Electric+ASHP Underfloor Heating in passivhaus bathrooms - madness? =)

[puntloos]

I have an 8.5kW EcoDan ashp planned, and of course can do underfloor heating anywhere I like. 

The interesting part is bathrooms. So many weird pro/cons:

 

- Amount of heat needed to heat a few tiles in summer is miniscule, the ASHP will surely short cycle unless it's already running for other duties

- Heating electric off grid is somewhat costly. (I suspect it'll cost me about 40/year for all bathrooms, based on a friend's usage stats)

- If I install both ashp and electric my entire first floor probably needs lifting by 10mm, non-zero cost and some minor annoyances and hey, sliiightly reduced head height on an entire floor..

- Electric responds much faster to turning it on, as long as I cleverly turn it off when not in the room, if I use eg 240W mat (the highest I could find.. or will that melt my porcelain tiles/glue etc?)

- Doing both - ASHP when it's running and electric when ashp is not running seems elegant...

 

My sense is end of the day just going electric mats is probably the most straightforward?

[Nickfromwales]

A 240W mat will probably give you overshoot tbh, I'd stick at 100-150W max. Nothing will melt as you need to cap the floor temp at 27^oC max anyways, and if you're anywhere near 27 then you've done something VERY wrong.

Electric mats don'y usually turn on/off, they usually go from one temp to another (comfort & economy / aka setback) so maybe just allow them to cool to (x)^oC instead of down to frost setting? If you have PV the maths (running costs with using setback temp vs "off") won't be too uncomfortable.

FF bathrooms yes, just electric. GF with slab go for both afaic, as electric mats are cheap enough to install, less controller if you insist, and then retro-fit the controls later if so necessary. Always separates the miserly from the luxury-feel folk  Give me a little 'luxury' every day of the week!

[puntloos]

5 minutes ago, Nickfromwales said:

A 240W mat will probably give you overshoot tbh, I'd stick at 100-150W max. Nothing will melt as you need to cap the floor temp at 27^oC max anyways, and if you're anywhere near 27 then you've done something VERY wrong.

Interesting point.. I didn't realise but it makes sense these devices are 'on/off', not necessarily cleverly regulating the ramp.. 

Still, turning off the moment you hit what, 22C might be doable if you time it right or (gasp) have a sensor in the floor..

 

 

[Nickfromwales]

19 minutes ago, puntloos said:

or (gasp) have a sensor in the floor..

That's mandatory for every job I do, as I cannot risk 3rd party injury. Self-builders can burn their own toes, but I cannot  FYI, nearly every single UTH controller will come with a floor probe, certainly all the Warmup stuff does.  It would be daft not to install this tbh, and they supply a small conduit for the probe to be replaced retrospectively.

[markocosic]

I'm grappling with similar. 

 

 

You can't run a heat pump at tiny load.

 

 

It will need to either dump a shedload into a thick slab then shut down or you'll need to use a volumiser/buffer type setup to allow (a) a high enough minimum flowrate for the unit to operate and (b) sufficient active system volume for it not to short cycle.

 

 

Burn and coast into a slab is only viable on the ground floor. First floor needs something else.

 

 

Run the math on how much heat the heat pump will lose (in the case of an outdoor mounted unit), the volumiser/buffer will lose, all the spaghetti from heat pump to/from the buffer will lose, and all the pipework from the buffer to the UFH zone will lose...vs the actual heat delivered into the floor...and I doubt you'll beat a temperature-controlled direct electric setup for a small bathroom. Especially if you then need to take that heat out of the rest of the house with active cooling!

 

 

There's some argument for using the DHW cylinder as a buffer for the heat pump. You have one anyway. You could run water through the DHW cylinder coil to pinch some heat into the UFH, and allow the heat pump to reheat the DHW in the usual way. That avoids you having a dedicated buffer. I think @Radian toyed with similar. It's less sketchy than using a DHW recirculation loop through a potable rates brass/stainless towel rail for heating bathrooms...

 

 

I think I'm going to do that AND fit an electric mat / sensor (given the stuff all cost of it now) before laying the floor.

 

 

Then I'll probably use a wooden floor finish (on top of a suspended wooden floor), in what is otherwise a warm building, and never ever use the UFH. (that's what happened in the house in Cambridge - if you do wood rather than tile you don't need the underfloor heat)

 

 

So yeah. What @Nickfromwales says. As usual. But perhaps do some head scratching about how you would suck heat out of the DHW tank for running the wet UFH (and the towel rail) in the summer. The COP isn't amazing when heating DHW to 50C but it's better than 1.

[crooksey]

fit bathroom loops on the same adjoining areas, if its a new build well insulated house it will be no issue. Hallways and corridors tend to get the most UFH demand (if you set your stat correctly). So if you run them off the same loops they are next to, you eliminate the short cycling issue.

