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DHW and Heating Options for Passive House


Conor
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Hi. Firstly, not sure this is the correct place to post... never seen so many sub-forums in my life....

 

We're building a 245m2 passive(ish) house this year. South facing, plenty of solar gain when need (bris soleil and external blinds to minimise when not wanted).

 

https://forum.buildhub.org.uk/topic/9228-hi-icf-project-in-holywood/

 

We're in a town and on mains gas, so it seems by far the most logical source of energy for heating and hot water. As much as I want to de-carbonise, a heat pump doesn't make sense in our case. So, that part is sorted.

 

As we'll have triple glazing, ICF walls, large thermal mass, high air tightness, our energy demand will be low. I don't see the point in spending thousands installing a central heating system. Plus we'll have a wood burner - will be rarely used, so not fussed if there's a back boiler on it.

 

Our initial PHPP assessment attached.

 

I'm thinking plumbed towel rads in wet rooms and maybe 2kw post heaters (3x 150mm - one for each level - each with own control e.g. central warm water heater)

 

Any issues with the above? Would also consider electric under tile heating mats for the master en-suite for my other half's chronically cold feet...

 

My main query is hot water.... there's just the two of us and we both travel a lot for work so we've no regular routine. We have a combi boiler in our house which is great.... but hate the long delay for hot water to come out of the tap and the massive pressure and flow drops when somebody else flushes a toilet or turns on a tap. So I'm ruling that out. I'm also planning on a 3-4KW PV system with solar diverter to provide hot water in the summer... so that means some sort of storage.

 

What are my options? In summary, we need instant DHW on demand, high flow rates, low running cost, heatable by gas and PV.

 

Thernal store, unvented HW cylinder or sunamp type storage? Really unsure!!! 

 

Thanks

 

PHPP Summary.JPG

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Either Thermal Store or Unvented cylinder would provide what you want. Both would let you divert excess solar PV to hot water.

 

Sun  Amp may be an option but someone else will have to advise which type would be best as I am not up to speed on all the different variants.  The main advantage of the SunAmp over any form of tank, is lower standing heat losses.

 

I have an unvented cylinder. I chose that because the stored temperature is the temperature that it delivers to the tap, in my case set to 48 degrees so it is within range of an ASHP to heat it.  A thermal store generally needs to store water at a higher temperature than the delivery temperature.

 

Also a lot of thermal stores seem to be not so well insulated. There is no reason for this, but it just seems from those I see that the insulation could be better.

 

But the key to getting good flow rates even when someone flushes the loo, is a good mains water supply pressure AND flow.  If the flow rate of that drops when more than 1 tap is on, then you will need to consider an accumulator to maintain a decent flow.

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3 minutes ago, Conor said:

Plus we'll have a wood burner - will be rarely used, so not fussed if there's a back boiler on it.

 

My first thought is that you don't want to spend money on a heating system, but you want to install a wood burner? How much will that cost to supply and install, including sealed air system, flue, heat-proofing etc?

 

You say you won't use it much. In a house with passivhaus levels of insulation and airtightness, I suspect you'll use it once and then never light it again. They're also hugely unhealthy for you and your neighbours. 

 

Definite yes to low temp UFH in the bathrooms. We were talked out of bathroom heating and it was a mistake. Tiles at around 19-20 deg C are quite cold on the feet. I don't know how much warmer tiles would be with just a radiator. Maybe fine - others will share their experiences.

 

Do keep thinking about overheating (I know it was discussed on your other thread). It may be something as simple as placing ducts and wiring for a ducted aircon system for the bedrooms, which you can retrofit if you find you need it. At the moment you have an estimated 4% above 25 deg C. That's 350 hours per year, which is actually quite a lot (bearing in mind you'll be spending a lot more time above, say, 22-23 deg C, which is still subjectively very warm in a PH environment). Is this before you account for brise soleil etc?

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1 minute ago, jack said:

 

My first thought is that you don't want to spend money on a heating system, but you want to install a wood burner? How much will that cost to supply and install, including sealed air system, flue, heat-proofing etc?

 

You say you won't use it much. In a house with passivhaus levels of insulation and airtightness, I suspect you'll use it once and then never light it again. They're also hugely unhealthy for you and your neighbours. 

 

I agree fully... but my other half is determined to have a fire of some sort. And I've spent the last month chopping 5 large trees in to logs!!! it's on the list and in the budget.

 

3 minutes ago, jack said:

Do keep thinking about overheating (I know it was discussed on your other thread). It may be something as simple as placing ducts and wiring for a ducted aircon system for the bedrooms, which you can retrofit if you find you need it. At the moment you have an estimated 4% above 25 deg C. That's 350 hours per year, which is actually quite a lot (bearing in mind you'll be spending a lot more time above, say, 22-23 deg C, which is still subjectively very warm in a PH environment). Is this before you account for brise soleil etc?

