Yet another Heating + DHW design!

[Visti]

Due to some of the astounding quotes for UFH and DHW systems, I have decided to have a stab at designing and specing our own system! This is the first draft, possible due to this forum and the Building Services Handbook recommended by @curlewhouse - an awesome book, I've left in the page #'s from where I've done capacity calcs. Even so, a sanity check is very welcome!

 

It's currently a bog-standard unvented system + system boiler that is scaled for the DHW only. You'll notice I'm one of the converted to @Nickfromwales's Hep 20 Manifold setup, though I'm not necessarily sure that the 2nd hot water at 48'C manifold is necessary for us. Principle question here is whether it's safe to pass the output of the UFH back to the main cylinder, or will that have to be discharged?

 

We're unsure as to how we're going to supply heating however, whether we scale up the boiler / cylinder capacity, or whether we go via the ASHP or Solar Panels/Thermal Store. We're looking at ~2100 Watts for the UFH based on our heat losses during the winter here in Oxford. There'll be no other heating in the house save for some electrical UFH matts in the upstairs bathroom.

 

• ASHP with a good CoE running overnight to take advantage of the E7 scheme sounds grand, though we're concerned with the noise and another hole in our external wall for this. Could this be plummed directly into the UFH, or would a separate cylinder be needed to store the output?

 

• Solar Panels sound grand, but we're concerned the timing of the heating will be at odds with when we'll need to heat the floor. Also, the initial outlay appears quite steep so we'd likely need to install it retrospectively when we've the money.

 

• Scale up the Boiler to ~18kW to supply the UFH along with everything else

 

 

FYI: I used MS Visio 2016 which has several Pipes & Valves stencil sets out of the box. Very useful!

 

 

Heat Losses calculated thanks to @JSHarris's calculator.

Edited January 22, 2018 by Visti
Units!

[nod]

I thought I’d do the same so I contacted warm up for supply only

Who were happy to do a design free of charge While I was going through this process it was sugested that I get a quote from Cadman for the screed and also ask about a UFH quote

I have the actual prices if required 

But there wasn’t a lot in it

we put all the insulation down ourselfs Then one fitter arrived at 730 am and had 150 mtr floor done by 2pm 

[PeterW]

@Visti UFH doesn’t work like that - it needs a 3 way valve and would be connected to the boiler if that is an unvented cylinder. 

 

You also don’t need an anti vacuum valve on an unvented cylinder - only used on gravity cylinders with high pressure pumps. 

[Nickfromwales]

@Visti

Rough as toast sketch. 

 

You can omit the buffer but its its better to have it in to help regulate the delivery from the HP and mitigate against short cycling when the house is up to temp.  

[MikeSharp01]

Yes there is something a bit odd about your design @Visti as @PeterW says because your UFH return is going into your hot water tank so there is a chance that it could, depending upon demand / slab temperature, I suspect overwhelm the tank in temperature terms. @Nickfromwales seems to have shown the way using the ASHP to run the UFH and the UVC.

[SteamyTea]

2 hours ago, Visti said:

2100 Watts a day

Not sure what that really is.

watts (W) is power

day (24 hour) is time

 

So do you mean 2100 [W] x 24 [h] = 50.4 kWh/day

 

[le-cerveau]

You have mixed potable (drinking) with non-potable (not drinking) water!

The way you have it the boiler heats the UVC via a coil (Primary water non-potable), what is in the UVC is potable and supplies you domestic outlets.  The UFH system is non-potable so cannot be run directly off the UVC you need a buffer tank as @Nickfromwales diagram shows.

Alternatively if you want to run one tank then it would need to be a Thermal STore (TS) type where you heat the TS directly from the water (TS contains primary water), you take a tapping for the UFH from the TS (usually middle) and use Coils/Heat Exchanger for Domestic Hot Water (DHW).  These have their drawbacks (heat loss).