[Nickfromwales]

10 minutes ago, crooksey said:

fit bathroom loops on the same adjoining areas, if its a new build well insulated house it will be no issue. Hallways and corridors tend to get the most UFH demand (if you set your stat correctly). So if you run them off the same loops they are next to, you eliminate the short cycling issue.

In summer this will be nuisance heat anywhere other than the bathrooms, so for "chill removal" with tiled floors outside the heating season it's electric or nowt.

46 minutes ago, markocosic said:

But perhaps do some head scratching about how you would suck heat out of the DHW tank for running the wet UFH (and the towel rail) in the summer. The COP isn't amazing when heating DHW to 50C but it's better than 1.

All components would have to be potable quality, and this would need to be like a hot return HRC arrangement? By the time you factor in slaughtering your ASHP to save a few £, it just doesn't make any sense to me at all (even when excluding the cost of the labour, equipment and downstream maintenance to facilitate this).

[Nickfromwales]

48 minutes ago, markocosic said:

Burn and coast into a slab is only viable on the ground floor. First floor needs something else.

Yup. +1.

[JohnMo]

Why would you need to heat the bathroom in the summer in a passivhaus? We're not passivhaus and have just had 2 weeks of rain and very little or no sun, outside temperature around 14 and lower, house hasn't dropped below 20. We still get people coming into commenting how warm the house is. Had an hour or so of sun this afternoon and the UFH has just kicked in.

[markocosic]

34 minutes ago, Nickfromwales said:

All components would have to be potable quality, and this would need to be like a hot return HRC arrangement? 

 

Primary side @Nickfromwales

 

 

Pump through the DHW coil then then space heating circuit. (with all other zones off)

 

[markocosic]

3 minutes ago, JohnMo said:

Why would you need to heat the bathroom in the summer in a passivhaus?

 

Bare tiles vs women's feet.

 

Bare tiles at <25C feel cold barefoot. (some heat pumps have a "enable minimum return temp" setting for summer use that keeps the floor heating circuit at 25C for this purpose)

 

Wood on the other hand feels fine. So do rugs. Rugs on wood even more so.

[JohnMo]

1 minute ago, markocosic said:

 

Bare tiles vs women's feet.

 

Bare tiles at <25C feel cold barefoot. (some heat pumps have a "enable minimum return temp" setting for summer use that keeps the floor heating circuit at 25C for this purpose)

 

Wood on the other hand feels fine. So do rugs. Rugs on wood even more so.

Not sure my floor has ever hit 25 degs!

 

Why go to the bother of building a passivhaus and then have the heating on in the summer? Just madness 

[Nickfromwales]

2 minutes ago, JohnMo said:

Not sure my floor has ever hit 25 degs!

 

Why go to the bother of building a passivhaus and then have the heating on in the summer? Just madness 

I wouldn't ever design for that, as most of my clients will be in reverse with the heat pump, providing cooling  

The heating of bathroom floors which are tiled is far from madness, as whilst showering at 38-40^oC (or more if you're a woman with asbestos instead of skin) then stepping onto a floor at 20^oC will feel adversely cool / cold.

Some people do not want that, some don't care, which is why I ask each of my clients how they live, what their needs, wants & wishes are, and I make their home the way they want it. There is no right or wrong, just what people want, and there's over 14,000 people on here  

[JohnMo]

Fair point. Each to their own.

[joe90]

Unfortunately I no longer live in a passive house but one that’s fairly well insulated with gas central heating, I have just bought an electric towel radiator for my bathroom as drying a towel or having a warm towel when you get out of the shower is an pleasure I can afford, plus it’s nice to have a bathroom warmer than the house when you get out of a hot shower and even 20’ feels chilly with no clothes on and wet 😳 (it has a timer by the way).

[markocosic]

And your MVHR or exhaust air heat pump will pickup the excess heat and dump it into your incoming air or the DHW respectively; so meh?

[JohnMo]

 

9 hours ago, puntloos said:

have an 8.5kW EcoDan ashp planned

That's a big heat pump in a passivhaus. Even a very very big one.

[puntloos]

13 hours ago, JohnMo said:

 

That's a big heat pump in a passivhaus. Even a very very big one.

Thanks for the note, always good to doublecheck but my eng team assures me that this value rolled out of the PHPP cals and they've done this often enough so I assume it's correct.

[JohnMo]

How big is your house?

[Mr Punter]

Fit LVT in the bathrooms and there should not be an issue with cold feet.  Only a problem if you have ceramic tiles.