 

It has a single bris soliel, but no active cooling included. We've since had thoughts about the location of the kitchen, which will result in some of the south facing glazing going, and we'll be doing  a part buried basement so the big slider at that level will be going. I also think I might be able to use some sort of ventilation to use the large concrete volume in the basement as a heat dump. I will work with our architect more on this, I want it remodelled at 22c in winter. 

 

I had thought about those simple single unit heat pumps that work as aircon in summer and heating in winter... you know, the big fan units they use in houses in southern Europe.

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23 minutes ago, ProDave said:

Either Thermal Store or Unvented cylinder would provide what you want. Both would let you divert excess solar PV to hot water.

 

Sun  Amp may be an option but someone else will have to advise which type would be best as I am not up to speed on all the different variants.  The main advantage of the SunAmp over any form of tank, is lower standing heat losses.

 

I have an unvented cylinder. I chose that because the stored temperature is the temperature that it delivers to the tap, in my case set to 48 degrees so it is within range of an ASHP to heat it.  A thermal store generally needs to store water at a higher temperature than the delivery temperature.

 

Also a lot of thermal stores seem to be not so well insulated. There is no reason for this, but it just seems from those I see that the insulation could be better.

 

But the key to getting good flow rates even when someone flushes the loo, is a good mains water supply pressure AND flow.  If the flow rate of that drops when more than 1 tap is on, then you will need to consider an accumulator to maintain a decent flow.

 

Thanks Dave. Yeah, I think the losses  due to our irregular water use from a thermal store would wipe out any savings we'd make from PV heating. And I'm already worried about over heating.

I've been reading a lot on here about sunamp... doesn't seem like the product is there yet - the controls at least. I'm 6 months away from installing a system so lets see what happens in that time.

 

Re flows and pressures.... my day job is an engineer in the water sector, so I've at least got this part covered! The reason why our current combi drops flow when other water is used in the house is old pipe configuration and the boiler has limited capacity. I'll be running a 32mm supply to the house, and the area has decent enough mains pressure.

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I went to school in Holywood, so I had to respond to this :) We're in a very similar position to you, although not living in the house yet, so I can't give you real-world experience, unfortunately. 

 

We have 2 young children, both fond of baths and long showers, so had a very low space heat requirement and a large DHW requirement, which was tricky to balance. With some great advice from the people here, we decided on an oversized gas boiler (35Kw) and large sealed thermal store (500L), which has an extra coil in case we decide to add PV at a later date. It's a non-starter for now, but a probability in the next 5 years or so. The low temp UFH (in the slab throughout the ground floor) is fed from the bottom of the tank and the DHW will come off the top. The theory is that this setup will basically work as a massive combi, supplying all the DHW we need and the boiler can run at full whack (in condensing mode) for chunks of time, rather than cycling on and off inefficiently. The thing that put us off the combi was the flow rate as we have possibly multiple outlets and high flow showers running at the same time, and the fact that we couldn't add a hot water return to make sure that we don't have to wait for hot water at the outlets furthest from the boiler/tank. 

 

The tank does have losses, but it is placed in the utility room, next to a hanging/drying area and there is na extract for the MVHR in there so the heat won't go to waste. We can also open the back door if it is particularly bad. Our house is 240sqm area, not quite passive but close. 

 

Good luck! 

dj 

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Oh - we also went for 100w electric UFH in the master bathroom under the tiles, and I really really really wanted a wood burner but previous posters are right, it will be uncomfortably hot within minutes and unusable. We compromised with an outdoor fireplace and pizza oven to get my wood burning fix! Honestly, it will be insanely hot. I LOVE being warm, but after having our UFH on for 2 days during February at a temperature where i couldn't even feel the difference between the flow and return on the manifold, I was breaking a sweat, just walking into the house. It took 4 days to cool down! 

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26 minutes ago, Conor said:

but my other half is determined to have a fire of some sort. 

 

Have you asked her what the point is in having a fire you literally can't light? Take a look at that Grand Designs episode in Bath. That was a well-insulated, but not to PassivHaus standard, German prefab. They tried to talk the owner out of a woodburner but she wouldn't be convinced. Lit it once, never lit it again. You say it's in the budget, but if you never use it, you've still effectively thrown thousands of quid away. Why not spend that money on a heating system?

 

I also don't get the general obsession people have with burning stuff in their houses (I didn't get it even before the health and environmental risks were known). I think it's often a hangover of living in cold drafty houses, where standing in front of a fire or woodburner is the only way to get properly warm. That feeling of never properly being warm won't happen in your house. The desire to stand in front of a hot woodburner just goes away when the whole house is a nice temperature.

 

As for heating with post heaters only, we have PH level insulation and airtightness, but not a lot of solar gain. I turned off the heating in the hot spell a couple of weeks ago, and the slab has slowly cooled down. It reached 18 deg C the other day and the house was definitely feeling a little frostier than desirable. Much of the perceived coldness was due to the cold floor - it's much more comfortable if you put shoes on. I wouldn't be without UFH now I've lived with it.

