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Author Topic: cold air hot water  (Read 1928 times)
eabadger
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« on: August 03, 2018, 03:30:34 PM »

Hi,
just a thought, we have a successful off grid setup with DHW provided by wood in winter and an air to water heat pump in summer, works well, but during this very hot spell i have got to thinking...
on very hot days we run a split air to air pump to cool the downstairs, but why cant i duct the expelled cold air from the water heat pump back in to the house? thus heat and cooling at the same time, the dogs would be annoyed as they make for the cold air to cool down.
any thoughts?
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kristen
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« Reply #1 on: August 03, 2018, 03:55:34 PM »

I have similar question:

I'm thinking of installing an air-source heatpump to put cold water through the UFH in summer, and I was wondering how hot the exhaust would be, and whether I can use that for something?

We have Solar Hot Water (both for DHW and Pool) and if I was doing again I would use heatpump and PV instead. The once a year-ot-two Plumber call-out for Solar Thermal have negated any possible cost-saving
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eabadger
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« Reply #2 on: August 03, 2018, 04:01:24 PM »

our heat pump comes on at 3 and off at 6, it heats 300ltr of water to 52* and runs 5 towel radiators, 3 oclock was set as batteries are normally full by then and it seemed we were wasting afternoon sun, so the 2kw input heat pump runs straight off our pv leaving batteries to absorb then float.
but cooling inside of house as a byproduct would be great, rig up a summer duct system in to the heat recovery unit to distribute around the house?
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1600w PV main array at 24v, excide 2v 1000a forklift cells now x 2, 320w PV secondary array at 12v. Enfield 1944 ex RAF 5.6kw diesel genset (now in pieces, big ends gone), Petter AC1 28v diesel charging set at 2.8kw.
1kw wind turbine.
26kw wood stove back boiler to underfloor heating and dhw
kristen
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« Reply #3 on: August 03, 2018, 04:07:26 PM »

rig up a summer duct system in to the heat recovery unit to distribute around the house?

We have a RAD in upstairs MVHR for winter. Its pretty pathetic (and the designer told us it would be ...) so not convinced that MVHR is the right way to distribute it.

We have UFH (wouldn't have bothered in a  new build Passive House, but ours is extension and we already had boiler) and now I've got over "don't use any energy" and replaced it with "have some comfort" along with "endeavour to generate 100% of your own energy" I would be happy to use electricity for cooling the UFH - as you say, we too have loads of PV on days when it is hot enough to need cooling.

The Night venting has been fine (to maintain temperature), but it lets the flies in ...
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kristen
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« Reply #4 on: August 03, 2018, 04:12:07 PM »

P.S. heatpump probably reversible? So if we had boiler-failure we could run heatpump for winter-heat in UFH ?
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eabadger
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« Reply #5 on: August 03, 2018, 04:33:50 PM »

ours is reversible, as  i already port output to towel rads i could do same to UFH and put on cool, but our house has massive oversized slab and floor never seems to get warm in summer, old stone building with shutters down stays cool, just some days a cold breeze from our aircon goes down well with my beer, which is where i am now.

steve
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1600w PV main array at 24v, excide 2v 1000a forklift cells now x 2, 320w PV secondary array at 12v. Enfield 1944 ex RAF 5.6kw diesel genset (now in pieces, big ends gone), Petter AC1 28v diesel charging set at 2.8kw.
1kw wind turbine.
26kw wood stove back boiler to underfloor heating and dhw
Tinbum
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« Reply #6 on: August 04, 2018, 12:18:06 AM »



I'm thinking of installing an air-source heatpump to put cold water through the UFH in summer,

Just be careful that the floor doesn't get too cold that on these humid days water condenses on it.
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kristen
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« Reply #7 on: August 04, 2018, 07:51:14 AM »



I'm thinking of installing an air-source heatpump to put cold water through the UFH in summer,

Just be careful that the floor doesn't get too cold that on these humid days water condenses on it.

Thanks. So what temperature is a good one to shoot for?

Presumably some sums that I can do in order to work out "how much cooling needed" and "how much it will lower the room temperature" ?

This is passive house, max temperature gain during a day is about 1C .. so Presumably "compared to normal" I have far less heat to shift ...

But I do have big thermal mass - all internal walls are dense-block, for example
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Tinbum
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« Reply #8 on: August 04, 2018, 09:04:54 AM »

Hi sounds ok. Look up dew point.
« Last Edit: August 04, 2018, 09:52:34 AM by Tinbum » Logged

85no 58mm solar thermal tubes, 28.5Kw PV, 3 x Sunny Backup 5048, 3x Sunny Island 5048, 2795 Ah (135kWh) (c20) Rolls batteries 48v, Atmos wood gasification boiler, Brosley wood burner, 2000lt buffer tank and 250lt DHW
Philip R
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« Reply #9 on: August 05, 2018, 11:20:47 AM »

Increase air temperature by approx 8 degC, you will increase the water carrying capacity if the air by about 100 %.
Eg air at 10 degC at 100% Relative humidity. Warm to about 18 degC, its Relative humidity will have fallen to 50%.