[markocosic]

9 hours ago, puntloos said:

Thanks for the note, always good to doublecheck but my eng team assures me that this value rolled out of the PHPP cals and they've done this often enough so I assume it's correct.

 

Mmmhmmm

 

They're using an ecodan too.

 

So not the most performance oriented outfit.

 

https://heatpumpmonitor.org/

 

Or ecological outfit (it's an fgas unit and not even a particularly low gwp fgas)

 

I'd ask for a 2nd opinion. Most of those ecodans appear to get sold into use cases where the Mitsubishi rep has done all the homework) badly) for some eco grant funding application etc.

 

It wouldn't be the first choice unit of building a passive house where performance and low modulation etc are important.

[JohnMo]

10 hours ago, puntloos said:

assures me that this value rolled out of the PHPP cals and they've done this often enough so I assume it's correct.

 

A passivhaus has a max heating demand of 10W/m2, so that makes your house circa 800m2?

[Dan F]

37 minutes ago, JohnMo said:

 

A passivhaus has a max heating demand of 10W/m2, so that makes your house circa 800m2?

 

Technically it's a maximum heating load of 10W/m2 or max heating demand of 15Kw/m2.yr.  The heating load is typically always <12W/m2 though.

 

You have to allow some headroom though IMO because:

i) house might not perform as well as PHPP says

ii) outdoor temperature used for PHPP is quite modest.  In our case (SE England) it's only -1.6C and we've just had a winter where it was much colder than that for multiple days.

 

The next Ecodan down is likely 5kW which could well be too small for a large house if you were to allow for e.g. max 20W/m2. 

 

 

Edited July 7, 2023 by Dan F

[Gus Potter]

On 06/07/2023 at 13:16, puntloos said:

I have an 8.5kW EcoDan ashp planned, and of course can do underfloor heating anywhere I like. 

The interesting part is bathrooms. So many weird pro/cons:

 

- Amount of heat needed to heat a few tiles in summer is miniscule, the ASHP will surely short cycle unless it's already running for other duties

- Heating electric off grid is somewhat costly. (I suspect it'll cost me about 40/year for all bathrooms, based on a friend's usage stats)

- If I install both ashp and electric my entire first floor probably needs lifting by 10mm, non-zero cost and some minor annoyances and hey, sliiightly reduced head height on an entire floor..

- Electric responds much faster to turning it on, as long as I cleverly turn it off when not in the room, if I use eg 240W mat (the highest I could find.. or will that melt my porcelain tiles/glue etc?)

- Doing both - ASHP when it's running and electric when ashp is not running seems elegant...

 

My sense is end of the day just going electric mats is probably the most straightforward?

This all sounds very complex.. a few watts here and there?

 

In my bathrooms I have UFH, but going for an electric heated towel rail, say 150 -250 watts so that will be on and off in the summer, UFH off.  Enough to make the bathroom feel warmer, actually quite a bit warmer as well insulated.

 

But are you off grid?

 

Look this thing about UFH.. we sit here doing theoretical stuff about weather compensation, floor coverings , all sorts of techy stuff, flow temperatures etc. All of that takes complex controls that break down, valves get sticky after a few years and the folk that supply the say Hive controls keep updating their software. Yes it may look great on paper just now and you may think I'm a philistine. but I was doing this stuff 30 -35 years ago! So I know about the hidden long term costs. Just have a think about how much it is going to cost you to replace the moving parts in five years time say! A twelve way manifold.. the flow control stats! Software updates.. the usual replacement of zone valves.. and if you need to drain down the system.. the inhibitor cost.. it seriously racks up the running cost. But in real life folk try and make do so you end up paying for something technical that works for a bit and then is crap.

 

The intrinsic beauty of UFH is it's simplicity. In my current house I have UFH. The water goes into the manifold at about 30- 40 deg C. The flow through the loops is controlled by some hand turned gate valves. I have a couple of room thermostasts. I have adjusted the flow in the loops maybe twice in the past four years. The maintenance costs are well so far well pretty much none. Yes, on the odd occasion it gets too hot or not hot enough.. but it really is on the odd occasion. When I balance that with the low maintenance / environmental costs I do get the last laugh!

 

What I'm saying is that folk are trying to make UFH into something it is really not suited for.. by introducing all these controls that also need future maintenance. Then folk try and calculate things to some finite degree for flow temperature and zoning.. but they then stick a big sofa over the floor or a rug.. which make all the calcs invalid.

 

If you have a well insulated external envelope then the heat is trapped inside.. you don't lose it.. but you can open some internal doors !

 

 

 

 

 

 

 

 

 

 

 

[Nickfromwales]

On 07/07/2023 at 12:52, puntloos said:

my eng team assures me that this value rolled out of the PHPP cals

Did they show you a copy of this?