 

As you have a boiler the simplest is to add a buffer and plumb the UFH (upsize boiler if required).

 

If you want an ASHP, then again it needs a separate buffer (though you can potentially not have one) and you should consider DHW pre-heat to take advantage of the high COP of ASHP at 40^oC, however you system will be infinitely more complex (for a normal plumber).

 

If you want to run the ASHP overnight for E7 then you will definitely need a descent buffer, to store adequate heat for the day (and DHW pre-heat if you go that route).

[willbish]

15 hours ago, Visti said:

ASHP with a good CoE running overnight to take advantage of the E7 scheme sounds grand, though we're concerned with the noise and another hole in our external wall for this

 

Insulated pipe work for a Monobloc ashp can leave the thermal envelope through the floor slab. If the alternative is a flue from a gas boiler then I reckon in terms of heat loss the ashp would be preferred.

 

With regard to noise, have you looked at a split type ASHP? Where the external condenser and internal compressor are separated. This gives you the option of placing the condenser around 30m away.

 

I think you need to decide how you are going to power your systems before getting into the detail of design.

[ragg987]

I would say you need to revisit some fundamentals as per the comments above.

 

Q1: if you have mains gas boiler why add an ASHP. Waste of money you cannot recover the extra capital. I posted some examples on this forum. I would stick with gas. However, if you add a PV system then it swings in favour of ASHP over gas.

 

Q2: Why 15 or 18kW from your system boiler? We have 6 people and manage with a 7kW ASHP and 300l DHW. Our space heating load is higher than yours at about 3 to 3.5kW.

 

Q3: Why have a water softener in-line with the UFH / heating circuit? The heating circuit is normally closed-loop so never needs fresh water after the intial fill. Just add inhibitor. Water softener is then reserved for all other uses apart from drinking.

[Nickfromwales]

@Visti

Just as a reality check, that having a flue penetration in the wall of the house is of very little consequence. You can still make it airtight and you can fill the sleeve ( duct ) with intumescent expanding foam for a bit of insulation. Yes it'll be a slight cold bridge, but the latent loss off the boiler will offset that by a huge factor. 

ASHP or gas boiler ?  GAS BOILER!

Smaller tanks, less installation complexity, no antifreeze, common and simpler controls ( so setting up and operating it is childs play ) and still a reasonable unit running cost per kWh. What boiler do you have now? Do you need to replace / upsize ?

The 'beauty' of the manifold setup only comes into play if it's centrally mounted and sees short-ish runs from the manifold to the hot outlets. ( the cold is not an issue ). If the plant room is off side and leaves you long runs to two or more outlets then it may be a candidate for a standard series plumbed configuration, and the introduction of a hot return circuit and pump.

I agree, if you have mixer taps then the secondary TMV is a luxury item that can be dropped from the design, but as they're so bloody cheap I choose to install where possible as a comfort measure as much as for safety. If there are a house full of little ones who will grow up there, it's a no brainer. Fwiw, the image where you see two TMVs is one where a TS has been fitted, so comes factory supplied with a TMV to cap the output temp ( usually 50-55oC ) as a measure for anti scald and to mitigate any excess energy wastage. A TS will typically store water ( primary heating grade water ) at a much higher temperature conmpared to an UVC so these are two different beasts with two entirely different uses and installation requirements.   An UVC is full of drinking quality potable water, stored ready to be drawn off at the tap, and a TS is full of non potable ( primary heating grade ) water which has an instantaneous DHW coil sat in the water which instantly converts incoming cold water to hot DHW, so basically a giant combi that uses heated water instead of gas as the energy medium. An UVC does not require the primary TMV, in fact it requires no TMV at all. If you decide on the hot return and series plumbing you definitely want the UVC, sized between 250L - 300L ( 300L better if you ever decide on solar pv ( NOT solar thermal )) and to order the UVC with a hot return tapping to accept the pumped DHW return. 

