flued. My bad, scan reading again

 

Classic System aka sealed and pressurised  

Classic Regular aka 'heat only' / open pipe / open vented

 

Which one do you have?

 

Classic regular. The first post had a PDF showing me original idea for the system, but at the time I had no idea on the heat load for the DHW so couldn't size the cylinder. Got 140L cylinder since I was 100% sure it'd work for UFH if it wasn't big enough for the DHW. ?

[Nelliekins]

3 hours ago, JSHarris said:

Here's a link to the heat loss calculator spreadsheet, just download it, rename it to change the .txt suffix to .xls (the forum software has a problem with .xls files) and open it in Excel, LibreOffice Calc or something similar: Heat loss calculator - Master.txt

 

If you also want to look at your UFH efficiency etc, then this spreadsheet might be useful, too: Floor heat loss and UFH calculator.txt

 

Thanks for this. Run through the heat loss spreadsheet and it says for 25C delta, I have a heat loss of 3651W, so the fag packet (3.6kW) is borne out! ?

 

Will try the floor heat loss next. 

[Nickfromwales]

17 minutes ago, Nelliekins said:

 

Classic regular. The first post had a PDF showing me original idea for the system, but at the time I had no idea on the heat load for the DHW so couldn't size the cylinder. Got 140L cylinder since I was 100% sure it'd work for UFH if it wasn't big enough for the DHW. ?

The 140L cylinder will work, but only if you maintain it at 75-80^oC which will be ridiculously high on losses with a vented setup. That kind of flies in the face of building to a low energy standard, so I think I'd look at reverting back to sealed and pressurised in one much larger cylinder and get a G3 sign off.

You can go 'all Sunamp' to reduce losses, ( expensive additional costs ), but the header tank will still remain because you bought the open vent boiler. Can you get an MVHR extract duct to the location of the F&E ( header ) tank? May as well recycle that heat as much as the MVHR will afford you to.

 

edit : the MVHR will also help manage the excess humidity local to the F&E tank, as that may well become an issue too.

[Nelliekins]

30 minutes ago, Nickfromwales said:

Ok.

There is no way I'd put an open pipe, high temp ( therefore high loss ) cylinder in my new house. Just goes too far against the progression of mankind, sorry. 

 

Hmm. Doesn't that rather depend on if you have ready made uses for the heat loss? For example, our cylinder will sit in the laundry room, which will:

 

1. Be used to dry clothes, and

2. Act as it's own heat source for the MVHR (which has an extract above the cylinder) 

 

Contrived I know, but it's the best defence I can muster!! ?

 

30 minutes ago, Nickfromwales said:

All you need to do here is provide a better medium for transferring PV and gas into DHW, purely because 140L is just under half of the size of TS you'll need IMO.

 

Somethings gotta give, so I shall stick with my previous recommendations, to fortify the existing tank with an ASHP ( only because of PV and the side effect of gaining cooling which is nice to have at the end of the day ) so you can keep that at a lower temp = lower losses, transfer ASHP > DHW to provide much of your bathing at max CoP and from 'free' PV generated electricity where available, thus giving you a lot of very cheap / free background heating and DHW, and retain what you have.

 

The alternative is to bin the 140L tank cylinder and replace with a 350-400L unit which will give much more sustain at lower set temperature. It would give you the single cylinder solution you seek ( and remain DIY ) and have sufficient size to absorb all of your excess PV.

 

[Nelliekins]

23 minutes ago, Nickfromwales said:

The 140L cylinder will work, but only if you maintain it at 75-80^oC which will be ridiculously high on losses with a vented setup.

 

So are the losses from the cylinder (which we are fine with) or the header tank (which only helps us if the tank is also inside the MVHR area as opposed to just the thermal envelope)? 

 

23 minutes ago, Nickfromwales said:

 

That kind of flies in the face of building to a low energy standard, so I think I'd look at reverting back to sealed and pressurised in one much larger cylinder and get a G3 sign off.

You can go 'all Sunamp' to reduce losses, ( expensive additional costs ),

 

Looking online the Sunamp units start from about £2k, is that right? 

 

23 minutes ago, Nickfromwales said:

 

but the header tank will still remain because you bought the open vent boiler. Can you get an MVHR extract duct to the location of the F&E ( header ) tank? May as well recycle that heat as much as the MVHR will afford you to.

 

Yes, it's about 3m from the extract manifold, which has 7 spare ports IIRC.

 

23 minutes ago, Nickfromwales said:

edit : the MVHR will also help manage the excess humidity local to the F&E tank, as that may well become an issue too.

 

That is genius, love it! 

[Nelliekins]

4 hours ago, JSHarris said:

If you also want to look at your UFH efficiency etc, then this spreadsheet might be useful, too: Floor heat loss and UFH calculator.txt

 

OK, seeing 21.8W/m2 required power for UFH, and 22.3C floor surface temp. Of course that's the worst case, because we have UFH on upstairs floors as well as above slab... 

 

We have around 280m2 of heated floor area, which as a whole needs to generate (3651-400-750)=2501W (deducting heat of 4 people and 3x250W towel rails) for equilibrium (in the worst case). Does that make sense? 

 

So if we can generate a (moderate) excess above that amount, we have a system that'll work (we need the excess to heat it up from cold). 

 

So will the cylinder deliver that much heat (I guess that's a lot less than the DHW demand because its spread over a longer period) ? Is there a way to calculate the losses from the vented side of things? 

 

If the cylinder is a non starter, I will look at a Sunamp solution because I prefer that to a UVC... If I have to spend big bucks it may as well be on something that is largely maintenance free... 

[Jeremy Harris]

29 minutes ago, Nelliekins said:

2. Act as it's own heat source for the MVHR (which has an extract above the cylinder) 

 

 

Why would you want a heat source for the MVHR?  The fresh air that the MVHR feeds into the house will always be colder than the air in the house, no matter what you do in terms of adding a bit of local heat by one extract terminal. 

 

For example, right now my MVHR control panel is telling me that the extract air is at 23.1 deg C and the fresh supply air to the rooms is 19.2 deg C.  The room temperature is currently 21.9 deg C and the outside air temperature is 8.6 deg C.

[Jeremy Harris]

5 minutes ago, Nelliekins said:

 

OK, seeing 21.8W/m2 required power for UFH, and 22.3C floor surface temp. Of course that's the worst case, because we have UFH on upstairs floors as well as above slab... 

 

We have around 280m2 of heated floor area, which as a whole needs to generate (3651-400-750)=2501W (deducting heat of 4 people and 3x250W towel rails) for equilibrium (in the worst case). Does that make sense? 

 

So if we can generate a (moderate) excess above that amount, we have a system that'll work (we need the excess to heat it up from cold). 

 

So will the cylinder deliver that much heat (I guess that's a lot less than the DHW demand because its spread over a longer period) ? Is there a way to calculate the losses from the vented side of things? 

 

If the cylinder is a non starter, I will look at a Sunamp solution because I prefer that to a UVC... If I have to spend big bucks it may as well be on something that is largely maintenance free... 

 

Are you wishing to use the 140 litre cylinder as a thermal store solely to supply the heating?

 

If so, then if charged to, say 65 deg C, with a UFH flow temperature of, say, 28 deg C, the cylinder would store around 6 kWh.  This would run the UFH at 2.5 kW for around 2 hours 24 minutes before it would need to be recharged.