MVHR & combined ASHP for 240m2 new build

[Dreadnaught]

15 minutes ago, PeterW said:

Really surprised at that price tbh as if you have a passive slab with rebar in it, the pipe will be £250 at most. 

  

Have you asked for a breakdown of the price ..?

 

Not yet Peter, no. I have spent the best part of a year preparing for my build and I still haven't even broken ground and I have already encountered other examples of suppliers inflating prices when they consider that the client does not know better.

[TerryE]

39 minutes ago, PeterStarck said:

It's also worth considering whether UFH is the best form of 'wet' heating. There's a lot more to UFH than just the pipework and it is less efficient than radiators or skirting heating. It does have it's advantages but I wouldn't say it was a no brainer.

 

Like we've got no radiators or the like an any wall in our house so no buying and fitting rads or having them clutter up walls.  The major advantage -- if you need it -- is the responsiveness of having 2-3 kW wall mounted heaters in every room, but why bother if your house temperature is always within ½°C of your target?

 

16 hours ago, Dreadnaught said:

MBC have quoted me £1,952 for UFH piping

 

Yes but that is the single most expensive bit of the CH system.  Try costing out the supply and fit of rads everywhere.  Another poster was talking about it costing him £17K for his DHW and CH.

[jack]

29 minutes ago, Dreadnaught said:

... so it is also what the person-in-the-street will expect to see and any future buyer will understand it. With the level of insulation I am planning, there won't be many radiators and, as you say, they are more efficient than UFH.

 

When I get to this stage, I will do the costings and I suspect that radiators will win out.

 

I think the person-in-the-street increasingly expects to see UFH in a new build. Everyone I know who's done a significant extension, refurb, or new build has included UFH. 

 

Re: costings, I'll be interested to see how you get on. I'd happily pay more (a lot more, actually) for UFH. The heat it gives is far more even and pleasant than any radiator-heated house I've been in. We did actually heat our house for a period last year with a single 2kW electric column heater in the kitchen. Amazingly, it was fine even when it was very cold, but the temperature wasn't evenly spread through the house by any means. With a low energy house, how do you distribute heat if your heating requirements mean you can only have a very small number of radiators? 

If you have a lot of solar gain in any particular area, don't underestimate the advantage of UFH pipework moving heat around the slab. We have a huge south-facing slider that gets quite a bit of solar gain in the shoulder months. Without running the UFH pump, I suspect that the floor beside the slider - several square metres of our dining area - would become uncomfortably hot.

 

There's nothing stopping you from getting someone else in to do the UFH heating pipework, or doing it yourself.

[Gone West]

1 minute ago, jack said:

With a low energy house, how do you distribute heat if your heating requirements mean you can only have a very small number of radiators? 

If I needed central heating in a low energy house I would consider skirting heating not conventional radiators.

[Jeremy Harris]

Me too.  Some friends have skirting heating and it works very well indeed, and you wouldn't know there was any heating system fitted unless you were told about it. 

[jack]

3 minutes ago, PeterStarck said:

If I needed central heating in a low energy house I would consider skirting heating not conventional radiators.

 

I seem to recall that skirting heating is relatively expensive compared to regular rads. If price is the main driver of choosing radiators over UFH, isn't that advantage eroded by using skirting rads?

[Gone West]

4 minutes ago, jack said:

 

I seem to recall that skirting heating is relatively expensive compared to regular rads. If price is the main driver of choosing radiators over UFH, isn't that advantage eroded by using skirting rads?

I was comparing skirting heating to radiators in a low energy house.

[Dreadnaught]

Interesting comments. Very helpful. I think I will price up UFH, radiators, and skirting heating and see where it ends up. I won't base the decision solely on price unless the price difference is large.

 

On DIY for laying UFH pipe, I have watched the pipe go in on a couple of builds and on video. With preparation and care (and the right equipment, such as a coil unwinder), it looks highly DIY-able.  Food for thought.

[Jeremy Harris]

DIY pipe laying takes a fair bit of time, and that means arranging for a break in the middle of the slab laying process.  At a guess I'd say you need to allow a couple of days to mark out, lay and tie in the UFH pipes.  This would be a fair bit quicker if there were two of you doing it.  The guys that laid ours had a well-worked out system where they spent an hour or so accurately spray marking the locations of walls, doorways, the stairs, kitchen and utility room units, the WC etc, on to the EPS, just after the steel fabric and the ring beam steel was  laid and tied in (a bit over a days work for our smaller slab). 

 

The UFH pipes were laid and tied in the afternoon before the concrete pour.  When laying the pipes, one chap held the coil of pipe and paid it out as the other tied the pipe to the steel.  Seemed to be an efficient way of working.  Here's a photo showing how they did it:

 

 

 

[JanetE]

Having lived in our MBC house with underfloor heating for just over a year I personally would never choose to go back to radiators. Nothing compares to the comfort and utility of underfloor heating and I am sure you will regret it if you decide to go down the conventional route.