As I know you'll be wondering what the hot return is all about  :

 

 

Lol. Another way of doing things  

[Nickfromwales]

Crossed with @ragg987 and agree. 

Id still stick with gas and a non MCS pv install though. 

[Jude1234]

47 minutes ago, Nickfromwales said:

Crossed with @ragg987 and agree. 

Id still stick with gas and a non MCS pv install though. 

Just following this as it relates a bit to my solar PV question.  Although we have mains gas we had decided to go ASHP on our new build to lower our carbon footprint and that we should get a lot of the investment back from RHI payments.  Is this a naïve opinion? 

[Nickfromwales]

14 minutes ago, Jude1234 said:

Just following this as it relates a bit to my solar PV question.  Although we have mains gas we had decided to go ASHP on our new build to lower our carbon footprint and that we should get a lot of the investment back from RHI payments.  Is this a naïve opinion? 

Quite often the cost of an installed package which is eligible for RHI is very expensive. There appears to be a high premium applied for the luxury of not fitting this on a DIY basis, but the payments appear to muddle peoples judgement when costing this. 

For ASHP to be economically viable on PV, you ideally need to be charging a large buffer vessel so when you need to inject some top-up heat into the slab, say around midnight maybe, there is some heat in the buffer ( so the buffer is basically a heat battery ) as at midnight you won't get much sunlight  

That is a generalised statement though and based on a typical, reasonably well sealed and airtight aka not draughty dwelling. Each instance is nigh-on unique, so no one-stop solution / easy answer I'm afraid. 

What is the standard of your build ? MVHR?

[Jude1234]

2 minutes ago, Nickfromwales said:

Quite often the cost of an installed package which is eligible for RHI is very expensive. There appears to be a high premium applied for the luxury of not fitting this on a DIY basis, but the payments appear to muddle peoples judgement when costing this. 

For ASHP to be economically viable on PV, you ideally need to be charging a large buffer vessel so when you need to inject some top-up heat into the slab, say around midnight maybe, there is some heat in the buffer ( so the buffer is basically a heat battery ) as at midnight you won't get much sunlight  

That is a generalised statement though and based on a typical, reasonably well sealed and airtight aka not draughty dwelling. Each instance is nigh-on unique, so no one-stop solution / easy answer I'm afraid. 

What is the standard of your build ? MVHR?

It is not passive house but insulation greater than standard building regs.  Yes we are having MVHR

[Nickfromwales]

Just now, Jude1234 said:

It is not passive house but insulation greater than standard building regs.  Yes we are having MVHR

Well, with reduced ventilation and fabric heat loss you may well be able to use the slab as a storage heater, topping up during the night from a buffer heated by pv and fortified by the ASHP.

Sizing the system will be down to the amount of heat energy required to offset the losses of the dwelling and start from there. Your not naive as both would work well, and tbh you'll need a buffer tank with either option, regardless of the fact your having PV. If fitting the larger buffer ( as your not fully 'passive' ) id certainly look at DHW preheat from the buffer too, killing two birds with one stone then. 

[ragg987]

13 minutes ago, Jude1234 said:

 Is this a naïve opinion? 

I would say not so clear-cut and if I had the choice I would stick with gas. Here is a posting I made a while ago illustrating the options, though I did not factor in RHI payments. The cost of installation is likely to be higher for a certified install ("RHI premium").

 

 

Do note that standard ASHP is only really good for water at 50 or 55C - to raise it beyond that you either have a 2-stage ASHP or direct electrical heating. Gas offers higher temperatures so you could get away with a smaller DHW cylinder.

 

I also came across this article from a few years ago. Not sure how accurate it is, but I lean towards the same conclusion. http://www.narecde.co.uk/air-source-heat-pumps-vs-gas-boilers/

 

A lot will depend on your overall system efficiency. Our ASHP system is giving a whole system coefficient of >4 in a mixed DHW + space heating scenario, even in Dec and so far also Jan. This is for whole system energy so includes pumps, valves etc. Not the same as a COP. So factor in PV and I think this is a great combination financially. But mains gas without PV would save you £7k (ish) in capital costs and seems a more compelling case.