If you cool down your floor or rads with a heat pump. the water in the air will dew on its cold surface, like on a mirror.
Causing a damp floor or rusty bottomed radiators.

Also why dehumidifiers, fridges, air conditioners have a condensate collection sump.
Philip R
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kristen
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« Reply #10 on: August 05, 2018, 04:56:16 PM »

Useful, thanks.

Clearly I can just install some wet-finger-in-air (e.g. take the advice of my engineering-plumber) and install some equipment, and then control it such that I don't get condensation on the floor ...

But I have two things which i think may work in my favour:

On a hot summer's day the house will gain 1C (and the converse in Winter - lock the house up and go away for 24 hours in mid winter and it will cool around 1C too)

So I have not got a lot of heat dT to get rid of (other than the fact that the thermal mass is high, so quite a lot of "pumping" will be involved, and thus I guess that a bigger dT floor temperature will speed that up ... but also risk condensation).

The house has MVHR, so a steady supply of external, fresh, air. Not sure if that helps because, for example outside air at 33C will be cooled to close to 24-25C (internal temperature) through the heat exchanger, so its relative humidity will have increased already - before I expose it to a cold floor surface.
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pdf27
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« Reply #11 on: August 05, 2018, 05:14:06 PM »

The house has MVHR, so a steady supply of external, fresh, air. Not sure if that helps because, for example outside air at 33C will be cooled to close to 24-25C (internal temperature) through the heat exchanger, so its relative humidity will have increased already - before I expose it to a cold floor surface.
It should help a lot - the floor surface will most likely be less than a degree cooler than the air temperature above the floor in a Passivhaus (less than 5W/m2 heat transfer for just about all likely cooling scenarios means the two will be really closely tied together plus no drafts meaning air flow will be very laminar). That leaves very little water capable of being extracted from the air by the floor - if the floor is at 22C and the air at 23C, the absolute maximum water that could be extracted from the air is 1g/m3 - so the potential maximum condensation rate is pretty small.
It's really easy to guarantee a fix though - just fit a water-to-air heat exchanger downstream of the MVHR with the same water supply being used for the floor passing through it. The water will be 3-4C cooler than the floor surface temperature because of the thermal resistance of the concrete/screed, so it should extract a fair bit more water from the air and ensure that the dew point is always a little bit cooler than the coldest floor surface temperature.
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kristen
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« Reply #12 on: August 05, 2018, 06:11:14 PM »

Thanks. The MVHR equipment is not ideally suited for that - long way from where the heat pump and the flow to UFH manifolds would be, and no easy pipe runs.  But I'll keep it in mind.

A narrow dT sounds right, in the sense that the dT of UFH water in winter is only a few degrees more than ambient. It suits us to run the pump for many hours, but small dT, rather than a shorter, hot, burst to heat the slab.  So small dT for UFH cooling "feels right" too, except that ideally I would only want to run it during hours of self-PV generation - PV generation coincides with need for cooling, of course.

I've also been mulling over the old part of the house. That has a much bigger MVHR unit (and no UFH). It tends not to heat up so much in Summer, as faces East/West (and none of it South), and we don't seem to get much evening heat gain, and we close curtains in morning for the Easterly gain (which seems the greatest to me, although that feels illogical as I would expect the West side to be more - might be the shape/size of the West facing rooms perhaps ... I think there is less window area on West side)

Anyway, although that MVHR is also not ideally placed adding cooling to the air flow supplied into the house would definitely be an advantage. Maybe it would be worth just dedicating a heatpump to it, at the point of flow into the house
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rogeriko
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« Reply #13 on: August 06, 2018, 12:34:52 AM »

Just add a simple split unit air to air airconditioner like you see in every european hotel. they are very cheap, only a couple of hundred pounds, do what they are supposed to do and have no condensation problems. I can see a huge business looming for air con installers in the future. Most of these units are even DIY installs, they come precharged with gas just connect 2 pipes through the wall and turn on.
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eabadger
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« Reply #14 on: August 07, 2018, 08:15:13 AM »

already got one of those, am happy with air cooling, but the similar unit heating my water is byproducting freezing cold air, i initially suggested possibility of ducting this air back in to my house to cool it for free.
i am going to test it soon, a fanned duct in to the soon to be livingroom currently workshop, if it cools it down i can think about using the MHRV system to move it round the house in summer.

started thread wondering if anyone had tried it?
the diy install of the aircon units is a doddle, the gas isnt in the units its in the pipes, position both units connect pipes and open all valves, i had to top mine up after moving it but got a kit from italy for about 70.
as a by product of the aircon i produce distilled water for my batteries, i like free byproducts!

steve
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1600w PV main array at 24v, excide 2v 1000a forklift cells now x 2, 320w PV secondary array at 12v. Enfield 1944 ex RAF 5.6kw diesel genset (now in pieces, big ends gone), Petter AC1 28v diesel charging set at 2.8kw.
1kw wind turbine.
26kw wood stove back boiler to underfloor heating and dhw
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