It was a no brainer as far as we concerned to get them to lay the pipes before the pour and if I recall correctly it was more than a half days job for the three guys as the pipes have to be cable tied to the rebar.

I am sure it is also the cheapest and most convenient way to heat your house. Worth every penny.

[lizzie]

+1 for UFH so nice to have 

[UncleQ]

On 10/03/2019 at 10:51, JSHarris said:

 

If you haven't yet modelled it with PHPP, but want to do a "quick and dirty" estimate of heat loss, then I wrote a spreadsheet a few years ago that simplifies things a lot.  It's nowhere near as refined as PHPP, but is a lot simpler, and people here who have used it have suggested that it gets within around +/- 10% of the real heat loss, which is usually good enough for sizing a heating system:  Heat loss calculator - Master.xls

 

 

 

I used your calculator spreadsheet with met office data for my area and came up with a maximum heat requirement of 2,765W in February. I'll ask my architect to remodel his calculations. Thank you for your tool @JSHarris!

[UncleQ]

How do you size an MVHR flow rate? Is it as simple as the total air volume of the house divided by the ACH figure you need to achieve? e.g. in my case about 350m³ / 0.43 ACH = a flow rate of 150m³ per hour is required?

[Jeremy Harris]

6 minutes ago, UncleQ said:

How do you size an MVHR flow rate? Is it as simple as the total air volume of the house divided by the ACH figure you need to achieve? e.g. in my case about 350m³ / 0.43 ACH = a flow rate of 150m³ per hour is required?

 

 

The key bit is that it has to be able to comply with the ventilation rates in the building regs, even though most people find these a bit high.  It also needs a bit of additional capacity to be able to boost ventilate if required.

 

You can download both Part F and the compliance guide here: https://www.gov.uk/government/publications/ventilation-approved-document-f .  The bit you need is the section covering continuous mechanical ventilation, and the applicable rates for this in Table 5.2 and in the notes underneath that refer to the whole house minimum ventilation rate.

[Sue B]

We laid all the UFH pipes ourselves in the last house.  We bought a designed system (too scared of getting the wrong things to buy the components separately ) from nu-heat so paid a premium for their design and supply but it was all really straightforward.  Yes, the pipe was unwieldy and because it was a designed system, the pipe was cut to length which did make it more manageable.  

However, even now, 10 years older, I have no qualms about doing this ourselves again, buying all the bits we need separately, other than finding a cheap way to hold the pipe reel to make life easier.  I intend to talk to Insulhub (Isotex supplier) about doing the passive slab largely by ourselves, buying in some labour from them to check what we have done before the pour.

[Nickfromwales]

36 minutes ago, Sue B said:

other than finding a cheap way to hold the pipe reel to make life easier

Sue. We have a pipe decoiler in our Buildhub tool loan stock. Let me know when you are ready and you can reserve it. £10 donation to the BH coffers is the price, plus P&P to send onto the next 'borrower' or to return it to a staff member to hold.

 

 

[UncleQ]

1 hour ago, JSHarris said:

 

 

The key bit is that it has to be able to comply with the ventilation rates in the building regs, even though most people find these a bit high.  It also needs a bit of additional capacity to be able to boost ventilate if required.

 

You can download both Part F and the compliance guide here: https://www.gov.uk/government/publications/ventilation-approved-document-f .  The bit you need is the section covering continuous mechanical ventilation, and the applicable rates for this in Table 5.2 and in the notes underneath that refer to the whole house minimum ventilation rate.

 

Much appreciated! I also double checked my figures and spotted a mistake I made. Our total house air volume is actually 650m³ which ups the heating load required to 3.1Kw.

[Jeremy Harris]

5 minutes ago, UncleQ said:

 

Much appreciated! I also double checked my figures and spotted a mistake I made. Our total house air volume is actually 650m³ which ups the heating load required to 3.1Kw.

 

 

Still a bit less than the 7 kW figure that you had originally, though.  You can subtract a bit from that 3.1 kW, as the spreadsheet takes no account of incidental heat gain.  Each occupant will contribute around 80 W or so, there will be a couple of hundred watts of gain from all the electrical stuff in the house and there will be some heat that comes from the losses from the hot water system.  However, sizing the heating based on the worst case heat loss from that spreadsheet should give you a bit of headroom.  If you opt for an ASHP, then the smallest models available tend to be around 4 to 5 kW anyway.

[UncleQ]

7 minutes ago, JSHarris said:

 

If you opt for an ASHP, then the smallest models available tend to be around 4 to 5 kW anyway.

 

 

We'd also look to power the DHW cylinder or Sunamp from the same external ASHP, so I guess something around the 8kW would be more than sufficient with a bit of headroom.