[Nickfromwales]

It can give you quite a headache, this plumbing lark......

31 minutes ago, ragg987 said:

Our ASHP system is giving a whole system coefficient of >4 in a mixed DHW + space heating scenario, even in Dec and so far also Jan. This is for whole system energy so includes pumps, valves etc. Not the same as a COP.

Excellent real life figures there. 

Theres also still the Heat battery ( like a Sunamp ) to consider. No boiler, no ASHP, and almost zero maintenance / moving parts etc but also with near zero latent losses too. 

Im currently specifying one with 18kw/hr of Sunamp heat battery, and have steered away from the originally assumed ASHP altogether. PV will feature but with enough capacity to be recharged every evening on E7 /E10, its a VERY slick installation with masses of DHW available that could easily be used without PV. Grid electricity will fortify any short fall / provide boost for higher occupancy ( guests ). I'm awaiting @AndyT to update on things 'up and coming' before going into more depth on that setup. 

Edited January 22, 2018 by Nickfromwales

[Visti]

Awesome feedback, thanks all! Thank goodness it's early days and I've got time to spend on this learning curve... I have decided with the good wisdom of you all not to poison the family with the UFH using wholesome water! The SO will be proud!

 

On 1/21/2018 at 20:01, SteamyTea said:

So do you mean 2100 [W] x 24 [h] = 50.4 kWh/day

Thanks for pointing that out, I've fixed the original post

It's just Watts, so that'd 50.4 kWh/day, or ~1512 kWh/month. 

That's worst case however. On average it'd be about a 3rd lower, at about 32 kWh/day, or 950 kWh/month.

 

11 hours ago, willbish said:

I think you need to decide how you are going to power your systems before getting into the detail of design.

Agreed, I got ahead of myself. It was useful to think about the pluming but it is clear I need to settle on the principle systems

 

8 hours ago, Nickfromwales said:

What boiler do you have now? Do you need to replace / upsize ?

None as of yet. We've got nothing but the land so far, foundation is going in march, so it's a blank slate thankfully. No limitations at present!

 

8 hours ago, Nickfromwales said:

The 'beauty' of the manifold setup only comes into play if it's centrally mounted and sees short-ish runs from the manifold to the hot outlets. ( the cold is not an issue ). If the plant room is off side and leaves you a long runs to two or more outlets then it may be a candidate for a standard series plumbed configuration and the introduction of a hot return circuit and pump.

 

Good point. The system wont be centrally mounted (utility room in red), however the majority of the runs will be under 4m in the areas I've highlighted green. Only exception will be the Kitchen sink which will be about a 10m run. LHS Ground, RHS First Floor:

 

 

Great diagram @Nickfromwales, I believe I get what a hot return is now. Given that I'm still getting my head around much of the terminology here, I'll summarize for myself: a hot return is basically a loop with a pump that constantly recycles the hot water feed. This eliminates the water from getting cold prior to the outlets, thus reducing the amount of time waiting for the water to get warm when you turn on the tap?

 

Ultimately it looks like the gas UVC is here to stay, ASHP is out, and PV is a maybe (the capital costs may push it back to some years after the build is done!). The Sunamp heat battery comes up a lot on the forum, so I'll have to do a bit more reading into it - I take it they have to be combined with PV to enable you to forfeit the boiler?

 

I know Andy from Sunamp will be at the NSBRC convention this weekend, so I'll update my design following everyone's input and shall see what he thinks.

 

Edited January 22, 2018 by Visti
spelling

[Nickfromwales]

By Jove........I think he's got it !

 

[ragg987]

Any reason you cannot swap the utility room with the office? Makes all runs (water, gas, electricity, ventilation ducts) that bit shorter.