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Thanks Jack, you make a lot of sense. I will have an outdoor pizza oven so that'll take care of the wood....

 

How effective is an MVHR and distributing warm air through a house? I understand how they work, but if you are extracting 25c air from a warm room, how much of that heat will go in to another living space? I'm wary of these high efficiency claims!

 

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Well we have a wood burner as her and me would rather watch a real fire than most of the crap on tv. It’s a real feature in our “cottage” we live in a very rural location and most around us have fires (not as efficient as ours I bet). With the doors open the heat circulates quite well, the huge brick fireplace acts like a thermal store, yes it gets a bit warm but is only lit fir a couple of hours when the weather is cold. Not sure how much the MVHR retains its warmth and distributes it but the whole house benefits and the effect lasts till the morning which is nice in winter.

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26 minutes ago, Conor said:

How effective is an MVHR and distributing warm air through a house? I understand how they work, but if you are extracting 25c air from a warm room, how much of that heat will go in to another living space? I'm wary of these high efficiency claims!

 

Oddly they are pretty efficient and it is well

documented on the method of heat exchange and how it works. 

 

The issue is rate of change, as the air changes per hour of a room are less than optimal for heating purposes. You may also find you’re the “wrong way round” if you have a stove in a living room as that would normally be an input room not an extract room, so heated air flow would need to be via doorways and corridors first. 

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23 minutes ago, Conor said:

Thanks Jack, you make a lot of sense. I will have an outdoor pizza oven so that'll take care of the wood....

 

How effective is an MVHR and distributing warm air through a house? I understand how they work, but if you are extracting 25c air from a warm room, how much of that heat will go in to another living space? I'm wary of these high efficiency claims!

 

When I light my wbs the heat travels throughout the house by convection. I have a big open-plan kitchen sunroom next to through living room and a large open hallway and staircase so the heat travels round the house this way. Mhrv won't move any decent amount of heat.

If it's a flame effect you could look at bio Ethanol type units. They give of the look but no heat or fumes.

My house isn't passive but if you close the living room door it can quickly get close to 30 degrees which is unbearable. If I light the wbs in the winter the heat will stay in the house for 3-4 days depending on how cold it is outside. I love mine and haven't bought any wood in 5 years as I also had a lot of trees on my site plus all the scrap wood I gathered up during the build.

In use a 350l thermal store for my water needs. It gets heated to 67 degrees and from this I draw all my heating and dhw. I have very gd water pressure so both my showers are really powerful. I do loose heat and the hotpress does get warm but it's never that bad. We did start of with the water temp at 80 degrees , plumbers!!, but after living in it for a few weeks we got to 67 which works for us. 

I also have pv and a diverter so from around now till October the tank gets heated for free with maybe me needing to fire up the pellet stove 3 to 4 times when we get our usual run of crap weather.

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19 minutes ago, joe90 said:

Well we have a wood burner as her and me would rather watch a real fire than most of the crap on tv. It’s a real feature in our “cottage” we live in a very rural location and most around us have fires (not as efficient as ours I bet). With the doors open the heat circulates quite well, the huge brick fireplace acts like a thermal store, yes it gets a bit warm but is only lit fir a couple of hours when the weather is cold. Not sure how much the MVHR retains its warmth and distributes it but the whole house benefits and the effect lasts till the morning which is nice in winter.

I am another one in a house with (just about) passive house levels of insulation, and a WBS.

 

The No 1 thing is it is a low power (4KW) stove with air intake ducted straight in from outside (room sealed)

 

It works very well. Only used in very cold weather.  Burn it for a few hours and yes it will warm up the house, typically to about 25 degrees (that is the point we stop refueling it)  some will say that is "cosy" some will say that is overheated.

 

The bigest benefit for us is that gets a bit more warmth upstairs (no heating upstairs) on the few occasions it gets a bit cool up there.

 

However I am convinced it works for us because the layout of the house, with a central stairwell, and double doors from the living rooms to that stairwell, creates and easy route for heat to circulate and the stove to heat the whole house.  If we shut the doors and the heat from the stove was confined to just one room, that room would get uncomfortably hot very quickly.  so if you are still considering a stove think about it's location and how heat will circulate the the whole house.

 

And like @joe90 we are in a very low density rural location. I would think again if we were in the middle of a town.

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2 hours ago, Conor said:

I agree fully... but my other half is determined to have a fire of some sort. And I've spent the last month chopping 5 large trees in to logs!!! it's on the list and in the budget.

you have mains gas --plenty of pretty looking gas fires that look like wood burners ,but with no cleaning out and having to store logs to dry out etc ,etc ?

I   gone half way with an  belling optimyst stove --no other flame effect can compare cos its water vapour you are illuminating + it has  a 2kw elec fan heater as  well

If its  effect your looking for go see one in the flesh -video is good --even better when you see it

 

just a suggestion   for you 

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