 

On the MVHR front, I guess that it's preferable to slightly oversize the air volume it can shift to keep the noise to a minimum?

[Jeremy Harris]

An ASHP is going to really struggle to provide water at a high enough temperature to charge a Sunamp for hot water, as the charging temperature has to be around 65°C, which is hotter than pretty much any ASHP will be able to run at, and even if you did get one to run as high as this it would struggle a fair bit.  Around 50°C is a reasonable maximum for hot water from an ASHP, perhaps as high as 55°C at a pinch.  This is OK if fed to a well-insulated UVC though, although the UVC will need to be fitted with a high efficiency coil to allow reasonably quick water heating.

 

I wouldn't oversize the ASHP too much, as you will probably find that it won't modulate down enough to meet your heating requirement, and that will lead to it short cycling.   A small amount of over-sizing is OK, but look carefully at the lowest modulation level that the ASHP will run down to.  We have a 6/7 kW ASHP and that will only modulate down to about 2 to 2.5 kW, which isn't low enough for our heating requirement, so I had to have a buffer tank, to allow the ASHP to run for a reasonable length of time at it's lowest modulation level.  This isn't ideal, as the tank takes up space, but the ASHP was one of the smallest I could find at the time (a 3 to 4 kW one would have been a much better bet).

 

Because (with luck) your passive house should have a long thermal time constant (good insulation, high decrement delay, good airtightness and efficient MVHR) then having the heating off for three or four hours whilst the ASHP charges the UVC won't have any noticeable impact, so the ASHP doesn't need to be sized to deliver heating and hot water simultaneously (none will do this anyway, AFAIK, they all turn off the heating when they switch to hot water mode), and I suspect the size may be a compromise between the mean heating requirement and the desired UVC recharge time.  Having a larger UVC increases the recharge time but also gives a larger DHW buffer, and you can often just charge the UVC with hot water overnight, when there probably isn't any need for heating.

[ProDave]

I know different ASHP's work in different ways, but ours is configured to heat the DHW in half hour bursts, with a half hour gap before the next DHW burst *.  So it will stop feeding the under floor heating during those periods.  Trust me you will not notice that it has stopped feeding the UFH for that time.

 

Yes buy an UVC with the high capacity "heat pump" input coil.

 

 

* The default setting was heat DHW for half an hour, then wait 90 minutes before doing the next burst of DHW heating, but as it typically takes 3 half hour bursts to heat the tank, that would have taken 6 hours to get the tank up to temperature which I thought was way too long, so I reduced the wait time to 30 minutes.

[le-cerveau]

You can get High Temperature ASHP, Daikin do one, this is a 2 stage device, the water is heated by a conventional ASHP, this is then raised by a second indoor stage to up to 80^oC, the downside being you cannot set it up for cooling!

The original SunAmp Stack used to be specified with this.

Not sure how efficient they are at High Temp though.

[Big Neil]

Not sure if these are the only supplier.

 

https://www.discreteheat.com/thermaskirt/products-and-information/what-is-thermaskirt.aspx

 

The Guy I spoke to was nice regarding info.

 

It strikes me that if a house in insulation terms is near passive, them upstairs at the least the heating option is really only there for extreme weather situations and occasional variations. To that end the skirt options seems quite reasonable as it's discrete and the cost (albeit maybe not a lot of it) can be offset against the skirting you probably would have had anyway.

 

...but I am the forum dunce so just cover me in Jam and leave me at the back....

 

 

[Jeremy Harris]

10 minutes ago, Big Neil said:

It strikes me that if a house in insulation terms is near passive, them upstairs at the least the heating option is really only there for extreme weather situations and occasional variations. To that end the skirt options seems quite reasonable as it's discrete and the cost (albeit maybe not a lot of it) can be offset against the skirting you probably would have had anyway.

 

...but I am the forum dunce so just cover me in Jam and leave me at the back....

 

 

 

You're spot on. 

 

We have no heating upstairs, just towel rails in the bathrooms, and the bedrooms never get below about 19°C, and are usually around 20°C, which we find OK; if anything I think we might prefer them to be slightly cooler.  I did include switched FCUs in the bedrooms, so we could fit electric panel heaters if we felt we needed them, but I doubt that we'll ever bother to fit them now.

 

The only thing I'd have done differently would have been to install very low power electric heating mats under the stone flooring in the bathrooms, not to provide room heat, but just to take the chill off the stone.  It's not a major issue though, and we've just got used to it now.

[Big Neil]

5 minutes ago, JSHarris said:

The only thing I'd have done differently would have been to install very low power electric heating mats under the stone flooring in the bathrooms, not to provide room heat, but just to take the chill off the stone.  It's not a major issue though, and we've just got used to it now.

 

Would it be: Sub floor - Kerdi(or whatever other type of tile backer board) - Adhesive - Tiles?